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VOL. 4, NO. 2 JUNE 2016 ISSN 2325-5987

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IEEE Electrification Magazine (ISSN 2325-5897) (IEMECM) is published quarterly by the Institute of Electrical and Electronics Engineers, Inc. Headquarters: 3 Park Avenue, 17th Floor, New York, NY 10016-5997 USA. Responsibility for the contents rests upon the authors and not upon the IEEE, the Society, or its members. IEEE Operations Center (for orders, subscriptions, address changes): 445 Hoes Lane, Piscataway, NJ 08854 USA. Telephone: +1 732 981 0060, +1 800 678 4333. Individual copies: IEEE members US$20.00 (first copy only), nonmembers US$123.00 per copy. Subscription Rates: Society members included with membership dues. Subscription rates available upon request. Copyright and reprint permissions: Abstracting is permitted with credit to the source. Libraries are permitted to photocopy beyond the limits of U.S. Copyright law for the private use of patrons 1) those post-1977 articles that carry a code at the bottom of the first page, provided the per-copy fee indicated in the code is paid through the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA; 2) pre-1978 articles without fee. For other copying, reprint, or republication permission, write Copyrights and Permissions Department, IEEE Operations Center, 445 Hoes Lane, Piscataway, NJ 08854 USA. Copyright © 2016 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Periodicals postage pending at New York, NY, and at additional mailing offices. Postmaster: Send address changes to IEEE Electrificaton Magazine, IEEE Operations Center, 445 Hoes Lane, Piscataway, NJ 08854 USA. Canadian GST #125634188 PRINTED IN U.S.A.

MISSION STATEMENT: IEEE Electrification Maga-zine is dedicated to disseminating information on all matters related to microgrids onboard electric vehicles, ships, trains, planes, and off-grid applica-tions. Microgrids refer to an electric network in a car, a ship, a plane or an electric train, which has a limited number of sources and multiple loads. Off-grid applications include small scale electricity sup-ply in areas away from high voltage power net-works. Feature articles focus on advanced con-cepts, technologies, and practices associated with all aspects of electrification in the transportation and off-grid sectors from a technical perspective in synergy with nontechnical areas such as business, environmental, and social concerns.

Digital Object Identifier 10.1109/MELE.2016.2556199

F E A T U R E S

D E PA R T M E N T S & C O L U M N S

Solar-dc Microgrid for Indian HomesA transforming power scenario.Ashok Jhunjhunwala, Aditya Lolla, and Prabhjot Kaur

10

Next-Generation Shipboard DC Power SystemIntroducing smart grid and dc microgrid technologies into maritime electrical networks.Zheming Jin, Giorgio Sulligoi, Rob Cuzner, Lexuan Meng, Juan C. Vasquez, and Josep M. Guerrero

45

DC Microgrid ProtectionUsing the coupled-inductor solid-state circuit breaker.Atif Maqsood and Keith Corzine

58DC Local Power DistributionTechnology, deployment, and pathways to success.Bruce Nordman and Ken Christensen

29

Voltage-Level Selection of Future Two-Level LVdc Distribution GridsA compromise between grid compatibility, safety, and efficiency.Enrique Rodriguez-Diaz, Fang Chen, Juan C. Vasquez, Josep M. Guerrero, Rolando Burgos, and Dushan Boroyevich

20

Are Microgrids the Future of Energy?DC microgrids from concept to demonstration to deployment.Luis Eduardo Zubieta

37

2 ABOUT THIS ISSUE4 VIEWPOINT

65 DATES AHEAD

66 NEWSFEED 72 TECHNOLOGY LEADERS

Cover image: Background house by Nerthuz; power lines by Serz72; and sky, garage solar, and wind power by petovarga.

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A B O U T T H I S I S S U E

IEEE Electr i f icat ion Magazine / JUNE 20162

RECENTLY READ A LETTER TO THE EDITOR IN ANIEEE publication in which the statement was made that “dc is already used where it makes sense; the

same goes for ac.” Although this is an argument for maintaining the status quo of how power is distributed, the fact is that the number of applications in which dc makes sense is increasingly growing. The growth in dc microgrids and distribution world-wide is a testament to this fact. A universal concurrence of the articles in this issue of IEEE Electrification Magazine is that a widespread adoption of dc distribution is impeded mainly by a lack of standardization, a lack of experience, and most notably by insufficient solutions to dc distribution-system protection.

In recent years, the most significant technological develop-ments with regard to dc have been in the area of multiterminal high-voltage dc (HVdc) distribution. ABB [Allmänna Svenska Ele-ktriska Aktiebolaget, Västerås (ASEA), Sweden Brown Boveri,

Zurich, Switzerland] has devel-oped a commercially viable hybrid HVdc circuit breaker that will enable the extension of offshore wind power from HVdc transmission lines to a terrestrial HVdc distribution network. Voltage source con-verter (VSC)-based intercon-nections between offshore

wind farms in the North Sea and Germany are already in place with planned implementations of multiterminal HVdc distribu-tion across Germany, throughout Central Europe, and into Northern Africa, as well as China. Embedding dc into the ac grid will result in a more controllable and precise power exchange and will enable significant mixing of renewable energy with regional power sources. On a global scale, the potential is for the reduction of greenhouse gases, increased energy independence, and improved resilience of electrical power delivery systems.

By Rob Cuzner

Does DC Distribution Make Sense?

Digital Object Identifier 10.1109/MELE.2016.2543978Date of publication: 31 May 2016

I


EDITORIAL BOARDIqbal HusainEditor-in-ChiefNorth Carolina State UniversityNorth Carolina, [email protected]

Eduard MuljadiCoeditor, Electric VehiclesNREL: Wind ResearchColorado, [email protected]

Herb GinnEditor, Electric ShipsUniversitiy of South CarolinaSouth Carolina, [email protected]

Robert CuznerCoeditor, Electric ShipsUniversity of Wisconsin-MilwaukeeWisconsin, [email protected]

Eduardo Pilo de la FuenteEditor, Electric TrainsEPRail Research and [email protected]

Jose Conrado MartinezCoeditor, Electric TrainsDirectcion de Estrategia y [email protected]

Bulent SarliogluEditor, Electric PlanesUniversity of Wisconsin-MadisonWisconsin, [email protected]

MohammadShahidehpourEditor, MicrogridIllinois Institute of TechnologyChicago, [email protected]

Steve PullinsCoeditor, MicrogridGridIntellectTennessee, [email protected]

IEEE PERIODICALS MAGAZINES DEPARTMENT445 Hoes Lane, Piscataway, NJ 08854 USA

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The fact is that

the number of

applications in which

dc makes sense is

increasingly growing.

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IEEE Electr i f icat ion Magazine / JUNE 2016 3

Since the 1960s, HVdc transmis-sion has been in practice worldwide as an effective means for bulk point-to-point transmission over electricity over long distances. In the late 1970s, the application of solid-state devices for HVac-to-HVdc conversion at both ends for using thyristor devices for voltage levels of nearly 600 kV was the most impressive demonstration of power-conversion technology on the planet. A field trip I took as an electrical engineering student to the source of HVdc transmission between Utah and California—the Intermountain Power Plant in Delta, Utah—was what introduced me to the field of power electronics and convinced me that this was what I wanted to do. Although the main focus of this issue is on low-voltage (LV) dc, the relevance of both this trip down memory lane and the opportu-nity to be in awe of ABB’s recent achievements is to point out that the most significant advances in dc transmission and distribution have occurred in systems in which per-haps the challenges are greatest—at voltage levels in the hundreds of thousands!

So, the question to ask is whether the advances in HVdc are foreshadow-ing a “dc revolution” in the area of LV distribution that will bring the dc distri-bution closer to the daily lives of every-day people. The “Viewpoint” column from Ewan Pritchard and Daniel C. Gregory of North Carolina State Univer-sity illustrates that we may very well be heading in this direction by highlight-ing several technological trends that are driving a shift toward dc.

It is my hope that this issue of IEEE Electrification Magazine will give readers a compelling opportunity to look at where we are today in the dc versus ac discussion. Many of the authors have chosen to bring us into the present by revisiting the past “war of the currents” between Edi-son and Tesla at the beginning of the 20th century to show that, after a hundred years of ac infrastructure,

there is good reason to revisit dis-tributed energy delivery using dc. These reasons run the gamut from the role of dc in enabling electrifica-tion of rural homes in India (where both the need and opportunity are great), to LVdc distribution in green buildings and dc-enabled smart homes, to a plausible evolution from managed dc systems like universal serial bus (USB) and power over Ethernet, to nanogrids that have enabled increased plug and play of native dc electronic loads. Although many concepts still seem futuristic, “Solar-dc Microgrid for Indian Homes” by Ashok Jhunjhunwala, Aditya Lolla, and Prabhjot Kaur dem-onstrates that local dc distribution of solar power is happening today, transforming and lifting individuals,

families, and communities. This issue also illuminates architectures, the integration of systems using commercially available hardware, lessons learned from highly surviv-able shipboard medium-voltage dc systems that are self-healing, and, of course, new developments in dc pro-tective devices. All of these activities and achievements are paving the way toward more widespread inte-gration of dc microgrid concepts at all levels of electrical power delivery with its attendant benefits. I hope you enjoy this issue of IEEE Electrifica-tion Magazine, that you will find it educational, and that, perhaps, it will provoke new ideas, innovations, and solutions.

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V I E W P O I N T

The dc RevolutionBy Ewan Pritchard, Daniel C. Gregory, and Srdjan Srdic

ITH THE ADVENT OF advanced power electron-ics, distributed renewable

generation, and direct-current (dc) distribution, the power industry has been anticipating a massive revolu-tion in the way power is used in the United States. Although this expected local shift has been gradual to date, it may be approaching quickly and qui-etly from the periphery. According to a recent report from Navigant Research, the dc product marketplace is expect-ed to total US$33 billion between 2015 and 2024 in four key market seg-ments: off- and bad-grid telecommu-nications, data centers, commercial buildings, and off-grid military opera-tions. A similar developing trend is taking place in the microgrid market, where a study by Markets and Mar-kets forecasts a US$35 billion annual marketplace by 2022. With such a wide open market, power industry experts are all left wondering: What percentage of these microgrids will be alternating current (ac) or hybrid ac–dc? Will one of these trends loosen the firm grip that ac has in our legacy system? If this grip loosens, will it spread as fast as cell phones and mobile computing?

The major trends that are driving this shift to dc include

1) transportation electrification2) the developing world

3) military operations4) telecommunications5) data centers.

Transportation ElectrificationShipboard power, electrified rail, die-sel locomotives, and submarine power systems have been using dc for the past century. However, recent advances in power electronics have required each of these industries to respond. Traditional power engineers have substantial intellectual invest-ment in frequency-based control; as a result, they use frequency imbal-ances to signal when net power transfer does not match. On the other hand, modern transportation power engineers control the system

by monitoring voltages (and, thus, field excitation).

The Centre for Solar Energy and Hydrogen Research in Baden-Würt-temberg, Germany, claims that the number of electric vehicles (EVs) worldwide more than doubled in 2015, bringing the total on the road to more than 1,300,000. In electric and hybrid vehicles, the Centre’s automotive engineers at Tesla, Nis-san, Toyota, Mitsubishi, BYD, BMW, Ford, and General Motors are build-ing small dc microgrids, by the thousands, with unparalleled safety systems. The transportation engi-neer now has the ability to better control the loads, essentially throt-tling back on the power demand as

W


Figure 1. The plug-in hybrid developed at North Carolina State University operates as a 380-Vdc microgrid. (Photo courtesy of North Carolina State University’s FREEDM Systems Center.)

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voltages begin to drop. The chal-lenge for traditional power engi-neers becomes one of confidence in controls and the ability to regu-late the bus voltage without having frequency as a control that they can use. The combination of new smart grid controls and some of the learned practices in the auto-motive industry may be able to solve these challenges.

Each of these EVs has tens of kilo-watt hours of energy stored in its bat-tery packs, and, when fitted with bidirectional power converters that have the ability to communicate with the grid, each will eventually serve as a load, power source, or distributed energy storage that can support the grid; contribute to load peak shaving and reactive power compensation; and improve system efficiency, sta-bility, and power delivery reliability and security. Large public or private

parking areas are particularly suited for the application of dc distribution systems, where many privately owned or fleet vehicles can be indi-vidually charged or discharged according to the cost of electricity, battery state of charge, and vehicle-owner preferences. When parked, electric school buses and university on-campus service EVs with enabled vehicle-to-grid technology will also help with peak-load shaving and electricity cost reduction for the institutions.

Developing WorldAccording to the World Bank, approx-imately 1.1 billion individuals world-wide do not have access to electricity; many are in areas that the grid sim-ply cannot reach. A common practice (often for a fee) is to charge a 12-Vdc battery with jumper cables connected to the output of an alternator, install

the charged battery in a home, and feed 12-Vdc loads until the battery is discharged. In these areas, a dc grid is an obvious choice. Often, simple dis-jointed systems of solar panels, bat-teries (e.g., car batteries), and efficient dc loads (e.g., light-emitting diode light bulbs, well pumps, and motors) quickly scale. Such systems safely power entire villages without the need for sophisticated controls, tech-nical experts, or formal training. Rural villagers find many ways to power cell phones, televisions, and lights with safe low-voltage (<50 V) dc systems. This dc power system solution is fossil fuel based (gaso-line), dangerous (mechanically haz-ardous), expensive (~300 depth of discharge cycles/lead acid battery), and toxic (acid/gas/fire exposure). However, this disjointed approach works and is thus commonplace throughout the developing world.

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V I E W P O I N T

Diesel generators are often found in the developing world, but they have negative economic, health, and envi-ronmental impacts. Negative eco-nomic impacts of diesel electric gen-eration include the fact that business-es and wealthy homeowners typically operate a diesel gen erator to supply electricity at as much as US$1/kWh (life-cycle cost). Recent advancements in systems engineering, software, power electronics, materials science, and energy storage combine to enable clean, quiet, affordable, and reliable dc electric systems. Such systems are capable of delivering high end-to-end efficiency, with more than 99.999% availability at less than US$0.20/kWh, and can be rapidly set up anywhere in the developing world. Applying high-efficiency buck converters to convert and regulate dc voltages, high-effi-ciency lithium titanate oxide (LTO) batteries for electric storage, low-cost solar photovoltaic panels, and select

dc products minimize power conver-sion losses and enable highly effi-cient operation. Depending on the system configuration and layout, resistive losses (e.g., line losses) may be greater than cumulative losses due to power conversion and storage. Another advantage of this architecture is that it enables a solid-state power system configura-tion that ensures reliability, durabili-ty, and portability.

Military OperationsMilitary forward o perating bases in conflict zones present unique elec-trification challenges for which dc microgrid architecture is ideally suit-ed and can be rapidly deployed. A number of dc solutions are being used for such operations. Integrated systems based on a simplified archi-tecture can be assembled in hard-ened seismic-rated enclosures to facilitate a portable, containerized

power system. Solar and small wind generation coupled to LTO battery storage can provide a dc grid any-where that a ship, truck, or helicop-ter can reach. Pos-En, of the FREEDM Systems Center at North Carolina State University, and Schneider Elec-tric, supported by the U.S. Trade and Development Agency, have devel-oped a containerized modular elec-tric generator (MEG) capable of deliv-ering up to 180-kW peak output at 380 Vdc with 45-kWh onboard LTO storage and onboard dc generation. Schneider is currently developing a commercial version of the MEG.

Electric power systems for mili-tary applications must meet federal building standards, National Electric Code (NEC) standards, and applicable military specifications (MILSPEC) standards. A MILSPEC-compliant ver-sion of the MEG has not yet been designed, but it is feasible. Mission-critical military assets (e.g., ships, air-craft, weapons, and forward operat-ing bases) require compliance with extreme MILSPEC standards. A num-ber of MILSPEC standards are appli-cable to dc grids, with a baseline of MIL-STD-810, that define environ-mental requirements impacting the grid system, component selection, and overall design. For example, cer-tain (but not all) MILSPEC-compliant electronics must operate properly within the temperature range of –55 °C to 125 °C. Certain aspects of MILSPEC dc system design require-ments may be classified.

A MEG can be transported using a Skycrane (Figure 2) to a forward oper-ating base. The MEG would include all needed equipment, tools, and sup-plies to operate for an extended peri-od without refueling and could be disconnected, moved, and reinstalled at another location in a few hours by minimally trained personnel.

TelecommunicationsThe telecommunications industry has always used dc, and with the explosion in mobile, there is a need

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for higher power and voltages. A recent study by the Pew Charitable Trusts estimates that 80% of the popu-lation of sub-Saharan Africa carries a cell phone, and another study by the World Bank shows that only 24% of the same population has access to electricity. The remaining population is likely getting power from bat-teries, as described above, but this also speaks to the level of off-grid telecommunications towers in the region. The telecommunications industry has estab-lished its own standards for dc voltages, such as the 380-Vdc standard ETSI-EN-300, and powers much of its own equipment using small dc microgrids.

Data CentersThe data center market is expected to grow from US$14.59 billion in 2014 to US$22.73 billion by 2019, at a compound annual growth rate of 9.3%. With this growth comes an increased energy demand on grid infrastructure. Power distribution, cooling, and energy costs comprise 31% of a typical data center’s monthly operating expenditures. This percentage is based on an electric rate of US$0.07/kWh, which is currently attainable in the United States through careful site location within certain regions; however, this is unre-alistic in highly populated areas such as New England (US$0.16/kWh) and California (US$0.14/kWh). Many other regions offering low electric rates do not have sufficient networking infrastructure to support current data center communications requirements. The risk of electric rate increases or data bandwidth limits during the 10–15-year life of a typical data center investment is concerning.

Reliability, efficiency, and cost benefits result from applying dc power system architecture versus ac or hybrid power systems architecture throughout data center design. Reliability is improved by elimi-nating the need for ac power supply to the data

Figure 2. A conceptual image of the Sikorsky CH-54b Skycrane. (Image licensed from: http://www.turbosquid.com/3d-models/sikorsky-ch-54b-skycrane-helicopter-max/691548 by Dan Gregory.)

Thermal Clad® Manages Heat In Surface Mount Power Applications.

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V I E W P O I N T

center, thus eliminating transform-ers, switchgear, inverters, and rec-tifiers throughout the entire data center infrastructure. Capital expenditures are reduced by sim-plifying power system design through the elimination of ac equipment and reduction in cooling capacity.

Researchers at the Lawrence Berkeley National Laboratory have been investigating efficiency in data centers since 2004. Poor efficiency of power distribution brought about the idea of eliminating some of the power conversions by using 380 Vdc. Results of their research were outlined in the November 2012 issue of IEEE Power & Energy Magazine, appropriately titled “Edison Redux: 380 Vdc Brings Reli-ability and Efficiency to Sustainable Data Centers.” According to the arti-cle, replacing legacy power distribu-tion with 380 Vdc was 28% more effi-cient than with 208 Vac. In addition, findings estimated up to a 15% decrease in costs due to elimination of components and simplification of the power distribution system. These findings were peer reviewed and con-firmed by the open industry consor-tium The Green Grid.

Obstacles to dcThere are some key issues keep us from widely adopting dc as a stan-dard in the United States, and until these issues are addressed, we can-not expect the dc revolution to take root. Some of the issues include:

understandingstandardsproducts.

Lack of dc KnowledgeThere is a popular misconception that dc is more dangerous than ac, despite countless articles to the contrary.

(Consider the electric chair widely believed to be an invention financed by Edison to prove that ac is more dangerous.) There is also the miscon-ception that power losses from dc are greater than those from ac, which is often true because dc is usually at such a low voltage, but at the same voltage dc is always more efficient. In addition, there is a feeling that dc can-not be used to run long distances despite ample evidence from long-distance high-voltage dc lines.

In the “war of the currents” between Tesla and Edison, there was a very strong argument that ac could be inexpensively and efficiently transformed to higher voltages. At the time, this was the only transfor-mation that allowed for high-voltage transmission and distribution. But as groups such as the FREEDM Systems Center and their industrial partners develop advanced power electronic solutions to transform any electricity to ac or dc using solid-state trans-formers, this argument becomes less valid, and all of these elements devolve into a fear of the unknown.

dc StandardsThere are a few standard dc voltages, such as the 12-V standard in vehicles or the 5-V standard for universal serial bus devices, but for higher power, widely adopted higher standard volt-ages must be developed. Owing to the 50-V standard for touch safe opera-tions, there is a clean breakpoint at 48 V, and many products already con-form to this standard. Additionally, forklifts and the telecom industry have settled on this as a standard volt-age. Both the European Telecommuni-cations Standards Institute (ETSI) and the EMerge Alliance have standard-ized on 380 Vdc and produced guide-lines for dc power distribution. These standards have been in use for several years. But even if a user wanted one of the standard dc distribution voltages in their home, business, or industry, few engineers could lean on existing codes and standards to select

Figure 3. MEGs used for data center power. (Photo courtesy of Pos-En.)

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products or approaches for their designs. In the United States, restrictions are imposed on electrical systems by the requirements of NEC (NFPA-70), Section 210.6, branch-circuit voltage limitations. Briefly, this code limits the voltage in residential and similar occupancies to 120 V (root mean square) between conductors for typical cord-and-plug connected loads. For specific loads, the limits are raised to 277 V among conductors. The voltage limitations imposed by 210.6 effectively exclude the use of dc at efficient voltage levels from the majori-ty of applications in this country. In other countries (e.g., Canada), there are similar restrictions from the Canadian Electrical Code (CSA C22.1).

Lack of ProductsAlthough the vast majority of products in the home today use dc energy, each one containing a small (lower-effi-ciency) converter, not many can plug into a 380-Vdc out-let or even a 48-Vdc outlet. However, a number of groups are working to provide widespread products, such as the FREEDM Systems Center and EMerge Alliance.

For Further ReadingL. Lorenz. (2015, Nov. 17). Revenue from direct current distribution network implementation is expected to total $33 billion from 2015 to 2024. Navigant Research. [Online]. Available: https://www.navigantresearch.com/newsroom/revenue-from-direct-current-distribution-network- implementation-is-expected-to-total-33-billion-from-2015-to-2024

The World Bank. (2016, Apr. 5). 2015 World Access to Energy. [Online]. Available: http://www.worldbank.org/en/topic/energy/overview#1

Pew Research Center. (2015, Apr. 15). Cell phones in Africa: Communication lifeline. [Online]. Available: http://www.pewglobal.org/2015/04/15/cell-phones-in-africa-commu-nication-lifeline/

E. Holodny. (2014, July 9). Thomas Edison secretly financed the first electric chair to destroy his rival. Business Insider [Online]. Available: http://www.businessinsider.com/edison-financed-the-electric-chair-2014-7

G. AlLee and W. Tschudi, “Edison redux: 380 Vdc brings reliability and efficiency to sustainable data centers,” IEEE Power Energy Mag., vol. 10, no. 6, pp. 50–59, Nov.–Dec. 2012.

BiographiesEwan Pritchard ([email protected]) is an applied auto-motive fellow and associate director of the FREEDM Sys-tems Center, North Carolina State University, Raleigh.

Daniel C. Gregory (dan.gregory@ pos-en.com) is the chief executive officer of Pos-En, FREEDM Systems Center, North Carolina State University, Raleigh. He is a Member of the IEEE and the International Coun-cil on Large Electric Systems.

Srdjan Srdic ([email protected]) is a visiting assistant professor at North Carolina State University, Raleigh.

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https://www.navigantresearch.com/newsroom/revenue-from-direct-current-distribution-network-implementation-is-expected-to-total-33-billion-from-2015-to-2024

http://www.worldbank.org/en/topic/energy/overview#1

http://www.pewglobal.org/2015/04/15/cell-phones-in-africa-communication-lifeline/

http://www.businessinsider.com/edison-financed-the-electric-chair-2014-7




http://www.ieeeusa.org/policy/govfel


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BACKGOUND AND SUN IMAGE LICENSED BY INGRAM PUBLISHING

2325-5987/16©2016IEEEIEEE Electr i f icat ion Magazine / JUNE 20161010

T IS WELL ESTABLISHED THAT ACCESS to energy is closely linked with socioeco-nomic development. India houses the largest share of the world’s population deprived of electricity with about 237 mil-

lion people lacking access (International Energy Agency). At the same time, in India, many households that do have access to electricity lack an uninterrupted and quality power supply. A recent study con-ducted by the Council for Energy, Environment, and Water (CEEW) across six states (Bihar, Jharkhand, Madhya Pradesh, Uttar Pradesh, West Bengal, and Odisha), found that about 50% of the households had no electricity despite having a grid con-nection. This indicates that there is an immediate need to address the quality, affordability, and reliability of the power supply in addition to extending the grid footprint.

As shown in Figure 1, the CEEW report suggests that, despite thou-sands of villages being electrified over the past decade under the Gov-ernment’s rural electrification pro-gram, over 85% of the households in five of the six states considered in


By Ashok Jhunjhunwala, Aditya Lolla, and Prabhjot Kaur

Solar-dc Microgrid for Indian HomesA transforming power scenario.

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the study had electricity for less than 8 h (maximum load of 50 W) or no electricity at all. Moreover, these house-holds were found to be experiencing blackouts for two to four days a week (A. Jain et al.). The situation in other states may be slightly better, but low-income homes in most parts of India have poor access to electricity. Overall, the problem of energy access in India is quite unique and requires a new approach that leverages modern techno-logical innovations. One way India can overturn the whole narrative of energy access for low- and medium-income households is by adopting novel solar and energy storage technologies and promoting innovations, such as a direct current (dc) microgrid, which benefits the energy poor.

Analysis of the Existing Problem

Poor Economics of Power Connections The apparent reason for a large number of homes remain-ing off the grid and those with the grid having long hours of load shedding is a shortage of power. India generates less power than it would like to consume, but this has been changing over the past few years. Power generation capacity is increasing, whereas consumption has not been increasing as fast. As a result, the demand–supply gap has been narrowing, and power shortages may no longer be the primary reason for the current situation. Even during peak-demand hours, the gap is no longer severe. The limi-tations of the power-transmission grid in some regions of the country were another reason that power did not reach power-deficit areas. Even this issue has been considerably rectified, as the power grid in the country has expanded

its capacity. The shortage of power is therefore no longer the primary reason for a number of Indian homes having no power or power for limited hours.

Another apparent reason for a large number of homes being off the grid is that, even when the village has grid connectivity, the power lines have not been extended to each home. In fact, as per the definition provided by the Ministry of Power, India, “a village is considered electrified when 10% of the homes in a village are connected to the grid.” Even though it is a serious problem, this bottleneck could be overcome by extending the existing power lines to all since some remote homes in a connected village should not be very expensive or difficult.

The real reason for the current power situation may lie in the economics. First of all, can these homes afford to pay for power even at the currently subsidized power tar-iffs? Second, can the power distribution companies (DIS-COMs) afford to supply power to these homes at subsidized rates? The answer to both these questions may not be in the affirmative, and, unless this issue is addressed, many of these homes may remain without access to electricity for a long time.

The power tariff for homes in India is about 5 per unit. (An exchange rate of US $1 = 70 is assumed in this article.) A small home that in a day uses two tubelights for 6 h, two fans for 12 h, two bulbs, a 24-in TV for 10 h, and a cell phone being charged for 4 h consumes a little over three units of power a day, costing about 500 a month. This would be expensive for at least 50% of all Indian homes and an even larger percentage of rural homes. Therefore, in many parts of the country, the electricity tariff is further

Figure 1. The households’ distribution across different electricity tiers (A. Jain et al.). Tiers 0–3 represent progression in the path to energy access. Households with no electricity fall under Tier 0. Households with capacities of 1–50 W and having power only for 4–8 h a day fall under Tier 1. Those with 50–500-W capacity and 8–20 h of power are Tier 2, and the rest are classified under Tier 3.

Bihar Jharkhand Madhya Pradesh Uttar Pradesh West Bengal Odisha

1% 0% 4% 0% 16% 3%

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subsidized for the first 50 or 100 units. However, even half of 500 a month is not affordable for many of these house-holds. It is possible that homes in the lowest-income group may manage with less power and use one tubelight, one bulb, and one fan instead of two. That will help, but the quality of life would suffer. Many of the households would still not be able to afford their power bills. At the same time, slightly better-off homes would like to add refrigera-tors, mixers, and computers, adding to the power bill and making it more unaffordable.

On the other hand, DISCOMs lose money even when they supply power at 5 per unit. The cost of power from plants using oil and gas (most of which is imported) is quite high in India. However, India can produce power from coal at a cost of 2 to 3 per unit (T. Buckley). Therefore, even though coal is a pollutant, power production using coal has been increasing rapidly in India. Even then, DISCOMs can-not break even when they supply power to homes. First, they have to take into account transmission and distribu-tion (T&D) losses (for rural homes it varies from 40% to 70%). Second, coal power takes time to ramp up and to ramp down. Therefore, one cannot size coal plants for peak loads. One needs other power sources with faster respons-es. These are usually oil/gas-based plants, where the cost of generation is higher. This increases the cost of power for DISCOMs. With regard to this, once the costs of meter read-ing, billing, collections (for large numbers of homes, each paying small amounts), and the overhead costs are added, DISCOMs start losing money. When state governments push DISCOMs to supply power at subsidized rates (for example, lower tariffs for the first 50 or 100 units), DISCOMs lose even more. They have no incentive to continue to sup-ply power or expand connections to homes not on the grid, as they know that these homes can afford (and pay) even less. Hence, at peak hours, they find one reason or another to carry out load shedding. One retired chief engineer of an Indian DISCOM remarked, “We are happy when there is load shedding as we lose less money.” This sums up the reality faced in India.

Can Rooftop Solar Panels Address These Issues?Recently, rooftop solar panels have been touted as an alternative source for power generation. A 500-W solar panel in most parts of India could generate most of the power required. As there would be no T&D losses, the solution looks promising. At an installed cost of 50 per Wp, the rooftop solar photovoltaic (PV) would amount to a little over 3 per unit, assuming a depreciation over 20 years and an interest rate of 7%. (In India, the commer-cial interest rate varies from 13% to 16% today. Homes may be able to put their savings in fixed deposits and earn about 7%.) This would be attractive. However, solar power is available only during the daytime, and even then, it fluctuates. On the other hand, DISCOMs face peak demand both in the daytime and in the evening. Hence, they are likely to resort to load shedding mostly

during these times. Thus, a rooftop solar installation would require a battery, which increases costs consider-ably by almost four times, and as a result, solar power no longer remains attractive.

Furthermore, a solar PV produces dc power that needs to be converted to alternating current (ac) and synchro-nized to the ac grid. When 10-kW of dc solar power is con-verted and synchronized to ac, the conversion loss could be as low as 3%. However, when 250–500-W solar- dc power is used, these losses could be as high as 15% as long as the converter cost is a small percentage of the solar-panel cost (P. Kaur et al.). The problem gets further compounded as input and output power of a battery is only dc. Alternating current power needs conversion to dc before it charges the battery, and the battery output needs conversion to ac before it drives the load. Each of these conversions is also likely to have a 15% loss. In addition, there is battery loss (as high as 8%–10% for low-cost lead-acid batteries) and over half the solar power is lost before it reaches the load.

The approach looks more absurd when one examines the load to be driven. Some 62% of India’s home load is composed of ceiling fans and lighting (Global Buildings Per-formance Network). With the advent of brushless dc (BLdc) motors, a dc-powered ceiling fan consumes only 40% of the power consumed by conventional ac-powered induction-motor-based ceiling fans (Global Buildings Performance Network). If one uses an ac-powered ceiling fan, another converter with about a 15% loss will be required. Similarly, conventional ac-powered compact fluorescent lamp light-ing is being replaced by light-emitting diode (LED) lighting. LEDs use dc power and are best powered by dc. Electronics (such as TVs, cell phones, and computers) are increasingly being used in homes, and all electronics need dc power. Taking solar power through multiple conversions to power the ac home grid and then converting it to dc to power each of these devices is indeed ridiculous. The stage is set for dc microgrids for homes powered by rooftop solar panels hav-ing batteries and connected to the incoming ac power through a converter as shown in Figure 2. This is the solar-dc microgrid for a home.

One of the key challenges in designing such a microgrid is to keep the losses low. The problem is not as straightforward as it appears. The solar-PV panel’s voltage [at a maximum power point (MPP)] would vary during the day. The battery’s voltage would vary depending on its state of charge. The grid power would be converted at an independent voltage, and the load would be expected to operate at some fixed voltage. If one uses dc–dc converters to connect these units together, the losses may not be very different from that of an ac home grid. The design would therefore require some smart power electronics such that the solar PV operates at its MPP and the battery is charged and discharged optimally while driving the load with min-imal losses. As discussed in the subsequent section, this has been achieved such that for power in the range of 100–500 W, solar-PV power aided by a battery drives the

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dc-powered load with losses as low as 8% (in addition to the battery losses). Such products could make the solar-dc microgrid very attractive for homes.

However, Can Solar-dc Microgrids Break the Logjam?A dc microgrid for a home with a solar PV, a battery, and an incoming ac grid to drive dc loads can indeed help over-come many a problem. First, it would be highly cost effec-tive for off-grid homes as discussed in the “Economics of Off-Grid Homes” section. Furthermore, as and when the power grid reaches these homes, it can be connected to the dc home microgrid. Second, for homes that are already on the power grid but suffer long hours of power cuts, a solar-dc microgrid would ensure uninterrupted power. As shown in the “Economics of On-Grid Homes with Load Shedding” section, the power costs would be much less as compared to that for today’s home using an ac power line along with an inverter providing power backup. Third, as shown in the section “Economics of On-Grid Homes Without Load Shed-ding,” rooftop solar panels would lower the cost of power for most homes, possibly to a level that they can afford. This is achieved by first reducing power consumption by use of dc-powered dc appliances in place of today’s ac appliances and second by a rooftop solar PV, which produc-es dc power at lower costs than the current domestic power tariff. This is used to an advantage by keeping the conver-sion losses to a minimum. The key here is to use the battery minimally. A smart controller on the dc microgrid helps pri-oritize the usage of the solar panel, the grid, and the battery in that order. When this is followed and losses are kept to a minimum, the total cost of power to home owners comes down considerably, even though a battery is used. Finally, as rooftop solar panels start getting used widely, DISCOMs have to supply less and less power to homes. This would reduce the subsidy that they need to provide to the domes-tic sector and will help them break even more easily. Healthy DISCOMs can then expand their grid to most homes faster to ensure that no home remains dark.

Thus, a solar-dc microgrid could help in breaking the logjam that the domestic power supply currently faces in India. Over time, the solar power cost is likely to reduce, and better batteries will become available. The solution will therefore become more and more affordable. Finally, as 250 million Indian homes start adopting energy-efficient dc-powered appliances and rooftop solar panels, India is likely to become a green nation. This could alter the debate on climate change significantly.

Power Usage and Costs in a Solar-ac and a Solar-dc HomeIn this section, the technoeconomic viability of a dc microgrid supplement-ed by solar power in juxtaposition with

similar systems in homes running completely on ac is assessed. We present the simulation results of power usage in a solar-ac home and a solar-dc home backed up by actual data obtained from deployment to show that:

1) For off-grid homes, the cost of power per day is much lower for a solar-dc home as opposed to that for a solar-ac home. In fact, the results obtained will show that the per day power costs for a solar-dc home is comparable to the per-day power costs for an on-grid ac home today. As and when the grid is connected to these off-grid solar-dc homes, the per-day power costs will further reduce.

2) For an on-grid home with power cuts, the use of solar-dc power not only enables 24 /7 power but also makes it available at a fairly low cost as compared to that for a solar-ac home.

3) Even for on-grid homes with no power cuts, consider-able savings are possible using solar-dc power.

Assumptions and MethodologyConsider low- and midincome homes with the above power scenarios. To make sure that the systems are com-parable, the following assumptions are made.

a) As a solar panel in India produces 4–4.5-kWh/kWp power per day (http://www.solarmango.com/faq/5), a

Figure 2. A rooftop solar-powered dc microgrid for homes.

Solar Panel

dc

dc Load

Battery

Grid

ac

dc

dcdc

~=

Figure 3. The available solar power from a typical 125-Wp solar panel over a day.

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125-Wp solar panel was assumed to be producing 540 Wh of solar energy every day. Solar power is available from 6 a.m. to 6 p.m., as shown in Figure 3.

b) Table 1 presents the loads assumed in ac and dc homes along with their power consumption. It also gives the number of appliances used in each home along with their operational hours.

c) Let us now take a deeper look at the solar-cum battery systems that an ac home (using ac appliances) and a dc home will use. Battery backup systems called inverters

are widely used in Indian ac homes today. Rooftop solar PVs can be added to such systems, and we will call them solar-ac systems. These systems, shown in Fig-ure 4, would have an ac-grid input, a solar-PV input, a two-way connection to the battery for charging and drawing out power, and an ac output to drive the home load. Table 2 presents the losses associated with such systems assuming the power used is less than 500 W. Such a system has zero GtoL loss when the grid directly drives the load. However, when solar power directly drives the load (without going through the battery), the StoL loss is 15% primarily due to an ac- to dc-converter, which also ensures that solar panel operates at MPP and is synchronized to the ac-grid power. When the grid charges the battery, the GtoB is 15%, and when the solar panel charges the battery, the StoB loss is 30% (due to two converters). Finally, the BtoL loss is about 25% with a 10% loss being contributed by the lead-acid battery. Measurements were carried out on several available commercial systems with these features at the Indian Institute of Technology Madras (IITM) ( Center for Decentralized Power Systems), and the loss-es, given in Table 2 for the solar-ac system, are the best results obtained from any of these systems.

A dc home, on the other hand, will use a system, such as inverterless, shown in Figure 5. The system has been designed at IITM and is being commercialized by Cygni Energy Private Limited (see http://www.cygni.com/

products-2/ for product-related information). Special care has been taken to minimize the losses and yet keep the

Figure 4. A solar-ac system with an inverter for an ac home.

Solar Panel

dc

Inverter

ac Load

Battery

ac ac

dcGrid

=

==

~~

~

TABLE 2. Power losses within the system.

SystemStoB1

(%)GtoB2

(%)BtoL3

(%)StoL4

(%)GtoL5

(%)

ac system 30 15 25 15 0

dc system 5 15 10 3 61Solar-to-battery (StoB) loss.2Grid-to-battery (GtoB) loss.3Battery-to-load (BtoL) loss.4Solar-to-load (StoL) loss.5Grid-to-load (GtoL) loss.

Appliance

Wattage (W)

NumbersNumber of Operational Hours/Day

AC-Powered Appliances in ac Homes

DC-Powered Appliances in dc Homes

Fan1 67 24 2 6

Bulb 40 5 2 12

Tubelight 36 18 2 10

TV 40 30 1 4

Phone 6.5 5 1 101ac-powered induction motor fan at an average speed was used in an ac home, and a dc-powered BLdc fan was used in a dc home.

TABLE 1. Device power consumption, the typical number of appliances in a small home, and usage.

TABLE 3. Efficiencies in delivering power from the source to the load at homes.

AC Home Efficiency

(%)

DC Home Efficiency

(%)

GtoL 100 94

GtoL via a battery 63.7 76.5

StoL 85 97

StoL via a battery 52.5 85.5

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costs low. The ac-GtoL losses in this system are 6%, and StoL losses come down to 3%. GtoB losses are still 15%, but StoB and BtoL losses are down to 5% and 10%, respectively. The loss values are confirmed using exten-sive measurements.

These losses will amount to a decrease in efficiency, as power is delivered from a source (at the inlet of a home) to the load. Table 3 presents these efficiency numbers. Thus, if an ac home is powered 30% through the grid without a bat-tery, 20% through a battery, 25% from a solar panel without a battery, and 25% through a battery, the total power effi-ciency will be 77%, whereas that for a dc home will be 89%. As the available grid power decreases (more load shedding), the efficiency of the ac home will become worse, whereas, that for the dc home will improve.

a) In the case of off-grid homes, it was assumed that the battery is consumed only when the available solar power is not enough to drive the load. For on-grid homes with load shedding, the priority of source con-sumption assumed was a solar panel, the grid, and a battery in that order.

b) The battery used here is a special valve-regulated lead-acid (VRLA) high-performance battery developed by Amararaja (see http://www.quanta.in/products.asp

for product-related information) in technical collabo-ration with IITM. The VRLA are low-cost batteries used widely in India. A typical 1-kWh battery, priced at around 6,000 per kWh, can be charged and dis-charged 800 times at 0.1 C and at a depth of discharge of 50% at 30 °C. The high-performance VRLA 1-kWh 48-V battery was especially designed to double its life and the number of charge–discharge cycles. The cost is about 15% higher than the conventional batteries.

c) Per unit costs of power are assumed as per Table 4. The corresponding solar-ac and solar-dc costs include the costs of deployment and balance of systems. [Depreciation of 20 years for solar panels and interest rate of 7% are assumed in computa-tions of costs. A 500-W solar system including roof-top deployment and cables is assumed to be at 50 per watt. The cost of the balance of systems for

a 500-W solar panel is assumed to be 3,000 for inverterless (solar-dc system) and 10,000 for the solar-ac system but with a life (depreciation) of five years only. The costs for a lower wattage system may be slightly higher.]

Results and Discussions

Economics of Off-Grid HomesTo begin with, an off-grid ac home and a dc home with the same load and solar profile were considered. The simula-tion results obtained are presented in Table 5. As shown, the per day load requirement in the dc home was about 37% of that of an ac home due to lower consumption of dc appliances in comparison to ac appliances. Further solar

power is delivered to a load (directly and through a bat-tery) at 91% efficiency in dc homes as compared to 65% in ac homes. As a result, the per day cost of power was found to be around 12.6 for the solar-dc home, which is signifi-cantly lower than 50.6 for a solar-ac home. More interest-ingly, as shown in the section “Economics of On-Grid Homes Without Load Shedding,” the per day cost of power in a grid-connected ac home with zero load shedding is 16.3 for the same load, implying that for an efficient off-

grid solar-dc home the power costs are lower than even the current grid-connected ac homes.

Recognizing that solar-dc technology has the poten-tial to revolutionize and transition India’s power sector to a more sustainable one, the Government of India recently started supporting its deployment in 4,000 off-grid homes in Rajasthan. In December 2015, deploy-ments started in Bhomji ka Gaon, a village in Rajasthan located in a region where the terrain conditions are harsh with no road connectivity, marked by frequent sandstorms and lack of resources. Each home is pow-ered by a 125-Wp solar panel supported by a 1-kWh

Figure 5. A solar-dc inverterless system for a dc home.

Solar Panel

Grid230 V ac

Battery Cloud

dcHomeBluetooth

Low-EnergyInterface

125–500 W48 V dc

TABLE 5. Per day cost of power in off-grid ac and dc homes.

Type of Home

Load/Day (W H)

Cost/Per Day ( )

Efficiency(%)

ac home 3,266 50.6 65.1

dc home 1,212 12.6 91.4

TABLE 4. Per unit costs of power from different sources.

Source of Power Per Unit Cost ( )

Grid 5

Solar-dc system 4

Battery 12

Solar-ac system 5

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battery to drive a dc fan, a dc tubelight, a dc bulb, and a socket-cum dc mobile charger as shown in Figure 6(a) and (b). The power is remotely monitored with power data logged onto a server using Bluetooth and a mobile handset. Figure 7 presents the solar power, the power flowing in and out of a battery, and the load for one such home on 25 February 2016. The captured data from homes validate the simulation results presented for a dc home in Table 5. The deployments have, on one hand, marked the arrival of power to these homes, which otherwise would have continued to be without electricity for a considerable period of time and, on the other hand, established that the future of power is solar-dc power.

Economics of On-Grid Homes with Load SheddingNow, let us analyze grid-connected homes and understand the impact of load shedding on power costs in the two types of homes. Table 6 presents cost per day when the grid has a load shedding of 4 h (2 h during sun hours and 2 h during off-sun hours). The per day cost for a dc home is as low as 7.3 even with 4-h power cuts, whereas the ac home power cost is about four times this cost. This translates to cost savings of about 650 per month.

These cost savings become even more prominent as the duration of

load shedding increases as is evident from Figure 8. So for homes that have a grid supply only for a few hours a day as is the case with thousands of rural homes in India, solar-dc power becomes an efficient and more importantly an affordable solution. Also shown in Fig-ure 8 is the power cost per day in an ac home but with energy-efficient ac-powered dc appliances, consuming 15% more power than dc-powered dc appliances. Here too, the costs are considerably lower than those for a conventional ac home with ac appliances; however, they are 10%–60% higher than that of a dc home, depending upon the extent of load shedding. There is no reason to go halfway; going with solar-dc power will be the fastest way for India to get to 24/7 power in every home. These lessons were used to pilot solar-dc power for grid-connected homes with significant load shedding. A solar-dc inverterless system powering a cluster of 27 homes and a school was set up in the village of Tirmal, Odisha, whose entire population is below the poverty line. Figure 9(a)–(c) shows these deployments. Figure 10 cap-tures (via Bluetooth and a mobile phone) the solar power generated, the load, the grid power used, and the power in/out of a battery in a home for a few days. This data

Figure 6. Bhomji ka Gaon, Rajasthan. (a) A 125-Wp solar panel installed on a house, and (b) a family in a home with a solar-dc inverterless system. (Photos taken with permission of the family that owns the system.)

(a) (b)

Figure 7. The power data measured over a day in an off-grid home in Rajasthan with a solar-dc inverterless system on 25 February 2016.

TABLE 6. Per-day cost of power in ac and dc homes with a grid and 4-h load shedding.

Type of Home

Load/Day (W H)

Cost/Per Day ( )

Efficiency(%)

ac home 3,266.00 28.9 88

dc home 1,212.00 7.3 93.3

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captured from the homes validate the simulation results presented in Table 6 for solar-dc systems.

Economics of On-Grid Homes Without Load SheddingFinally, consider power costs for the two kinds of homes on the grid with zero load shedding. Table 7 shows that a dc home scores significantly again in terms of power costs. The monthly power bill would come down from about 500 for the current ac homes to less than 200 for a dc home. This is huge for low- and midincome homes as they are often unable to power their homes because of the lack of affordability. Direct current homes, therefore, will go a long way in enabling them to afford power. Midincome homes can now also afford to consume more and add more dc appliances.

Expanding the Solar-dc microgrid for multihomesSo far, we have discussed how a solar-dc microgrid deployment within a home is highly beneficial in terms of energy savings and savings in power costs. It becomes imperative to analyze the impact when such a solar-dc microgrid is scaled to a cluster of homes. One of the advantages would come from the fact that the most expensive elements of a solar-dc system, that is, solar panels (power sources) and backup storage, would now be shared. The Inverterless-500 system has been designed to power a cluster of four homes using common solar panels and batteries on a 48-V dc power line. Similarly, the Inverterless-2500 system uses common solar panels and batteries and extends the dc microgrid to a cluster of 12/24 homes. As it is unlikely that each home will use the peak load at the same time, the system can be designed for average loads rather than peak loads; in other words, such shared systems can benefit from the different ener-gy-usage patterns of individual homes by what is known as the law of averages. This also enables one home to draw more power when needed while other homes are not consuming much. Thus, in comparison to the sum of the sizing of the solar panels and batteries needed to power each home, the size of shared solar panels and

batteries can be less, or better quality of service could be provided to each home by keeping the size the same.

A multihome microgrid deployment requires the energy consumption of each home (from different sources) to be billed appropriately. The Inverterless-500system, shown in Figure 11, is equipped with an inverterless remote unit (IRU) deployed at each home, which measures energy consumption and enables cut-ting off the supply in case the consumption exceeds the (configurable) threshold power consumption after gen-erating a suitable warning signal. This system, however, measures only the total energy consumed by a home and does not separate the energy used from different sources (solar panel, grid, and battery). The Inverter-less-2500 system, on the other hand, performs these measurements. These systems have a normal line and an emergency (low-load) line at each home. At the time when the grid and the solar panel are not feeding power

Figure 8. The variation in cost of power per day with the load shed duration for different types of homes. System 1: an ac home, system 2: an ac home with ac-powered dc appliances, and system 3: a dc home.

60.00

50.00

40.00

30.0020.00

10.00

0.00

Per

Day

Cos

t

0.00 5.00 10.00 15.00 20.00Duration of Load Shed h

System 1System 2System 3

Figure 9. A deployment at Tirmal, Odisha. (a) A school running on dc power supplied by a solar-dc inverterless system, (b) a cluster of homes powered by a solar-dc inverterless system, and (c) a home in Tirmal, Orissa, with dc appliances powered by a solar-dc inverterless system. (Photos taken with permission of the family that owns the system.)

(a) (b) (c)

TABLE 7. Per day cost of power in grid-connected ac and dc homes with no load shedding.

Type of Home

Load/Day (W H)

Cost/Per Day ( )

Efficiency(%)

ac home 3,266.00 16.3 100.00

dc home 1,212.00 6.45 94

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and the battery goes below a minimum level, the normal line is cut off, whereas the emergency low-load line con-tinues to supply power for much longer durations.

The Inverterless-500 systems have been deployed at Kundithal, Neelgiris, where eight such systems powered 21 homes. The 48-V dc power used in the microgrid lim-its the maximum separation for homes in the cluster. As homes in Nilgiris were separated by long distances, each system could power between two and four homes. Each home is supplied 125 W with a cumulative peak con-sumption of 500 W. Every home is installed with an LED tubelight, a BLdc fan, a cell-phone charger, and a bulb. On similar lines, three units of the Inverterless-2500 sys-tems have been deployed at Tirmal village in the state of Orissa, India; each of these systems powers about 30 homes. The amount of power to be supplied to each home can be configured remotely through the management system.

Conclusions and the Way ForwardA large percentage of the Indian population, especially the lower- and the middle-class homes, are denied access to quality power most of the time. The shortage of power and the lack of connectivity to homes were traditionally under-stood to be the primary causes for this lack of access. However, recent developments in the country and studies show that the actual reason for this state of affairs could be the lack of affordability even when the

power tariff is subsidized. At the same time, power DIS-COMs are not eager to supply power to such homes as the supply is not financially viable; their own finances are in dire states. India seems to be stuck in this logjam as power cannot be generated at much lower costs, even though it depends on environment-unfriendly coal-fired power plants.

Disruptive technology is needed to get India to break this logjam. A solar-dc microgrid for homes seems to be an answer. In the article, we have illustrated that a 48-V dc microgrid for each home powered by a rooftop solar panel, connecting to the grid through an ac–dc converter wherev-er the grid is available, integrating a small sized battery, and powering dc-powered dc appliances, could overcome a multitude of problems. As a result, the power costs for homes could be drastically reduced and could become affordable in the presence of a 24/7 grid for grids with sig-nificant amounts of load shedding and in the absence of

the grid. In addition, homes will draw limited power from the grid, significant-ly cutting down the losses for DISCOMs. Finally, when homes use energy-effi-cient dc-powered dc appliances and generate power from a solar panel on their rooftops, the nation moves toward becoming truly green.

Solar-dc microgrids for homes may make immense sense in this context. Yet, they face tough challenges. First, there has to be a change in the mindset that the ac versus dc debate was settled a century ago and that we are going backward when we talk about the dc microgrid. Appliances in homes and offices have quietly become dc over the past few decades, and it will be a step forward into the future when we power them using a dc microgrid rather than using ac–dc converters for each of them. The second bottleneck is the lack of standardization. There is no accepted

Figure 10. The power data measured in an on-grid home with a solar-dc inverterless system from 23 February 2016 to 24 February 2016.

Figure 11. An Inverterless-500 system with IRUs at each home in the cluster.

Solar Panel

48 V dc

48 V dc

48 V dc

48 V dc

230 V ac

Grid

BatteryCloud

BluetoothLow-EnergyInterface

IRU

IRU

IRU

IRU

NENENE

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N: Normal LineE: Emergency Line

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dc-power standard for home wiring today, even though the telecom and automobile industries use 48-V dc widely. The IEEE Low-Voltage dc Forum in India proposed 48-V dc power as a standard for home power involving low-power appliances. The Bureau of Indian Standards is now work-ing on such standards and is veering toward the 48-V dc standard for homes with low-power appliances and 380-V dc for larger microgrids involving higher power usages. The International Electrotechnical Commission has set up a subgroup to examine dc-power standards for homes and other usages. But for now, there is no accept-able standard that can be used by appliance manufactur-ers and installers. This could be the primary reason for the third bottleneck—the lack of availability of standardized dc-powered dc appliances. Unless such appliances are widely available at prices comparable to their ac counter-parts, users are unlikely to bite. This is so even when dc-powered appliances have the benefit of avoiding ac–dc converters and, thereby, reducing costs and increasing reliability. Even the power-factor correction that ac-pow-ered appliances need will no longer be required. Despite these hurdles, countries, such as India, can benefit immensely by adopting the dc microgrid for homes and powering them using rooftop solar panels. Difficulties cannot stop the future, and solar-dc microgrids for homes is the future. India needs to lead rather than wait for the difficulties to be resolved.

For Further ReadingInternational Energy Agency, “World Energy Outlook 2015,”

Int. Energy Agency, Paris, France, annual report, 2015.

A. Jain, S. Ray, K. Ganesan, M. Aklin, C.- Y. Cheng, and J.

Urpelainen, “Access to clean cooking energy and electricity:

Survey of states,” Council on Energy, Environment, and Water,

New Delhi, India, 2015.

T. Buckley, “Briefing note: India power prices,” Inst. for

Energy Econ. and Financial Anal., Cleveland, OH, 2014.

P. Kaur, S. Jain, and A. Jhunjhunwala, “Solar-DC deployment

experience in off-grid and near off-grid homes: Economics,

technology and policy analysis,” in Proc. IEEE First Int. Conf. DC

Microgrids (ICDCM), Atlanta, GA, 2015, pp. 26–31.

Global Buildings Performance Network, “Residential build-

ings in India: Energy use projections and savings potentials,”

Global Buildings Performance Network, Ahmedabad, India,

2014.

Center for Decentralized Power Systems, “Technological

comparative study of solar lighting systems for homes,” Indi-

an Inst. Technology, Chennai, India, 2015.

BiographiesAshok Jhunjhunwala ([email protected]) received his B.Tech. degree from the Indian Institute of Technology (IIT) Kanpur, and his M.S. and Ph.D. degrees from the University of Maine. From 1979 to 1981, he was with Washington State University and has been at IIT Madras ever since, where he leads the Telecommunications and Computer

Networks group. He is the director of the Board of Tata Teleservices (Maharashtra) Limited, Polaris, Sasken, Tejas, Tata Communications, Exicom, Mahindra Reva Electrical Vehicles Private Limited, and Intellect Design Arena Limit-ed. He was also a member of the Prime Minister’s Scientific Advisory Committee from 2004 to 2014. He was bestowed with the Padma Shri in 2002, the Shanti Swarup Bhatnagar Award in 1998, the Millennium Medal at the Indian Science Congress in 2000, and the H.K. Firodia for “Excellence in Science and Technology” in 2002, among several others. He is chair or member of several committees, including the Technology Information, Forecasting, and Assessment Council’s vision 2035 for Electronics and Information Tech-nology; Information Technology Institute for the Tribes of India; Mobile Payment Forum of India; Technology Adviso-ry Committee of the Securities and Exchange Board of India; Biotechnology Industry Research Assistance Council; and the IIT committee for solar energy installation among many others. He is a Fellow of the IEEE, the World Wireless Research forum, and several Indian academies.

Aditya Lolla ([email protected]) received his B.Tech. degree in chemical engineering from Osmania University, Hyderabad, India, in 2012 and his M.Sc. degree in sustain-able energy systems from The University of Edinburgh, United Kingdom, in 2013. He is a project officer at the Center for Decentralized Power Systems, Indian Institute of Technology, Madras, and is a recipient of the Queen’s Jubilee Scholarship from the British government. In his previous roles, he worked on different projects encom-passing renewable energy, sustainable development, and energy policy. He researched microbial fuel cell technolo-gy with a specific focus on analytics and has worked as a research intern at FloWave TT, Edinburgh, specializing in tidal energy systems. His current work includes energy policy, data analytics, and decentralized solar power.

Prabhjot Kaur ([email protected]) received her B.Tech. degree from Punjab Technical University in 2001 and her M.S. degree in engineering from Punjab Universi-ty. She completed her Ph.D. degree at the National Insti-tute of Technology, Jalandhar, in 2013. She is currently working as principle scientist in the Department of Elec-trical Engineering at Indian Institute of Technology (IIT) Madras, and she also serves as deputy director at Tele-com Centre of Excellence, IIT Madras. Prior to this, she worked as associate professor at NorthCap University (NCU), Gurgaon, India, where she also served as deputy dean (research, development, and industrial liasoning). At NCU, she formed and led the research group on wire-less communication and was also a lead for Change Ini-tiatives for Education Reforms and practices in the university. She has been conducting faculty development programs and also delivered some invited talks on vary-ing fields of interest. Her recent research revolves around green technologies encompassing renewables, batteries, and electric vehicles.

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Voltage-Level Selection of Future Two-Level LVdc Distribution Grids

20

NVIRONMENTAL CONCERNS AND NEW ENERGY POLICIES are causing energy systems to shift toward decentraliza-tion and sustainability. Electricity generation has been his-torically based on large-scale fossil and nuclear sources, even though in the last decade, the share of renewables

has grown significantly. Microgrids (MGs) come as a suitable solution for the installation of distributed sources in the low-voltage (LV) grid, where most consumers are sparsely located. MGs ease the integration of distrib-uted generators (DGs) with energy storage systems (ESSs) at a consump-tion level, especially renewable energy sources (RESs), such as solar panels and small wind turbines (WTs). By decentralizing electricity gen-eration, it can now be produced in closer proximity to the consumer, thereby avoiding transmission and distribution losses and increasing the efficiency of the electricity grid, as well as higher power reliability.


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A compromise between grid compatibility, safety, and efficiency.

2325-5987/16©2016IEEE

By Enrique Rodriguez-Diaz, Fang Chen, Juan C. Vasquez, Josep M. Guerrero, Rolando Burgos, and Dushan Boroyevich

IMAGE LICENSED BY INGRAM PUBLISHING

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In line with the developments previously mentioned, direct-current (dc) distribution systems are making their way back to electrical grids. Worldwide, today’s electrical systems predominantly use alternating current (ac); however, the technical/technological problems that, over a century ago, made dc harder to transmit than ac have been solved. Both ends of the electrical spectrum, high voltage (HV) and LV, have seen the proliferation of dc systems and its implementation for trans-mission and distribution of electricity. For both ends, dc offers signifi-cant improvement regarding simplicity, efficiency, and cost reduction. DC systems are now viable options as a result of developments that have taken place in the power electronics industry, which have allowed converters to operate at dc voltage levels required for transmission, dis-tribution, and consumption.

Starting from the top end, there are over 100 HV dc (HVdc) systems already installed around the world, especially for long-distance and sub-marine connections. HVdc transmission systems allow higher efficiency, potentially lower costs, and enhanced environmental solutions. HVdc transmission lines are generally thinner than HVac lines for the same power capacity; also, HVdc allows long-distance transmission with under-ground lines, thereby considerably reducing the environmental impact. Moving to the bottom end, dc provides a promising solution for modern power systems to improve efficiency, power quality, resiliency, and reliabili-ty. Despite having no presence in grid applications, these benefits have been previously observed in several stand-alone applications, such as vehicular power systems, telecommunications stations, data centers, and aerospace, marine, and other electrical power systems in which reliability, efficiency, and cost are critical.

Potential Benefits of LVdc Distribution Systems for Building ApplicationsThe high penetration of installations with DGs at consumption level, especially solar photovoltaic (PV) panels, ESSs, and modern electronic loads, gives dc distribution a competitive advantage when compared with ac for residential/building applications. Most renewable energy generators, such as PV panels and fuel cells (FCs), are dc generators; however, even WTs, intrinsically ac generators, are more conveniently integrated into a dc grid since double conversions are avoided. ESSs as batteries are dc devices as well. Furthermore, modern electronics loads [e.g., TVs, light-emitting diode (LED) lights, phones, computers] are all internally dc loads, and the energy consumed by these devices is increasing every day. Moreover, as has happened with WTs, even appli-ances that are intrinsically ac loads (e.g., refrigerators, washing machines, dishwashers) interface better with a dc supply, because of the elimination of ac–dc conversion. In addition, the expected future inte-gration of electric vehicles (EVs) is going to inevitably increase the pres-ence of dc devices in buildings’ electrical systems because, basically, EVs have batteries that can be charged or discharged. Therefore, a dc distri-bution system is a more natural interface between mostly dc devices, which allows considerable power-conversion stage reduction, hence achieving a significant loss reduction, as well as simplicity and potential cost reduction in the power converter units.

On top of this, there are a few common benefits of dc when compared with ac for all applications. For example, in dc there is no reactive power loading the lines, and there is no need for synchronization. As a conse-quence, the system naturally becomes more efficient and simpler. It is important to highlight that dc efficiency or energy-saving improvement

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is strongly related to the presence of local RESs and ESSs. When examining the LVdc distribution system in Figure 1, if there is low or nonlocal generation and no storage, most of the energy consumed by the load would inevitably come from the grid rectifier. The grid rectifier would be sized to supply the approx-imate building-installed power; however, the loads are normally con-nected at different times. Hence, it is expected that the rectifier works mostly at low loads where its effi-ciency is very poor, thereby increas-ing system losses considerably.

Challenges and Safety ConcernsAs mentioned, LVdc distribution systems for residential/building applications have brought high expectations regarding simplicity, cost reduction, reliability improve-ment, and energy savings. The lack of commercially available products, standards, codes, and regulations for dc systems is a critical challenge holding back a wider implementa-tion of LVdc distribution systems. Such a lack of commercially available products is an issue for companies implementing dc systems and sys-tem users. When designing LVdc electrical power sys-tems, it would not be easy to find products (e.g., power

converters, protection devices, connectors, chargers) that comply with the system’s requirements, especially regarding the voltage level. Also, consumers would find it extremely difficult to find dc-compatible appliances and devices.

As for standards, several organiza-tions such as EMerge Alliance, the European Telecommunications Stan-dard Institute (ETSI), the International Electrotechnical Commission (IEC), the IEEE, and others, are working to develop the required regulation for the implementation of dc systems for building/residential applications. According to the IEC 60038 standard, LVdc systems are defined as those with voltage levels below 1,500 V. This range gathers several applications, from computer electronics to auto-motive, marine, and aerospace power systems. Figure 2 shows an overview of the voltages and standards used in the different applications.

Protection devices, fuses, and cir-cuit breakers (CBs) can sometimes be used directly in dc systems, but these devices are currently designed for ac

systems. The current-interruption mechanisms rely main-ly on the natural zero crossing of the ac current, with development of arc voltage being a secondary effect for CB clearing., In dc systems, because the fault current does not

Figure 1. A reduction in power-conversion stages enabled by dc.

PV Panels

Grid

dc dc dc dc dc dc dc dcac dcdc dcac dcdc ac dc ac

dc ac dc dc dc dc

dc ac

dc acdc ac

ac dcdc ac

ac dcdc dc

acdc

dc dc

WT

Appliance

ac VoltageDistribution

Line

dc VoltageDistribution

Line

Energy Path When LoadsAre Fed from the RES

Production Stored in the ESS

Energy Path When LoadsAre Fed from the Grid

Electronics LightingSystem

Appliance Electronics LightingSystem

Grid RectifierEfficiency

η(%

)

Battery EVs WT Battery EVs

P (W )

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acdc

Worldwide, today’s electrical systems predominantly use ac; however, the technical/technological problems that, over a century ago, made dc harder to transmit than ac have been solved.

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naturally go to zero, CBs must be connected in series to ensure sufficient arc voltage for clearing; otherwise a rede-sign of these elements is required for a reliable protection system. Recently, electromechanical CBs specifically appli-cable for dc systems with voltages up to 1,500 V have become commercially available. However, there are other issues that still must be addressed, such as CB coordina-tion and the fact that upstream power converters either limit their output current faster than the CBs respond, or let through fault current to their ac feeds, thus causing wider system-voltage outages during fault scenarios.

ArchitecturesIt has been shown that LVdc distribution systems are applied in a large variety of applications, consequently, different solutions and architectures have been proposed. Regardless of the power rating or voltage of the system, the system structure can be generally classified in three main categories, as follows.

Single bus is the simplest topology, as only two wires are used to supply the voltages at the point of load. Also, the power-electronic converters themselves can be relied upon for fault protection. The automotive and telecommunication industries have widely used this

configuration at 12 V and 48 V, respectively. Proposed single-bus distribution systems often differ, depending on whether the bus voltage is tightly regulated by a power-control unit, as shown in Figure 3, or a whether a battery pack is directly connected to the dc bus. For the latter option, the bus voltage depends on the state of charge (SOC) and the current of the battery; as a consequence, this configuration is used in a reduced number of applications because the battery SOC needs to be coordinated by all the power-converter units con-nected to the bus. A modified single-bus topology, in which the distribution is made by a three-wire (i.e., positive pole, neutral, negative pole) bipolar configura-tion, brings significant advantages for LVdc distribu-tion in building/residential applications. This topology allows a reduction in the distribution voltage with respect to the ground, thereby improving safety and offering three different voltages levels (+VDC, –VDC, and 2 VDC). Thus, the loads with different power rat-ings can be connected to the voltage that better suits them. This topology is depicted in Figures 4 and 5.Multibus configurations are used when redundant distri-bution buses are needed to enhance the reliability and availability of the system, as well as for interconnection

Figure 2. A collection of standards, codes, and applications using LVdc. SELV: separated extra LV; PELV: protected extra LV.

1,500 V: Limit of LVdc,IEC60038

400 V: Limit Telecom dcSource ETSI EN 300 132-3-1

380 V: Emerge Alliance(Data/Telecom Std)

120 V: Limit of SELVand PELV, IEC61140

75 V: Low Limit EU LDV 2006/95EC (WillBe Replaced by EU LDV 2014/35/EU)

50 V: IEEE 802.3bt, 802.3bu

24 V: Emerge Alliance (Occupied Space Std)

Standards and Codes Applications

dc Voltage

1,500

1,400

1,300

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1,100

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900

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Electronics

12 V: Automotive,Lighting

24 V: LightingSystems

380 V: DataCenter

48 V: Telecom,Rural PV Systems,

Trucks

50 V: PowerOver

Ethernet

400 V: EV

750 V: Trams Power Systems

1,500 V: TractionSystems, PV Systems

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of several MG clusters. The interconnection of MG clusters is an applicable concept for LVdc distribution between buildings, where the power exchanges between the different systems can be controlled by controlling the local voltage set points. A reconfi-gurable topology is used when the system requires higher reliability and flexibility during faults and maintenance periods. It usually consists of a mesh or ring distribution system, in which intelligent electron-ic devices (IEDs) are able to connect and disconnect

sections of the mesh or ring. All of the elements con-nected to the ring are bidirectional. When a fault is detected, the IEDs disconnect the faulty area, allowing normal operation in the unaffected areas. Further-more, a modified scheme is the zonal configuration in which each element is connected to different buses in a redundant system. The element can be fed by either one of the distribution buses, and when one bus gets compromised, the elements can switch to the remain-ing healthy bus.

Figure 3. A schematic of a hybrid ac/dc distribution system for building/residential applications.

WT

acdc dc dc

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LVacDistribution

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Voltage Levels for LVdc Distribution Systems for Building ApplicationsThe voltage-level selection for LVdc distribution systems is not a straightforward choice, and the discussion on appro-priate voltage levels has been going back and forth for some time. The lack of standardization is palpable when observing the variety of voltage levels used for LVdc distri-bution systems in different applications, as shown in Fig-ure 2. The distribution voltages adopted for different solutions basically depend on what features comprise the main design criteria. For instance, in the automotive industry, 12-V distribution systems are used in cars to guarantee their safety. Nevertheless, the increase in the number of electronic devices in modern cars leads to higher electrical consumption. For this reason, 24–48-V levels have been considered as an option to improve the efficiency and avoid excessive oversizing in the conduc-tors, because both weight and energy consumption are critical in vehicles. In applications with higher consump-tion, such as data centers that normally use 380–400 V, the increase of the voltage level is unavoidable because distri-bution losses need to be minimized. It is at these and greater voltage levels that more effective protections sys-tem need to be developed. When analyzing LVdc distribu-tion systems for building/residential applications, there are some voltage levels that come as natural choices,

depending on the regulation imposed by the standards, the availability of commercial solutions, or to ensure com-patibility with the ac grid. A summary is shown in Table 1.

Figure 4. A schematic of a two-level bipolar LVdc distribution grid.

ac Grid

Isolated VoltageBalancer

Isolated VoltageBalancer

Shopping Center

MIcrochip Fans Heat Pumps

HeatPumps

Lighting

Lighting Converters Process

G M

M

M 1,500V

–750 V +750 V

+375 V

–375 V

1,500V

PV Plant

Heavy Industry

House House

ApartmentBuilding

ApartmentBuilding

Voltage Level Advantage

≥565 V Direct interconnection with the three-phase, 400-V ac grid

380–400 V Standard in the data-center industry

325 V Minimum modification required for loads with input rectifier

230 V Compatibility with pure resistive loads

120 V Limit for extra-LV definition, no need for protection system against indirect contacts

48 V Standard in telecommunication industry

24 V EMerge Alliance Occupied Space Standard

12 V Standard in automotive industry

TABLE 1. The primary voltage levels for LVdc distribution for building/residential applications.

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The selected voltage for distribution is inevitably related to the power rating of the different elements in the power system if the same wire gauge is intended to be kept. In consequence, after an analysis of the location and power rating of the elements (i.e., DGs, ESSs, and loads) it becomes apparent that the elements with similar power ratings are mostly located together. Therefore, different voltages are more optimal for different groups while optimizing efficien-cy for each scenario, safety, and compatibility with other systems. The suitable voltage ranges would be as follows.

1) Low-power loads (24–48 V, <0.4 kW) mostly comprise electronic equipment and devices (e.g., Wi-Fi routers, phone chargers, computers, TVs, DVD players, Hi-Fi systems and LED lights), which account for most devices in bedrooms, living rooms, and outdoor areas. The distribution in most of the spaces in a home, therefore, could be performed efficiently while maxi-mizing safety.

2) Medium power elements (230–400 V, 0.4–10 kW) usu-ally account for the appliances in kitchens (e.g., stove,

Figure 5. A schematic of an LVdc distribution system for building/residential applications.

WT

ac dc dc dc

PrivateApartments

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LVdcDistribution

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0 V,

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dc dc dcdc

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dc dcac

dcdc

dcdc

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375 V

48 V

48 V

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oven, dishwasher) and laundry rooms (e.g., washing machine, dryer, iron). For a given power consump-tion, supplying the loads at 230 V would keep the same current loading in the wires as in a single-phase 230-VAC system. If high efficiency in the dis-tribution and/or wire-gauge reduction is required, the voltage can be increased up to 400 V. Beyond this level, the efficiency improvement should be negligible, and the protection system would be highly demanding.

3) High-power elements (≥538 V, ≥10 kW) are expected to be a part of the common-facilities elements for a building, including DGs (e.g., PV panels, WTs, FCs, μCHP), ESSs, and the building’s air-conditioning sys-tem, elevators, and EV charging posts.

Integration of Future LVdc Distribution SystemsThe following discussion analyzes the selection of the voltage and topology of the distribution system in building applications. The standards, guide-lines, and codes necessary to widely implement LVdc systems to supply the electricity to homes and buildings have not yet been developed, even though several organizations, such as the IEC or EMerge Alliance, are active-ly working in this area. In the near future, a reasonable first step to apply dc power to buildings, as depicted in Figure 1, is to use an LVdc system to interconnect the local RESs, ESSs, and loads common in buildings, such as lighting systems and elevators, whereas the ac distribution system is used to connect the building to the grid and supply the apartments. The purpose of this configuration is that not a single change is required in the apartments, and therefore there is no need for the consumers to change electrical equipment in their homes.

If we take a look forward, once the regulations and standards for dc system are developed and mature, hybrid ac/dc distribution systems for buildings, as the one proposed in Figure 3, will likely be widely accepted and implemented, and most important, there will be dc-com-patible devices broadly available in the market, such as appliances and electronics that can be used with either an ac or a dc system. In the literature, most of the pro-posed architectures for dc distribution in buildings use a unipolar bus running at 380–400 V. As mentioned previ-ously, this is the solution adopted by the data-center industry, even though it might not be the most optimized solution for buildings and homes. It is an easier step to take, rather than developing a new solution for the distri-bution system. This solution is simple, convenient, and more efficient than the traditional ac system. It is conve-nient because it has been tested in different applications,

therefore most of the solutions already developed in that field can be used for this particular systems as well. It is more efficient because first, the voltage level is higher, hence the losses in the conductors are lower, and second, using a dc system to distribute the energy between most-ly dc-based devices (e.g., PV panels, batteries, lights, motor drives) allows a reduction in the number of conver-sion stages.

It seems that unipolar 400-V distribution systems are widely accepted as a suitable solution for dc distribu-tion for building applications. However, aside from the convenience of adapting an already tested solution, the following question must be asked: Is the 400-V system the best option for distribution in buildings? For instance, regarding the topology of the system, we dis-cussed earlier the benefits and disadvantages of unipolar, bipolar, and multibus architectures. Bipolar configurations offer significant advantages, especially for this kind of application, which has different

elements (e.g., generators, loads, storage systems) with a wide range of power ratings. Therefore, the availability of different voltage levels to supply the elements in the sys-tems enables a better compromise between distribution efficiency and safety. As a consequence, a bipolar-type LVdc distribution system seems a better solution.

Then the discussion moves to which voltage levels should be used for the distribution. We have dis-cussed that dc voltage levels within 230–400 V are a good compromise

between, efficiency, safety, and compatibility with exist-ing ac systems. Furthermore, once established that a bipolar configuration is a more convenient solution for distribution in building/residential applications, it seems advantageous to apply the same topology for the distribution grid between the buildings themselves. In Figures 4 and 5, a two-level bipolar distribution grid is shown. The IEC 60038 standard sets the upper limit for LVdc systems at 1500 V, therefore, to maximize the effi-ciency in the distribution, a bipolar !750 V is proposed, from which it is possible to extract a second-level !375 V bipolar line, which fits the suitable voltage levels for both high and medium power-rating elements in buildings. It should be also noted that the use of a two-level bipolar distribution system restricts the voltage level to be used in the system, therefore doubling up twice the voltage level 380-400 V would exceed the LV limit set by the standards. The grounding and protec-tion scheme for the proposed two-level bipolar LVdc distribution system is not trivial, and some modifica-tions are needed to comply with the safety require-ments. The neutral conductor of the !375-V distribution

DC provides a promising solution for modern power systems to improve efficiency, power quality, resiliency, and reliability.

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lines is put to ground. In addition, it is required that the voltage balancer provide galvanic isolation, because the neutral conductor in a different area may have a differ-ent voltage level.

ConclusionsFrom where we stand today, it appears that a much wider scale adoption of LVdc grids as enablers for lower cost and more efficient integration of RESs and ESSs at the building and residential level is just on the horizon. The challenges lie mainly with effective and safe distri-bution protection and industry-accepted standards. However, the opportunities for further resilience at the customer end, because of increased decentralization of energy sources and significant increases in efficient energy usage, are compelling and will continue to moti-vate growth in this area.

For Further ReadingP. Fairley, “DC Versus AC: The second war of currents has already begun,” IEEE Power Energy Mag., vol. 10, no. 6, pp. 104–103, Nov. 2012.

E. Rodriguez-Diaz, J. C. Vasquez, and J. M. Guerrero, “Intelli-gent dc homes in future sustainable energy systems: When efficiency and intelligence work together,” IEEE Consum. Elec-tron. Mag., vol. 5, no. 1, pp. 74–80, Jan. 2016.

H. Kakigano, Y. Miura, and T. Ise, “Low-voltage bipolar-type DC microgrid for super high quality distribution,” IEEE Trans. Power Electron., vol. 25, no. 12, pp. 3066–3075, 2010.

T. Kaipia, P. Nuutinen, A. Pinomaa, A. Lana, J. Partanen, J. Lohjala, and M. Matikainen, “Field test environment for LVdc distribution: Implementation experiences,” poster presented at the CIRED Workshop: Integration of Renewables into the Distribution Grid, Lisbon, no. 0324, May 29–30, 2012.

B. T. Patterson, “DC, come home: DC microgrids and the birth of the ‘Enernet,’” IEEE Power Energy Mag., vol. 10, no. 6, pp. 60–69, Nov. 2012.

R. Adapa, “High-wire act: HVdc technology: The state of the art,” IEEE Power Energy Mag., vol. 10, no. 6, pp. 18–29, Nov. 2012.

BiographiesEnrique Rodriguez-Diaz ([email protected]) earned his B.S. degree in electronics engineering and his M.S. degree in sustainable transportation and electrical power systems from the University of Oviedo, Spain, in 2012 and 2014, respectively. He is currently working toward his Ph.D. degree in energy technology at Aalborg University, Den-mark. He is a member of the International Electrotechni-cal Commission System Evaluation Group SEG4 on Low-Voltage dc Applications, Distribution, and Safety for use in Developed and Developing Economies. His research interests are low-voltage distribution systems, control of power converter units, energy-management systems, and microgrids.

Fang Chen ([email protected]) received his B.S. and M.S. degrees from Zhejiang University, China, in 2009 and 2012, respectively. He is currently pursuing his Ph.D. degree at the Center for Power Electronics Systems at

Virginia Polytechnic Institute and State University (Vir-ginia Tech), Blacksburg. From 2015 to 2016, he was a guest Ph.D. student at the Department of Energy Tech-nology, Aalborg University, Denmark. His research inter-ests include operation and control of dc power distribution systems, net-zero-energy building, and design of ac/dc converters for utility applications.

Juan C. Vasquez ([email protected]) earned his B.S. degree in electronics engineering from the Autonomous Universi-ty of Manizales, Colombia, in 2004. In 2009, he received his Ph.D. degree in automatic control, robotics, and computer vision from the Technical University of Catalonia, Barcelo-na, Spain. In 2011, he joined the Department of Energy Technology, Aalborg University, Denmark, where he works as an associate professor and is coleader of the Microgrid Research Program.

Josep M. Guerrero ([email protected]) earned his B.S. degree in telecommunications engineering, his M.S. degree in electronics engineering, and his Ph.D. degree in power electronics from the Technical University of Catalo-nia, Barcelona, in 1997, 2000, and 2003, respectively. Since 2011, he has been a full professor with the Department of Energy Technology, Aalborg University, Denmark, where he is responsible for the Microgrid Research Program. In 2014 and 2015, he was recognized by Thomson Reuters as a highly cited researcher, and, in 2015, he was elevated as an IEEE Fellow for his contributions in distributed power systems and microgrids.

Rolando Burgos ([email protected]) received his B.S. degree in electronics engineering, his Electronics Engi-neering Professional degree, and his M.S. and Ph.D. degrees in electrical engineering from the University of Concepcion, Chile, in 1995, 1997, 1999, and 2002, respec-tively. In 2012, he joined Virginia Polytechnic Institute and State University (Virginia Tech), where he is currently an associate professor in the Bradley Department of Elec-trical and Computer Engineering and Center for Power Electronics Systems faculty. His current research inter-ests include multiphase, multilevel power conversion, grid power electronics systems, stability of ac and dc power systems, high-power-density power electronics, modeling, and control theory and applications.

Dushan Boroyevich ([email protected]) received his Dipl.Ing. degree from the University of Belgrade, Serbia, in 1976; his M.S. degree from the University of Novi Sad, Serbia, in 1982; and his Ph.D. degree from Virginia Polytechnic Insti-tute and State University (Virginia Tech), Blacksburg, in 1986. He is currently the American Electric Power Professor in the department and the codirector of the Center for Power Electronics Systems. He is a recipient of the IEEE William E. Newell Power Electronics Technical Field Award and is a member of the U.S. National Academy of Engi-neering. He was the president of the IEEE Power Electron-ics Society from 2011 to 2012.

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2325-5987/16©2016IEEE IEEE Electr i f icat ion Magazine / JUNE 2016 29

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IRECT-CURRENT (DC) POWER distribution has been used ever since electric grids were invented, but, for the last cen-tury, low-voltage dc has been

largely limited to a variety of niche applications such as rail transport, vehicles, telecommunications, and off-grid buildings. Recent years have seen a variety of innovations in dc distribution technology, notably, standards for 380-V dc cabling and connectors and increases in power that can be carried over Ethernet and universal serial bus (USB). There are increasing calls for much more use of dc dis-tribution and dc microgrids in buildings, and there are potential advantages of both. However, open questions remain about the directions this might take, what policy makers could and should do in this area, and technology devel-opments that would be most useful. This article considers potential pathways for increased use of dc and identifies those path-ways that seem most beneficial and likely to succeed. We limit the scope of consideration to distribu-tion within (or between) buildings.

DC Power in BuildingsWhen an alternating-current (ac) utility grid power is available to a building, directly distributing and using


By Bruce Nordman and Ken Christensen

DC Local Power DistributionTechnology, deployment, and pathways to success.

power as ac is the obvious initial choice. If costs and benefits are similar, ac has a great

advantage as the incumbent technology in product and parts availability and its familiarity with designers, electricians, contractors, building owners, and local planning departments. There has always been clear advantages of ac power within utility grids, particularly for the ease of changing voltages, and those advantages remain. Thus, for those who would like to see more use of dc distribution in buildings, the focus needs to be on cases for which the net benefits of dc are both substan-tial and compelling.

An increasing fraction of electrical load in buildings is natively dc, most recently with the rise of light-emitting

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diode (LED) lighting and increasing loads from electron-ic devices such as computers and displays. Some devic-es, including fluorescent lighting and variable speed drive motors, convert ac to dc (and then to high-fre-quency ac), so are also dc internally. A large portion of the remainder of loads in buildings could readily be converted to dc at equal or greater efficiency than with ac. Vehicle charging with dc is not the norm, but it is available.

DC distribution today is highly successful in specific niche applica-tions but is rarely used outside of them. It is used in 12- and 24-V dis-tribution in vehicles, USB for mobile device charging, Ethernet for desk-top phones in office buildings, and off-grid buildings; these are all very specific applications in which dc distribution and use have advantag-es over ac. In data centers, 380-V dc is seeing increasing uptake. The use of dc may not be competitive today for mainstream or generic use, but that is beginning to change with the rise of lighting powered by Ethernet.

The use of dc distribution in buildings is growing, pri-marily at the very edge of the building power-distribution tree. Power over Ethernet port shipments has consistently risen during the past decade, from under 10 million/year in 2005 to more than 100 million/year in 2015. With the expected increase soon in Ethernet power to nearly 100 W (more than six times the original power limit), many more devices will be brought into the scope of being dc pow-ered, and shipments should thus grow even more rapidly. USB annual port shipments are approaching 5 billion. With USB recently enhanced to provide 100 W, it also can now support many more product types than before. With this capability, USB can now provide 40 times as much power as it did when originally introduced more than 20 years ago.

There are three basic approaches to power-native dc end-use devices, with two involving dc distribution. Many buildings contain two or even all three of these architec-tures in different places. In the present paradigm of build-ing operation (ac distribution), the source of all power is

originally ac from the grid, with conversion to dc occurring inside of each dc device (with an internal or external power supply). There is no alteration or addition to the building wiring. This is shown in Figure 1(a). The second option, central dc distribution, moves the location of the ac–dc conversion to a central device and distributes dc to end-use devices, as shown in Figure 1(b). This may be

more efficient [i.e., save energy compared to the scheme of Figure 1(a)] in some uses because it reduces the number of front-end components that must engage in power condition-ing to deal with variations on the input ac bus and provide stable dc output. Central dc distribution is often used to obtain cost savings, greater reliability, and convenience. If any local power generation and/or storage are present, they are connected to the ac infrastructure.

More interesting and useful are power architectures in buildings with local generation [most commonly,

solar photovoltaic (PV)] and local storage, as shown in the right of Figure 1(c). Direct dc enables power to flow from generation to end use—through storage as needed—with-out ever having to be converted to or from ac. This saves energy from the avoided conversion losses and saves cap-ital by requiring less conversion hardware, thus saving money from both. Direct dc increases reliability by avoid-ing potentially unreliable conversion hardware and by the presence of battery storage, because storage is less expen-sive to include in an all-dc system. Savings estimates from direct dc vary dramatically, from 2 to 14%, in part due to differences in the application contexts and base-line assumptions.

The ac–dc conversion in direct dc can be bidirectional, to enable exporting excess power out of the dc domain; this provides operational flexibility. A simple alternative is to install a unidirectional interface, as shown in Figure 1(c), in which electricity is never exported from the dc domain; we call this “semidetached direct dc.” Power is imported across the ac–dc link as needed, but for long periods of time no power may be flowing across this link. In this model, a unidirectional link means that dc genera-

tion and storage are effectively invisible to the utility grid; the dc system is just another load. This approach should reduce permitting and regulatory burdens of installing direct dc systems. In cases in which more generation exists than can be stored or used for ordinary purpos-es, inexpensive resistance heating of space or water could productive-ly use excess power.Figure 1. The (a) ac distribution, (b) central dc conversion, and (c) direct dc.

ac Grid

ac Loadsdc Loads dc Loads dc Loads

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Storage

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Savings estimates from direct dc vary dramatically, from 2 to 14%, in part from differences in the application contexts and baseline assumptions.

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When generation and loads are compatibly sized, the vast majority of electricity generated in the dc domain can remain there, and the vast majority of energy used in the dc domain is also generated there. A result is that only small amounts of ener-gy cross the ac–dc link, and the electrical capacity of the link can thus be significantly downsized. Many end uses such as lighting and electronics require relatively consistent daily use of electricity, so genera-tion and storage for such local dc grids can be sized to the needed consumption level. In Figure 1(c), if either the generation or storage is eliminated, some of the benefits of direct dc are retained, but with decreasing costs, it is likely that systems with both generation and storage will be the norm.

DC Technology ArchitecturesAlthough direct dc is possible to implement today—and is effectively used in some installations—it is uncom-mon and lacks standard designs on which to draw. Some voltages (e.g., 12 V) lack standard connectors, although in recent years progress in the area of connectors has been made by the Emerge Alliance for 380- and 24-V dc (Figure 2 shows standard dc connectors for three tech-nologies). Even if common deployment schemes were available, it is likely that the use of direct dc would remain uncommon if the only benefit delivered was energy savings. Key to making direct dc more successful is to focus on other benefits and develop new technolo-gy to create more benefits and/or decrease costs.

DC power distribution exists today in two primary forms—and a third form (networked dc) could be devel-oped—in terms of how it is organized and structured and the way in which it operates within buildings.

1) Traditional dc moves power though circuits as determined by Kirchhoff’s laws, similarly to the way in which ac systems operate. For example, in vehicles, power distribution is organized into cir-cuits, similar to power distribution of circuits in ac building infrastructure.

2) Managed dc moves power across a single cable from one device to another, with characteristics (includ-ing voltage and current) determined principally by digital management as communicated between the two devices and implemented with modern power electronics. USB and Ethernet both perform this process and regularly increase capability. Managed dc derives from modern communications technolo-gies more so than from traditional power distribu-tion (ac or dc).

3) Networked dc extends managed dc from single power links to a network or mesh of power entities

(nanogrids) of arbitrary topology and scale. Products are unavailable today that implement networked dc, but the local power distribution (LPD) technology described below would make this possible. Similar to managed dc, on which it is built, networked dc has more in common with network communications technologies than with traditional power distribution technologies. Figure 3 shows an example network of dc nanogrids, including local generation and vehicles, with a connection to ac infrastructure.

Networked dc makes use of a relatively new feature of some managed dc technologies—bidirectional power—for which the direction of power flow may be different at different times. This exists in the latest ver-sion of the USB Power Delivery specification as well as with HDBaseT, a variant of Ethernet. Networked dc does not require any innovation in how electricity is transferred, but it does require new capabilities in com-munications and control. Figure 4 shows a USB hub and an Ethernet switch; with the addition of modest additional communications technology, each of these could become a fully functional nanogrid controller, although adding some electricity storage as well will likely be the norm.

A key principle of LPD is to decouple how power distri-bution is managed from functional control protocols in which end-use devices participate, similar to how physical layer and application layer protocols are separated from each other with the Internet Protocol (IP) in IP networks.

Figure 2. The standard dc connectors for (a) a USB, (b) an Ethernet, and (c) a 380-V dc.

(a) (b) (c)

Figure 3. An example network of dc nanogrids.

acInfrastructure

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These separations make each part simpler and more effective. In addition, the topologies of power distribution, data communications, and building structure can all be disjoint. Communication is needed to determine where power should flow in these networks. In networked dc, power flows are determined by computation.

What is LPD?LPD is a network model of power. Electricity is managed in arbitrary and dynamically changing topologies of sources, storage, and end-use devices. Power connections are peer to peer between two devices, not via buses with many devices attached. All power exchanges are digitally medi-ated. End-use devices are organized into nanogrids—sin-gle domains of power for voltage, capacity, reliability, and management. A nanogrid controller manages the power distribution to end-use devices connected to the controller and exchanges power with other nanogrids; local genera-tion (or a utility grid connection) is a special form of a nanogrid controller. Controllers almost always have local storage to aid in balancing supply and demand over time. In LPD, there are only two device types and two types of links, paralleling the architecture of the Internet. Figure 5shows a diagram of a nanogrid controller with attached end-use devices, internal storage, and connections to other controllers.

In data networks, all packets are different, addresses provide their destination, and routing mechanisms

determine the path required to get to the destination. In contrast, all electrons are the same, so how do they know where to flow in a power network? The answer is that they should flow from places at which they are more available (plentiful supply) to those where they are less available (more in demand). Balancing supply and demand is a basic function of any grid—and of our economy in general. Elsewhere in our economy, price is the basic mechanism used to accomplish resource allo-cation. LPD, based on a local price set by each nanogrid controller, directs electricity to flow toward nanogrids with higher prices, much as gravity forces water to flow downhill.

With generation and storage in many places in the net-work, power might flow in different directions at different times, as supply and demand change. The prices are local to each nanogrid. Local pricing is the core mechanism used for coordinating among devices on when to generate power, when to charge or discharge storage, and how much and when to use electricity in end-use devices. The price includes a nonbinding forecast of future prices to help inform decisions about modulating device behaviors over time.

LPD capabilities can be added to managed dc technolo-gies, such as USB and Ethernet, and to new physical layers of power that implement managed dc. Some will have higher power capacity. A key feature of LPD is that it will enable both storage and generation of electricity to become plug and play, i.e., able to be connected and disconnected at will by anyone without the safety risk otherwise present when using higher voltage levels. This makes changing power infrastructure something that can be done simply, inexpensively, and frequently, as local conditions require.

How LPD Can Enable DC SuccessDirect dc with LPD together provide a compelling value proposition and a plausible deployment path that can tip dc power from being a small player on the edges of elec-tricity distribution to a sizeable component—to become a success in the market by delivering value to users and saving energy. The following are benefits that LPD can bring to power systems in any building.

Optimal Operation with a Local PriceA core element of the definition of a nanogrid is a local price that correctly indicates the relative scarcity of power and so drives efficient operation of end-use devices, local generation, and local storage. It can also be used to opti-mize exchange of power with a utility grid. Analog voltage levels can be used to indicate scarcity, but this is not accu-rate and precludes including a forecast of future prices and negotiating between devices. Forecasts are essential to moving generation, storage, and end use across time for better system operation.

Communications are also needed for devices to know when they should switch the direction of power flow in a Figure 5. A nanogrid controller.

Storage

Nanogrid ControllerGateways to OtherControllers (nG or μG)or Local Generation

Loads

Figure 4. (a) A USB hub and (b) an Ethernet switch, devices that could be readily modified to be nanogrid controllers.

(a) (b)

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link. Some technologies allow for bidirectional power today; others could accomplish reversing power flow with two parallel links, one for each direction. Power links to vehi-cles will become a common example of bidirectional power flows. We can also expect vehicles to be able to con-nect directly to one another, so that one can charge another anywhere. Management of power distribution with and within vehicles should use the same technology as is used everywhere else.

Plug-and-Play End-Use DevicesIn existing managed dc technologies, initially only a small amount of power is provided to enable commu-nications. Each device communicates to the other about its capa bilities and preferences to allow for adjusting and optimizing the link. The voltage can be negotiated to the highest that can be provided and then used and is also appropriate to the cable (based on its capacity and length). This mini-mizes resistance losses and maximizes power capacity. Communications can be used to determine cable length. In some USB links, the cable itself reports its characteristics to the devices, and a variety of combinations of voltage and current are available that the electricity-providing and electricity-consuming devices might support. This allows for the most efficient combination that both devices can use to be selected. When a device is plugged in that is not compatible electrically for any reason, this can be detect-ed and reported.

Improved SafetyBy communicating before delivering power, conditions that would otherwise be unsafe can be avoided. If a cable is cut or improperly connected, this will be detected by an interruption in the communication. This signal can be used to terminate ongoing power delivery or to stop deliv-ery from the beginning. In such a case, one proprietary mechanism cuts power within 3 ms to enable 400-V deliv-ery over Ethernet cabling. Capacity and voltage limitations of the cable or either device can also be respected auto-matically. Cable overheating could be detected through the two devices comparing the supplied and received power levels and recognizing when the difference between these is inappropriately large.

Managed Limited SupplyEach electrical circuit has a maximum current or power level that it can manage, due to wire size, length, temper-ature, and other factors. In ac systems, a circuit breaker

will cut power to the entire circuit (and hence all devices on it) when the limit is reached. This is inconvenient if it occurs frequently. To avoid it, ordi-nary practice is to substantially over-size wires and circuits, and building codes generally require this, even on circuits that will never carry such high current. This is a waste of the copper and capital and increases installation costs.

With managed dc, actual power levels being used can be tracked. In addition, what devices can poten-tially use at maximum and what they are actually consuming at any particular moment can be consid-ered to then install equipment of appropriate capacity. That is, capaci-ty limits can be respected through pricing and communication rather than oversizing and blowing break-ers. In emergency situations, devices can be summarily disconnected on

an individual basis as needed to reduce total demand rather than interrupting power to the entire circuit.

Plug-and-Play GenerationTraditional electricity systems (ac and dc) use generators that can vary their output to follow electricity demand based on voltage changes, thus maintaining balance in the system. Communications enable systems in which demand can respond to supply conditions, and in sys-tems with more than one generator, communications can determine generator on-times and output levels. When connected to a utility grid, the best balance of utility power and local generation can be determined. Generation technologies are often nondispatchable (as with many renewables), lose power on conversion between ac and dc and between different voltages, and have variable part-load efficiencies, minimum times to be on or off, and losses on each cycle up and down. With communications, these factors can be coordinated to maximize efficiency and equipment use and improve reliability and safety. Communication can ensure that a generator can safely deliver power of the quantity and type desired, before it begins doing so. Without this abili-ty, safe and efficient operation requires careful system design and management, as well as extra hardware. Optimal system operation is impossible without effective communications.

Plug-and-Play StorageElectricity storage as a common feature in electricity sys-tems is only now emerging. Existing use of storage has been for reliable or disconnected operation of individual

A key feature of LPD is that it will enable both storage and generation of electricity to become plug and play, i.e., able to be connected and disconnected at will by anyone without the safety risk otherwise present when using higher voltage levels.

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devices or in uninterruptible power supply (UPS) systems; usually the battery is only ever used when the primary supply is lost, and then it supplies all demand. LPD enables the best use of batteries all the time, taking into consideration lifetime impacts of battery cycling. Without communications, a storage device will generally not know if it should be charging or discharging (or neither) and at what rate. Some systems use changes in voltage levels to communicate about system status, but this is not always reliable and greatly limits the scope of possible informa-tion passed. In complex systems with multiple local gen-eration and multiple local storage devices, only with effective communications can proper, efficient, and eco-nomic use of electricity storage be accomplished.

Privacy and CybersecurityCommunication in LPD is only ever point to point—there is no multihop communication as in data net-works. This dramatically reduces the scope for concern for security and privacy, because only devices with a direct wired connection can attempt to disrupt or spy on one another other. In addition, because the commu-nications are so simple, there is little opportunity for mischief. This is greatly different from the risks and security vulnerabilities that occur in ac grid-tied sys-tems, particularly as large numbers of devices owned by different people are connected.

New Powering ModelsIn the past, end-use devices were only ever connected to a single external power source. There are rare exceptions, such as electronic devices in data centers or telecom

facilities. However, managed dc creates the possibility of devices easily able to use power from more than one source, at different times, or at the same time. This is par-ticularly useful when resiliency is of concern, because a device can be powered through one means most of the time, but by another means when the first is not working or is more expensive. For example, it would be convenient if refrigerators could take in dc power from a vehicle or local generation or storage during times when the utility grid is down, or expensive, and use grid ac power otherwise.

AC circuits are usually multidrop—many devices can be attached to a single wire. DC topologies more common-ly provide only a single device per power port. This ensures that capacity limits on the cable, and cable length, are not exceeded. However, with communications, it could be possible to create multidrop capability for technologies such as Ethernet, with cost advantages for many devices, including lighting, to enable easy daisy-chaining.

The recent 380-V dc standard lacks any mechanism for communicating about power. A good option to remedy this is to use standard Ethernet links, in parallel to each power link. The Ethernet link could provide small amounts of power (relative to what the 380-V path could provide) to energize the end-use device for communications to negotiate aspects of the 380-V line before it is energized.

DC Deployment ArchitecturesDirect dc, particularly when combined with LPD, raises the question of how LPD should be deployed in buildings with most economy and benefit. The dc portion of building infrastructure can be treated as a single cloud of technolo-gy for this purpose, including generation, storage, and connections to vehicles. DC infrastructure can be integrat-ed with ac buildings in several ways that differ primarily in how reliable device operation is accomplished. Power reliability and quality are often key drivers for dc adoption.

The first way to integrate dc is to simply attach dc systems to the existing ac infrastructure with a rectifier or inverter/rectifier, as shown in Figure 6(a). The ac sys-tem is unchanged in topology or capability. The dc sys-tem can be reliable in the face of grid failure, as long as power is prevented from being exported to the ac side during these times. The data line between the meter and dc grids passes price information; it must come from the meter, not the grid, in cases when the price is different depending on whether the building is buying power from or selling power to the grid, as some regions are soon planning to do.

A second method is to create some reliable ac infra-structure as strictly subsidiary to the dc grids, as shown in Figure 6(b). The reliable ac power is always produced from dc and thus can also be insulated from power quality issues that may be present on the utility grid; needs for quality and reliability are highly correlated. The reliable ac is decoupled from the ac grid at all times. Typically, the amount of power in the reliable ac domain is small in

Figure 6. Alternative methods to integrate dc grids and reliable ac. (a) Attaching dc systems to the existing ac infrastructure with a rectifi-er or inverter/rectifier, (b) creating reliable ac infrastructure that is strictly subsidiary to the dc grids, and (c) carving out a portion of the ac infrastructure that can be powered from the dc side during times of grid failure.

dc Grids

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comparison to that in the dc grids. This is convenient in that the ac infrastructure is changed minimally.

A variant of this is to carve out a portion of the ac infrastructure that can be powered from the dc side dur-ing times of grid failure, through the inverter, as shown in Figure 6(c). This adds an islanding switch to assure that power does not flow to the utility grid when it is not oper-ating. There does need to be a mechanism to extend the price-based management of the dc grids to the reliable ac domain, to ensure that supply and demand can be best balanced. In all of these cases, dc systems are reliable, and easy reliability is a key benefit. In LPD, the degree of reliability can be varied from nanogrid to nanogrid and device to device. Changes to the ac infrastructure are minimized as much as possible to minimize costs. A long-term principle that can be adopted is that all devices for which power reliability and quality are of particular concern should be dc powered. This would mean that no capital or other expense would need to be expended to make ac infrastructure reliable, and over time, the utility grid itself could optimally be tuned to lower reliability and quality levels, saving considerable capital and energy.

DC Deployment PathsOne approach to dc deployment is to install a large amount of direct dc infrastructure at one time. This is possible for new construction or major renovations, but for most buildings it is impractical in the near term. An alternative is to slowly evolve toward major use of dc and direct dc in a building, through a series of many steps. DCs can thus be deployed incrementally and organically and can be introduced as opportunity or need dictate. The evolutionary approach can be used in circumstances such as those in the following examples:

spot reliability; when there is a need or desire to make some devices reliable in the face of a grid outage, installing direct dc infrastructure can be done for those particular devices, rather than for a large part of the buildingmodest remodeling projects; small projects can be used to introduce dc hardwarelarge device replacement, such as an appliance or cli-mate control system; although it may not make eco-nomic sense to replace ac devices that are functioning properly, it is a much smaller hurdle to shift to a new dc device rather than buy a new ac deviceoccupancy changes; occupant needs change over time, as do the occupants themselves, introducing more opportunities for changed infrastructure.

A building owner may be interested in dc power but reluctant to move a lot of infrastructure over to it at all or

all at once. The evolutionary approach allows modest investment and risk to allow dc to prove itself in cost effectiveness and other benefits, so that financial and other risks are minimized. DC technology is very much in flux, and so for many end uses in many buildings it is pre-mature to convert to dc. However, this need not preclude installation of dc devices for which the merit of doing so is clear today. DC devices can be introduced with external central or local ac–dc conversion, and dc sources added later to produce direct dc.

This model of technology deployment is familiar for information technology (IT) systems, where conversion of functions from analog to digital, or upgrading or adding new functions, is generally done on an ad hoc basis rath-er than through single large-scale upgrades. The nature of IT networking is such that swapping in new hardware and connectivity is not as burdensome in the way that it

is for modifying traditional electrical systems. This is another way in which networked dc is inspired by architec-tures and capabilities of IT.

Reliable CommunicationsAnother current opportunity for dc distribution is in reliable communica-tions. The U.S. Federal Communica-tions Commission (FCC) has been concerned in recent years with ensur-ing residential communications conti-nuity during utility grid outages. For many decades, telephones were reli-able in such circumstances because they were very low-power devices

powered through the communications wiring from the telephone central office. Today, communications is more commonly a combination of text messaging from mobile phones, e-mail from PCs, and voice-over IP phone calls, requiring a combination of modems, handsets, network equipment, mobile phones, and computers, all of which must be continuously powered or periodically recharged.

It is possible to supply ac power to these devices via a generator (or battery), but this would be cumbersome and relatively expensive, particularly given the relatively low power levels involved and the expense of hardware to automatically make the switch and ensure continuous power delivery. Another alternative is to place batteries into each end-use device, but this would make them cost-lier, bulkier, and more failure prone. A better solution is to power all devices needed for communication via USB from a central hub, eliminating ac–dc conversions within each device—because all of the involved devices are dc internally. The hub could have a battery recharged from the ac power and, even better, have a link to a PV panel to provide power when the utility grid is down and displace grid power when it is up. USB could add a mechanism to indicate when power is scarce so that the end-use

The economics of daisy-chaining devices on a power cable are significant, and developing ways to accomplish this in the context of LPD will be valuable.

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devices could scale back or power down less critical fea-tures; related to this, the current draft of the update to the Ethernet standard has such a feature, a local price index, showing the practicality of the LPD approach. This example of powering devices needed for reliable commu-nication is another example of how dc distribution can be introduced for specific needs to gain multiple benefits. In cases such as these, dc distribution for communications devices could be initially a dc island but later connected to other dc domains, including local generation, for great-er efficiency and reliability.

Challenges and OpportunitiesThe key near-term challenge is to do the research necessary to determine the communications needed between grid con-trollers and end-use devices and, secondly, between grid con-trollers. Once this is done, the features required can be added to all physical layers that implement managed dc. Managed dc link technologies above 100 W are needed; one implemen-tation of that is from Voltserver, Inc., which offers technology for managed dc over 1 kW, over relatively thin cable by using much higher voltages while maintaining safety. The econom-ics of daisy-chaining devices on a power cable are significant, and developing ways to accomplish this in the context of LPD will be valuable. LPD is based on a model of peer-to-peer power exchange. It may be possible in some contexts (per-haps for higher power levels) to create managed dc power buses, with many devices able to put power on or take power off at the same time. The electrical engineering and commu-nications aspects of this may be substantial but could offer compelling efficiency and other benefits. A final challenge will be to ensure that sufficient end-use devices are available that implement LPD. The electronics industry shows that quick innovation and product introduction are possible when manufacturers see a viable market.

SummaryDC power, particularly with direct dc, has the potential to save energy, and offers many other benefits. Making this a reality requires developing new technology. Existing managed dc creates the possibility for new mechanisms to choose how to manage generation, storage, and end use, but because it is limited to a single power links, it has great limits in how it can be leveraged. Only networked dc greatly expands the degree of capability and utility of dc over ac, and LPD is a simple and powerful way to cre-ate this. Although direct dc will sometimes be introduced into buildings in single large installations, more com-monly it may be added in small pieces over time, building an evolving network of power entities inside buildings.

For Further ReadingB. Nordman, “Local Grid Definitions,” prepared for the Home-to-Grid Domain Expert Working Group (H2G DEWG), Smart Grid Interoperability Panel (SGIP), Jan. 5, 2016.

B. Nordman, K. Christensen, and A. Meier, “Think globally, distribute power locally: The promise of nanogrids,” IEEE Com-puter, vol. 45, no. 9, pp. 89–91, Sept. 2012.

B. Nordman and K. Christensen, “DC local power distribu-tion with microgrids and nanogrids,” in Proc. 1st Int. Conf. DC Microgrids, Atlanta, GA, 2015, pp. 199–204.

B. Nordman and K. Christensen, “The need for communi-cations to enable DC power to be successful,” in Proc. 1st Int. Conf. DC Microgrids, Atlanta, GA, 2015, pp. 108–112.

USB Implementers Forum. (2012, July 5). Universal Serial Bus: Power Delivery Specification, Revision 1.0. [Online]. Avail-able: http://www.usb.org/developers/docs/

K. Garbesi, V. Vossos, A. Sanstad, and G. Burch, “Optimizing energy savings from direct-DC in US residential buildings,” Lawrence Berkeley Nat. Laboratory, Berkeley, CA, Rep. LBNL-5193E, 2011.

LAN/MAN Standards Committee. (2009, Oct. 2). IEEE Std. 802.3at-2009, DTE Power Enhancements. [Online]. Available: http://www.ieee802.org/3/at/

BiographiesBruce Nordman ([email protected]) received his B.A. degree in architecture and his M.A. degree in energy and resources from the University of California, Berke-ley, in 1984 and 1990, respectively. He is a research sci-entist in the Buildings Technology Department at Lawrence Berkeley National Laboratory. His research interests include energy use and efficiency of electron-ics and networks, low-power-mode energy use, user interfaces, energy policy, and power distribution tech-nology. He has contributed to many technology stan-dards organizations including the IEEE, the International Electrotechnical Commission, Ecma International, the Consumer Technology Association, the Internet Engi-neering Task Force, and Energy Star specifications and test procedures.

Ken Christensen ([email protected]) received his electrical and computer engineering B.S. degree from the University of Florida, Gainesville, in 1981 and his M.S. and Ph.D. degrees from North Carolina State Uni-versity, Raleigh, in 1983 and 1991, respectively. He is a professor and associate chair of the Department of Computer Science and Engineering at the University of South Florida, Tampa. His research interest is in perfor-mance evaluation of computer networks. In the past 10 years, he has made significant contributions toward proxying for network connectivity and energy efficient Ethernet. This research has contributed to Ecma Inter-national, IEEE standards, and Energy Star specifications. From 1983 to 1995, he was employed at IBM Research Triangle Park in Durham, North Carolina, as an adviso-ry engineer. He has written more than 100 journal and conference publications and holds 13 U.S. patents. He is a licensed professional engineer in the state of Florida, a member of the ACM and the ASEE, and a Senior Mem-ber of the IEEE.

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I C R O G R I D S A R E defined as groups of energy resources, both re newable and/or con-ventional, and loads

located and interconnected in a specif-ic physical area that appear as a single entity to the alternating-current (ac) electric grid. The use of distributed resources to power local loads combined with the capability to operate indepen-dently of the ac grid makes microgrids a technically feasible option to address the concerns of sustainability, resilience, and energy efficiency. Furthermore, microgrids can operate while completely separated from the grid, representing a lower-cost option to provide electrical power to regions in developing countries where con-ventional ac grids are not available or are too unreliable. When connected to the ac grid, microgrids appear as controlled entities within the power system that, instead of being a burden to the ac grid power-management system, represent a resource capable of supporting the grid. Energy storage as the element responsible for balancing generation with load is critical to the success of the microgrid concept, and it is more important as larger penetration of renewable resources is present in the microgrid. Accelerated improvements in performance and cost of energy-storage technologies during the last five years are making


By Luis Eduardo Zubieta

Are Microgrids the Future of Energy?

DC microgrids from concept to demonstration to deployment

©ISTOCKPHOTO.COM/DAVID MARIUZ

microgrids an economically viable option for power sys-tems in the very near future (see Figure 1).

Most microgrid projects have been implemented as ac microgrids, where the resources and loads are inter-faced by power converters to an ac micro bus that matches the characteristics of the conventional ac grid. In most cases, the microgrid ac bus is connected to the ac grid through a fast electronic switch. AC microgrids are a logical result of the extensive experience with large ac grids, the wide availability of ac loads, and the matu-rity of the inverter industry in converting direct-current (dc) power into ac power. However, ac microgrids have

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several technical challenges that are difficult and/or costly to resolve.

The large inertia of the machines used in large-scale ac power generation provides stability to the ac grid. AC microgrids are mainly based on distributed gener-ation connected through inverters. The small inertia of the system results in additional complications and control challenges to keep the system stable.In addition to voltage regulation, an ac system requires frequency and phase angle control to achieve real and

reactive power-flow control and balance. Frequency and phase regulation is more complex in small ac microgrids than in the large traditional ac grid.In microgrids connected to the ac grid, fast disconnec-tion is a requirement to maintain the microgrid and all the loads operational during and after a grid fault. This results in the need for a costly, complex, and expensive solid-state switch between the ac grid and a microgrid like the one in Figure 2. In developed countries with reliable ac grids, these devices are still required, although they are used very few times dur-ing the lifetime of the equipment.The island detection in ac microgrids can be more complex as multiple elements have to be connected to a bus that can be an island or not an island at dif-ferent times. Active anti-islanding detection methods may have to be centralized or shared among several devices using complex algorithms.

Benefits of dc MicrogridsIt is hard to argue against the fact that local distribution systems would greatly benefit from using dc rather than ac as an interconnection bus. Some of the facts validating this statement follow.

Most renewable sources produce dc power, and most modern loads—commercialized as ac loads such as LED lights, consumer electronics, and motor drives—are provided with a front-end rectifier while the load itself is served from an internal dc bus and can therefore be connected more efficiently to a dc bus by eliminating the front end and its losses. For example, large rectifier/inverter motor drives com-monly used in intensive manufacturing plants use topologies like the one in Figure 3. These drives could save up to 50% of their losses if powered directly from a dc bus.Many classic ac generation resources using rotating electromechanical machines can operate more effi-ciently if they are connected though variable-frequency power converters instead of being forced to operate connected to a fixed-frequency ac bus.Electric energy-storage devices, fundamental compo-nents for the operation of microgrids, are also dc in nature. Among the different battery storage technolo-gies, flow batteries have characteristics that are very attractive to microgrids, such as deep discharge capa-bility without affecting cycle life, fast response, decou-pled power and energy ratings, and no cell-to-cell equalization needs. However, these batteries have limitations to supplying high voltages without increasing the cost of the system, and therefore would need a dc–dc in front of the dc–ac converter in ac applications. These battery technologies that are already provided with an internal dc–dc converter would be easily integrated to a fixed dc bus with reduced cost and increased efficiency.

Figure 1. Vanadium redox flow batteries are a perfect fit for microgrid applications.

Figure 2. The 13.8-kV static transfer switch used in the Santa Rita Jail ac microgrid.

Figure 3. An ac-powered motor drive with a rectifier front end.

Energy Flow

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The expected proliferation of electric vehicles during the next few years may represent one of the most demanding challenges in electric power distribution. Fast battery chargers can be simplified and made more efficient if powered from a high-voltage dc bus.A dc microgrid is much less susceptible to failure due to main grid disturbances because the ac–dc convert-er interface between the ac grid and the dc microgrid provides an energy-storage buffer and can replace the functionality of fast active switches used in the ac microgrids.Power quality problems, such as sags, swells, imbal-ances, and flickering, among others, associated with the high penetration of renewable resources that show varying power output, are an important issue in ac microgrids but are easily mitigated in dc systems with robust control of the dc bus voltage.Stability depends on maintaining the dc bus voltage within the normal operating range for all the devices connected to the bus under every transient and steady state condition. Although stability is more chal-lenging as larger and more complex dc microgrids are implemented and different dc microgrid clusters are connected together, the solutions are simpler than for ac microgrid systems.Advances in dc–dc converter technology have resulted in highly efficient and reliable converters providing the “dc transformer” effect that counterbalances the decisive factor that favored ac systems in the 1900s. In fact, high-voltage dc transmission lines are now being used to link separated ac grids with different ratings where the ac power is rectified into dc power that is then converted to a higher dc voltage level for trans-mission. This provides space savings and removes the need for synchronization.

Consequently, strong arguments indicate that imple-menting microgrids by using a common dc voltage is sim-pler, more reliable, and more efficient than using ac to implement microgrids.

Obstacles to dc Microgrid Demonstration ProjectsAlthough, conceptually, dc microgrids are much simpler than ac microgrids, they are still seen by many as a futur-istic concept that needs technical advances in many fields. With the theoretical advantages of dc microgrids validated by calculations, analysis, and simulations, the next step is the implementation of dc microgrid demon-stration projects to validate the different concepts in real conditions. Several demonstration projects have been ini-tiated around the world during the last few years. Most of them are relatively simple projects focused on a specific application such as telecom or data servers, or photovolta-ic (PV) solar plus battery storage installations. Direct-cur-rent distribution systems are also being used successfully on ships and airplanes operating independent of the ac

grid. Larger, more complete demonstration projects including variety of loads are now being subsidized by governmental organizations mainly in Europe and North America. Some of the challenges that project developers interested in building dc microgrid demonstrations are facing are

difficulties in finding electric loads that will operate on dc due to the investment required to remove ac-rectifying elements that are an integral part of the ac products and due to the wide range of internal dc bus voltages in modern off-the-shelf loadsresistance from local authorities to approve dc instal-lations due to the limited experience with this type of project and the lack of references in safety codesthe need for custom power electronics equipment incorporating controls and communication features specific to dc microgrids that demands extensive R&D effort (this increases the cost and extends the project execution time)unknowns regarding the protections needed in dc sys-tems and the lack of protection devices designed spe-cifically for dc systemsa wide variance of ratings and specifications and the repetition of efforts instead of collaboration that would accelerate the goal of more practical imple-mentations of dc microgrids (this is because dc microgrids have been conceived and designed by small groups in academia and industry with very little cooperation among them).

Efforts are now being made to overcome these obsta-cles and limitations. DC microgrid conferences and work-shops are being organized to bring together different experts, to increase awareness, and to promote dc microgrids with the governments, the press, and the gen-eral public.

Engineers working on dc microgrid concepts start with a blank page, building a puzzle based on their own experi-ences and preferences without a set of basic common rules. As a result, components developers interested in dc systems are unable to find a common ground on the spec-ifications they receive from project integrators, leading to high cost and lack of interest in providing custom hard-ware. The EMerge Alliance is an industry association help-ing in accelerating the adoption of dc power distribution in commercial buildings. Among other functions, it brings together companies interested in dc microgrids with the purpose of finding a common ground that will result in standardized requirements and systems and ensure that all the players are aware of what other companies are pro-ducing (Figure 4). In addition, the EMerge Alliance reaches out to manufacturers of conventional electric loads to show them the benefits of using dc power and to stimu-late them to develop dc-rated loads.

At the same time, compliance organizations are work-ing on safety studies and standards that will reduce hur-dles and enable the implementation of commercial-scale

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projects. Committees are working on standards specifical-ly focused on dc systems and including clarifications and rules related to dc systems in present standards to help local inspectors understand the differences between dc and ac systems and give them the tools to identify which safety cautions should be required in dc systems.

As greater standardization is achieved and a larger variety of dc loads become available, hybrid systems com-bining ac and dc loads and generators will allow early implementation of dc microgrids. Some hybrid microgrids can be implemented with the dc bus as the backbone of the system where the power and energy management is executed, while ac loads and classical ac generators are integrated using inverters and rectifiers. Although this system has disadvantages with respect to an optimized dc microgrid using higher-efficiency dc loads, it enables the verification of control concepts and provides data about installation, maintenance, reliability, and performance that would validate economic models used to attract addi-tional investment and customers.

This effort is starting to show results, with more atten-tion on and additional financing of dc microgrid projects. The California Energy Commission announced in 2015 the funding of a demonstration project led by Robert Bosch LLC to implement a dc microgrid at a Honda distribution center. In March of 2016, the Canadian Federal Govern-ment announced the financing of a dc microgrid demon-stration project led by ARDA Power Inc. at an industrial plant in Ontario, Canada. Since energy storage is the cen-terpiece of microgrids, battery companies are getting an increasing number of requests from developers to inte-grate their storage units directly with a dc bus and to exe-cute microgrid-management functions embedded in the energy-storage controls.

Road to Commercialization of dc MicrogridsTo reach extended global use of a technology, in addition to the usefulness of the innovation, several other condi-tions have to be met. These include a cost that matches the market value of the proposition, mature off-the-shelf components, a minimum project-specific engineering and

approval effort, and easy installation and maintenance. Despite the early efforts for standardization and coopera-tion, dc microgrid demonstration projects will use a diver-sity of bus voltages and custom components. Solutions will be improvised to solve technical and compliance issues and many of these solutions will not be designed with immediate market expansion in mind. As more dem-onstration projects are built and interest grows in the market, components and procedures will naturally evolve toward standardized requirements.

Although demonstration and early adapters of microgrids will benefit from incentives and philanthro-pists, to achieve extended use of dc microgrids, the cost has to be competitive with other alternatives. To reach cost-competitive system hardware, all the components required in the installation have to be optimized for cost. The solar industry has reached a cost level that makes it competitive, but optimization and larger volumes are nec-essary in other dc-specific components (loads, protections, hardware, etc.). A significant effort in the generation of safety standards for dc installations and dc components is necessary, and this effort will be fed by results and experi-ences from demonstration projects. With safety standards in place, the components in a microgrid would be able to reach a level of standardization and manufacturability that would allow multiple manufacturers to compete for the market based on quality and price. As the volumes increase, more cost-efficient manufacturing procedures and lower-cost materials may push classical electrical components operating on dc toward the cost targets, turn-ing them into commodities.

The energy storage is the most critical and immature element in a microgrid. Battery technologies are evolv-ing quickly, moving toward the performance levels required for durable and economical microgrids. Howev-er, massive cost reductions in energy-storage technolo-gies are necessary to shift the microgrid concept from a niche market to a mass market. The cost reduction can-not be at the expense of performance, as microgrids will have to compete with traditional power sources in dura-bility and reliability. Improvements in performance and cost of lithium-ion batteries, as well as incoming tech-nologies that are a better fit for the deep cycles required in microgrids (such as flow batteries), provide hope that in short time, a competitive cost of energy storage will be reached.

Residential and commercial ac electrical installations, including loads such as heating, cooling, and lighting, are installed and maintained by electrical contractors and approved for safety using simple and low-cost procedures. The same model will be necessary in dc systems to make them cost effective. Regulated training of electricians has to be implemented, and guidelines and recommended practices have to be written and followed. In parallel, system standardization, off-the-shelf components, and mature regulatory standards will lead to simplified

Figure 4. The 98%-efficiency modular dc–dc converter components being developed for dc microgrids.

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installation and lower maintenance requirements for commercial installations.

Even if cost targets for the components, installation, and service in a dc microgrid are achieved, the project-specific engineering cost has to be minimized. The design and con-struction of large centralized power plants require consid-erable engineering effort that ends up representing a small cost on a per-watt basis. The microgrid market is foreseen covering a wide range of powers but with a large section in systems below 1 MW. The PV solar generation that mainly operates exporting power to the ac grid has been successful in achieving modularity and simplicity in commissioning for solar modules and power electronics components. Components are produced in series, tested at the factory, and require minimum tuning or configuration on-site. Communication is used for monitoring and is not critical for power production. However, microgrids include func-tionality that is much more complex than exporting power to a solid and stable source. A major difference between classical components used in commercial or industrial installations and those components that would be used in microgrids lies in the fact that most components in the microgrid require a level of intelligence to allow them to participate in the power and energy management of the microgrid. Most elements connected to the bus have to operate in harmony, interacting with each other and following specific algo-rithms to satisfy power management, energy-demand response, power qual-ity, and energy-management goals. Without regard to using centralized or distributed control philosophy, project-specific configuration and communi-cation have to be engineered and commissioned, increasing the capital cost of each installation. Even if the same control strategy and communication platform are used, different size instal-lations require redesign, tuning, and long commissioning processes. If components are modified or suppliers replaced, additional reprogramming would be needed. Such a cost burden may limit the market of microgrids in lower-power installations where most benefits can be achieved.

To get a wide deployment of microgrids, a different model is needed in which components are tested by the manufacturers, installed by electricians, and receive simple and quick approval based on certifications simi-lar to the installation of industrial equipment or to the level being achieved in the residential and commercial solar industry. Achieving standardization in the controls and energy management of a microgrid is much more difficult than reaching standardization in hardware and electrical safety. In early concepts and demonstration projects, different developers use different control prin-ciples, system management, and communication

platforms that, in many cases, include intellectual prop-erty and trade secrets. The variety of control methods would need to converge into a few concepts, and those approaching the plug-and-play concept successfully used in the computer industry have a better chance of success.

ARDA dc Microgrid Management SchemeARDA Power proposes a concept shown in Figure 5 in which dc bus voltage is used as the communication mean to provide information among the different elements con-nected to the microgrid. However, the dc bus signaling is not used with the purpose of power management. Instead, it is used for performance functions with slow dynamics that may include battery management, energy manage-ment, and operating-cost optimization. This differs from the classical droop control methods that fundamentally achieve power balance among elements on the bus.

In ARDA’s concept, the main control of the microgrid or regulating function, which requires fast response and quick decision making, resides in the energy-storage ele-

ments that execute power-manage-ment algorithms to maintain the dc bus voltage at a set point. If a load or source suddenly changes, creating a power imbalance on the dc bus, only the energy-storage elements respond in a fast and coordinated manner fol-lowing standard control algorithms such as the one in Figure 6. The power sources and power loads do not respond to the fast transients in the dc bus and are not involved in the instantaneous power management of the microgrid. Such a concept simpli-fies the system’s stability and the commissioning effort at the expense

of demanding a faster response and full power range for the energy storage. Even if the energy-storage units are operating at a high or low state of charge, they can be used to maintain the dc link as long as the average power in or out of the energy storage does not drive them to full discharge or overcharge conditions. The proposal of using the energy-storage elements to control the dc bus voltage is supported by the fact that the energy storage is the center of a microgrid capable of operating separately from the ac grid.

In addition to executing the power management of the dc bus, the energy-storage elements become the energy manager for the microgrid. Simultaneously to controlling the dc bus voltage, the energy manager changes the dc bus voltage set point following an algo-rithm that involves one or several secondary sustaining functions. The sustaining functions have the goal of ensuring that the microgrid will continue operating indefinitely by balancing the energy (or averaged power)

The next step is the implementationof dc microgrid demonstrationprojects to validate the different concepts in real conditions.

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in the system so that the components are not pushed to extreme operating conditions where they cannot respond to changes in demand or generation. Examples of sustaining functions include battery- and energy-management algorithms. The power sources and loads use the average measured dc bus voltage to gain infor-mation about what the energy manager is commanding

and respond following internal algorithms to change their power generated or consumed appropriately. Since all the components are connected to the dc bus and the operation of the microgrid depends on it, the dc-bus-based communication has a high reliability and a low cost. It makes sense to locate sustaining functions in the energy-storage devices since they have all the

Figure 5. The ARDA dc microgrid concept.

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information regarding the state of charge and the power balance in the microgrid.

One of the important advantag-es of the concept is that the energy manager does not need to have any knowledge about specific sources or loads connected to the bus. It simply changes the average dc bus voltage, understanding that other elements on the bus will react to the voltage deviations, cre-ating the desired outcome. If a drift in the dc voltage commanded by the energy manager does not result in a sufficient response from the elements on the bus, the ener-gy manager increases the dc volt-age deviation to trigger additional responses from the same or other elements connected to the bus. As a result, loads and generators can be added to or removed from the microgrid (within the installation rating) without affecting the global operation of the microgrid and with-out requiring any retuning or reprogramming. Since the sustaining controls are slow in nature, a fast response to power-demand changes from the different elements (with the exception of the energy-storage devices) is not important to the microgrid stability. In fact, the elements should react slowly to remove any effect of transient volt-age changes in the dc bus and to give the energy manag-er time to evaluate whether the response has been sufficient. Figure 7 shows a possible algorithm for an energy-storage device, changing the dc voltage set point as a function of its state of charge.

In microgrids with different types of sources and loads, each component would have different preferred operating patterns or performance requirements. Different compo-nents in the microgrid do not have to respond in the same way to variations in dc voltage. The smooth dc voltage communication principle enables programming different power response patterns as a function of dc bus voltage for the different components connected to the microgrid, adding a third level of control for the microgrid or a set of optimizing functions. Optimizing functions may include algorithms to minimize the operating cost for the microgrid, extend the lifetime of the components, provide energy-consumption reduction, etc. The function limiting or setting the power for a source does not necessarily have to follow a linear relationship with the dc bus voltage. Instead, it may include nonlinear terms (quadratic, expo-nential, steps, etc.) or discontinuities, depending on the specific properties of each distributed resource or load, and it may also be seasonal, depending on market or envi-ronmental factors. For example, a specific generating resource could be preprogrammed to ramp down the power production as the dc bus voltage increases, while

another can be programed in steps so it shuts down sec-tions of the generator as the dc voltage increases. Power converters used for generators and loads only need to be provided with the capability to adjust their production/consumption based on a control signal. The control signal can be embedded in the power converter or provided by an add-on controller. This would make any power convert-er capable of following a power reference compatible with ARDA’s microgrid-management concept. Figure 8 shows a representation of the microgrid-management concept with individual functions executed on each element con-nected to the dc bus.

By using the energy storage as the backbone of the microgrid, the ac grid is considered simply as a bidirectional source/load interfaced through a power converter. This means that the operation mode and control method are the same in a grid-connected or grid-independent opera-tion. This would decouple the microgrid from the ac grid and eliminate the risk of a large disturbance on the grid affecting the microgrid or resulting in power loss. Further-more, the power converter connecting the dc bus to the ac grid can be communicating with the utility and receiving requests for ancillary functions. These ancillary functions would represent the optimizing function for the grid power converters and would be executed in the same fashion as for any other component on the bus. ARDA’s concept can also be implemented as a hybrid microgrid by incorporating traditional ac loads and sources using inverters and rectifi-ers provided with algorithms to respond to average dc volt-age changes and with optimization functions. This way, they would appear to the energy management as any other dc source or dc load.

ConclusionsIncreases in the use of renewable distributed generation and improvements in energy-storage technologies make microgrids the best concept for addressing reliability, sus-tainability, resilience, and energy-efficiency concerns.

Figure 7. An example of dc bus signaling for microgrid battery management.

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Because of the dc nature of many renewable sources, energy-storage technologies, and modern loads, integra-tion of resources and loads in a dc bus is attractive in reducing the footprint of the installations, increasing the efficiency, and eventually reducing the cost of the microgrid. Lack of standardization, lack of dc-rated com-ponents, and obstacles in safety approval are factors impeding the implementation of dc microgrid demonstra-tion projects. Achieving widespread deployment of dc microgrids will require, in addition to standardization and lower hardware cost, effective means to reduce the engi-neering cost of microgrid projects. Concepts that approach the plug-and-play paradigm so successfully used in the computer industry will have a better chance of achieving a widespread market acceptance. ARDA Power’s proposed solution, based on using the energy-storage elements to regulate the dc bus and as the energy manager for the sys-tem, meets this requirement.

For Further ReadingS. N. Backhaus, G. W. Swift, S. Chatzivasileiadis, W. Tschudi, S. Glover, M. Starke, J. Wang, M. Yue, and D. Hammerstrom, “DC microgrids scoping study–estimate of technical and economic benefits,” Los Alamos Nat. Lab. (LANL), Los Alamos, NM. Tech. Rep. LA-UR-15-22097, Mar. 2015.

REbus Alliance. (2011). REbus microgrid specification. [Online]. Available: http://rebuspower.com/REbus%20Microgrid%20Specification%20v0_14.pdf

J. M. Guerrero, J. Vásquez, and R. Teodorescu, “Hierarchi-cal control of droop-controlled dc and ac microgrids—A general approach towards standardization,” in Proc. 35th Annu. Conf. IEEE Industrial Electronics, Porto, Portugal, 2009, pp. 4305–4310.

R. Singh and K. Shenai. (2014, Feb. 6). DC microgrids and the virtues of local electricity. IEEE Spectr. [Online]. Available: http://spectrum.ieee.org/green-tech/buildings/dc-microgrids-and-the-virtues-of-local-electricity

L. E. Zubieta, “Power management and optimization con-cept for DC microgrids,” in Proc. IEEE First Int. Conf. dc Microgrids, Atlanta, GA, 2015, pp. 81–85.

BiographyLuis Eduardo Zubieta ([email protected]) received his Ph.D. degree from the University of Toronto and is the vice president of development at ARDA Power Inc. in Oakville, Ontario, Canada. He has close to 20 years of industrial experience in advanced power elec-tronics topologies and controls for alternative energy applications.

Figure 8. The ARDA microgrid management based on dc bus voltage.

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©ISTOCKPHOTO.COM/TKPHOTOGRAPHY64

N RECENT YEARS, EVIDENCE HASsuggested that the global energy system is on the verge of a drastic revolution. The evolutionary development in power elec-tronic technologies, the emergence of

high-performance energy storage devices, and the

ever-increasing penetration of renewable energy sources (RESs) are commonly recognized as the major driving forces of the revolution. The explosion in consumer elec-tronics is also powering this change. In this context, dc power distribution technologies have made a comeback and keep gaining a commendable increase in research interest and industrial applications. In addition, the con-cept of flexible and smart distribution has also been pro-posed, which tends to exploit distributed generation and


By Zheming Jin, Giorgio Sulligoi, Rob Cuzner, Lexuan Meng, Juan C. Vasquez, and Josep M. Guerrero

Next-Generation Shipboard DC Power SystemIntroducing smart grid and dc microgrid technologies

into maritime electrical networks.

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pack together the distributed RESs and local electrical loads as an independent and self-sustainable entity, namely a microgrid. At present, research in the area of dc microgrids has investigated and developed a series of advanced methods in control, management, and objec-tive-oriented optimization that would establish the tech-nical interface enabling future applications in multiple industrial areas, such as smart buildings, electric vehi-cles, aerospace/aircraft power systems, and maritime power systems.

Maritime power systems can be traced back to the 1880s, starting with the earliest record of a dc-based onboard power system on the SS Columbia, where Edison’s dc lighting system was first installed. In the last century, maritime power systems have been greatly developed along with the increasing demand of onboard electrical loads. During this development, shipboard power trans-formed from Edison’s dc power system into Tesla’s ac power system, as the use of electricity extended from the initial lighting to almost every aspect aboard a vessel where it was necessary to build upon the advances in the ac distribution infrastructure. In recent years, government regulation of emissions has become increasingly strict, while customers’ fuel-efficiency requirements have risen. This has resulted in the current trend toward more effi-cient ships, the most emblematic of which is the all-elec-tric ship (AES), which exploits an electrical propulsion system instead of the conventional mechanical system. One of the significant features of the AES is the concept of

the integrated power system (IPS), which minimizes the number of generators in a ship by incorporating intelli-gent methods for meeting load demands through multiple paths and dynamically matching generational capability to loading needs. In broad terms, the shipboard IPS can be regarded as a large-scale, onboard microgrid with specific requirements. In recent studies, the current IPS research trend is turning to dc power distribution systems. This has resulted in advanced research outcomes in the dc microgrid field, especially in its advanced control, man-agement, and optimization methods, all of which can be attributed to a wide body of AES research.

DC Power ArchitectureThe Queen Elizabeth II, the world’s first cruise vessel with an electric propulsion system, is a high-profile example of an existing ac shipboard power system. The power archi-tecture of its shipboard power system is shown in Figure 1.The vessel, originally steam powered, was built in 1968 and was converted from steam to diesel-electric propulsion in 1987. The ship was refitted with nine diesel generator sets rated 10.5 MW at 10 kV. The electric power plant is con-nected with the vessel’s main bus, driving the two major 44-MW electric propulsion systems. The auxiliary loads and the hotel service loads are powered through trans-formers and power electronic converters. The conversion to a diesel-electric power system was expected to improve fuel efficiency by up to 35% at the vessel’s service speed of 28.5 kn and save £12 million a year in fuel costs. However,

Figure 1. The diesel-electric shipboard power system of the Queen Elizabeth II.

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as studies have progressed, researchers and engineers have noticed inadequacies in the ac power architecture that can be summarized as follows:

generator sets have to work in fixed speed and thus limit further improvement in fuel efficiencythe ac power architecture introduces unwanted reac-tive power flow and power quality problems (e.g., three-phase imbalances and harmonic currents)the bulky conventional transformers occupy too much valuable space and weight onboardthere is a potential risk of systemic disintegration when supporting emerging pulsed electrical loads.

These problems also plague terrestrial power distribu-tion systems, resulting in the current trend toward return-ing to dc-based power distribution systems. Edison’s dc power system has once again led the second Industrial Revolution and brought a new era of light as well as electrification to humankind. It was overshadowed for more than a century after losing the famous “battle of the currents” due to its inherent inability (at that time) to change voltage levels with-out the addition of multiple motor–generator sets, thus making the system uneconomical to operate compared with the ac power system (which had at its disposal the simple transformer for changing voltage lev-els). But thanks to the rapid develop-ment of modern power electronic technologies, the high-frequency dc–dc converter has already qualified for taking on the role of transformer in dc systems. It therefore may allow Edison’s invention to change the world once again. Just as Edison once strove to prove, it is becoming clear that the dc power sys-tem has several major advantages over the ac system, and even some newly recognized advantages, such as

replacing bulky ferromagnetic transformers with compact power electronic converterseasier parallel connection or disconnection for dc power sourceselimination of harmonic and imbalance problemselimination of synchronization problemselimination of reactive power flow.

Additionally, considering the specific needs of ship-board power systems, the dc-based IPS could bring a broad range of advantages for both commercial and mis-sion-oriented ships. Generally, the dc power architecture will eliminate bulky low-frequency transformers and reduce the rating of switchgear, thus reducing the occu-pied space and overall weight of the whole system, which may result in extra cargo space. The commercial sector focuses on the 15% fuel saving due to allowing variable-speed diesel generators, whereas the military

sector is interested in support for advanced electrical equipment and weapons, which are characterized by high-power pulsed loads. For these vessels (mainly war-ships), meeting these objectives requires a highly secured power supply. Moreover, a dc power architecture could provide better survivability, limitation of fault cur-rent, and reconfiguration capability. Besides that, the integration of advanced high-speed, high-efficiency die-sel generation (i.e., gas turbine generation) could also be easily achieved within the dc power architecture, which could effectively improve the fuel efficiency of the sys-tem. Due to the higher power levels required in AES applications, the only available design option for a dc-based IPS is the medium-voltage dc (MVdc) solution with a dc bus voltage above 1 kV.

The typical power architecture of terrestrial dc microgrids is shown in Figure 2(a), where the RESs, ener-gy-storage systems (ESSs), and local electrical loads are

packaged together with the dc bus to enable islanding operation, which makes the system fully resistant to major blackouts in the main grid. The elimination of reactive power and synchronization problems makes the whole system much simpler to design, control, and coordinate. More-over, with a well-selected nominal bus voltage, the overall efficiency will be generally higher than its ac counter-part. The three-wire, bipolar-type dc microgrid power architecture is shown in Figure 2(b). The architecture evolves from Edison’s three-wire dc power distribution system, which was initially designed to save conductors. Compared with the typical architec-

ture, the positive bus and negative bus can work indepen-dently if a fault occurs, which result in inherent redundancy and higher reliability. Moreover, it allows using a neutral bus with a low rated current if the loads on the positive bus and negative bus are roughly equal.

Figure 3 shows a ring-bus-based dc microgrid power architecture proposed for a critical load with higher secu-rity requirements (e.g., a data center). The ring bus allows energy flows along either the shortest path or a subopti-mal path. That is, wherever a single fault occurs in the sys-tem, it can be isolated by switching off the nearest circuit breakers, allowing other parts to work as normal. This fea-ture guarantees system survival from single-point failures. In addition, the ring bus allows the critical load to obtain energy from multiple nodes by applying either a conven-tional multiple-contact point switch or multiterminal con-verters. Accordingly, the critical load is highly secured to achieve uninterrupted operation.

A similar architecture can apply to the maritime power system, but the inner part of the system will be

Just as Edison once strove to prove, it is becoming clear that the dc power system has several major advantages over the ac system, and even some newly recognized advantages.

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divided into electrical zones corresponding to the feeds, and it will typically be laid out with generational sources on the two sides, designated as port and starboard and

with a simple cross-connect in the forward and aft parts of the system, as shown in Figure 4(a). Such a system is commonly referred to as a dc zonal electrical distribution

Figure 2. Typical power architectures of a single-bus dc microgrid: (a) a common architecture and (b) a bipolar architecture. PV: photovoltaic.

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system (ZEDS). Figure 4(b) shows the layout of the equip-ment in an electrical zone. Note that a large number of the loads in the zone are fed from both sides of the ship to enhance survivability. As opposed to a terrestrial power system, a maritime power system is inevitably restricted by the cabin structure of the vessel or offshore platform, so the size and weight of the overall system are impor-tant. To minimize the dc cabling size, voltage levels of greater than 6 kV are proposed for future combatants. For architectures as in Figure 4(a), the switches around the ring bus are there to isolate faults that may occur on the buses that distribute power to the zones. There are two approaches: breaker-based and unit-based. With breaker-based architectures, the switches must be actively con-trolled solid-state circuit breakers (SSCBs) combined with fast-acting no-load isolating mechanical switches. Such systems have the potential to provide a high quality of power during fault events (i.e., minimal power inter-ruption), but the SSCB at these levels are still items in development that carries with it considerable risk. Inter -communication between adjacent SSCBs is necessary to isolate the fault because the dc ZEDS must be able to pro-vide the same current from any direction.

With unit-based architectures, the power converters that interface with the electrical sources to the port and starboard buses play the primary role of driving the cur-rent to a fault on the bus to zero. The switches are all no-load switches. To be unit-based, the architecture in Figure 4(a) cannot have cross-tie switches between buses (i.e., where the battery-interfacing converters are), because when a fault occurs on a bus system, operation requires that critical loads within the zones autonomous-ly shift their power sources to the healthy opposite bus. This is accomplished by diode auctioneering of power sources fed from both sides of the ship into the loads. Intercommunication between the switches and convert-ers is necessary to determine where to isolate the fault. Once a switch isolates a fault, the power converters on the effective bus are reenergized, and all but the faulted

part of the system is restored to operation. Communica-tion is considerably more complex with the unit-based system, but the risks of implementation and system cost will be much lower when compared to the breaker-based model. The system in Figure 4(c) is an alternative archi-tecture that utilizes SSCBs of different current rating lev-els on two buses and may be able to isolate faults using SSCBs but with minimal intercommunication. If genera-tors are distributed between buses, this architecture pro-vides an opportunity for operation with a high power quality bus on the inside, dedicated to feeding the low-voltage systems in the zones under normal conditions, and a lower-quality bus on the outside that is dedicated to high power loads and pulsed loads. These two buses can operate independently of each other if the SSCBs have reverse current-blocking capability. The architecture offers an opportunity for efficiency improvement in the ship by allowing the output bus to operate at a lower volt-age than the inner bus when it is not necessary to oper-ate at full propulsion speed.

These different power architectures are all feasible choices for the design of onboard dc power systems. How-ever, there are always tradeoffs between reliability and complexity. Complicated power architectures require much more sophisticated control and coordination strate-gies, which need to be carefully evaluated during early-stage design. Generally, the crucial guidelines for power architecture design and selection should be the reliability and redundancy requirements and the shipboard mis-sion requirements.

Onboard Distributed ESSs

Enabling Smart Grid TechnologiesDue to the soaring price of fossil fuels and the practical need to integrate intermittent renewables into the future energy system, energy storage technology has been one of the hottest research directions in the last decade. With the presence of highly intermittent energy sources and

Figure 3. A ring-bus-based dc microgrid.

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loads, ESSs are necessarily needed to guarantee reliabili-ty, security, stability, and desirable power quality, espe-cially under islanding operation. However, ESSs were seldom a concern in traditional power systems. In recent

studies, the importance of ESSs in microgrids, especially in islanding ones, is being gradually elevated due to their potential to introduce a range of benefits. ESSs can be directly controlled as the master unit in the microgrid, and there-fore they ensure the uninterrupted operation of the entire system. In addition, the ESS charge–discharge cycle can be optimally scheduled according to variable energy prices, consumption prediction, and weath-er forecasts, aiming at achieving eco-nomic objectives.

In the case of maritime applica-tions, onboard ESSs are taking on a pivotal role in the IPS of the next-generation AES. For U.S. Navy surface combatants, the main rea-sons for an ESS are twofold: 1) to enhance survivability and 2) to enable high-energy pulsed loads. Congressional funding for research and development on the AES is motivated by the advent of and need for high-impact electric weap-onry. Without an ESS, the shipboard generators would need to be signifi-cantly oversized to support the high-energy, pulsed nature of elec-tric weapons. Even with an ESS, the growth of auxiliary loads and the capacity needed to support electric propulsion necessitates a capabili-ty to utilize the reserve capacity of online generators and the ESS to deliver the right amount of power to the right place in the ship at the right time—which is enabled by the IPS.

As for the commercial sector, fuel economy is the major con-cern. Considering the fact that diesel generation is still the major power source for all maritime applications, its efficiency charac-teristic in fixed-speed operation is as shown in Figure 5. In general, engineers will intentionally design and make the diesel generator sets work in their high-efficiency

area and modulate the number (K in Figure 5) of running engines to achieve optimal load matching. However, instantaneous fluctuations in the demand side (e.g., dynamic positioning) will break the balance between

Figure 4. Typical power architectures of ZEDS-based dc power distribution systems: (a) dc ZEDS, (b) zonal load center, and (c) dual-ring-bus dc ZEDS.

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power generation and power consumption, thus reduc-ing fuel efficiency. The presence of the ESSs can inject bidirectional, controllable power flow into the system to achieve load conditioning. Such a fact enables modifying fuel efficiency with the help of onboard ESSs. In this way, it is possible for diesel generator sets to work con-stantly with the modified fuel efficiency.

Along with the development of energy storage devices, a range of commercially available storage device options for stationary or mobile terrestrial applications have already appeared. A comparison of their instantaneous power density and energy density is shown in Figure 6. Heretofore, batteries, especially lithium-ion batteries, became the preferred choice for electric vehicles and hybrid electric vehicles. Electrical double-layer capacitors [(EDLCs) or, informally, superca-pacitors] have been applied for peak power shaving. Fly-wheels have found application in improving the low-voltage ride-through ability for wind farms. Besides that, there are several references involving sodium-sulfur (Na-S) batteries and superconducting magnetic energy storage (SMES), even though they have extreme-tempera-ture requirements. Since the AES IPS is a large-scale sys-tem with complex loads, one potential solution will be distributed ESSs, which is based on a cluster of large or small ESSs using different kinds of energy storage devices. At present, the most promising, dominant energy storage devices for maritime applications are batteries, EDLCs, and flywheels. With proper allocation and configuration, the onboard ESSs will be able to enable multiple functions, such as power backup, peak power shaving, and braking energy recovery.

From the perspective of control and decision making, the integration of ESSs also introduces a new dimension into the control and management of shipboard power systems, where efficiency and the emissions from the onboard generation could be actively optimized. By cooperative control of onboard ESSs and generators under the complex load conditions, the optimization toward lowest fuel consumption and/or lowest emis-sions, as well as the need to service highly dynamic load demands and pulsed energy requirements, can be achieved simultaneously. Currently, a new trend of installing PV panels and wind turbines on board vessels to reduce the cost of sailing is drawing industrial atten-tion. Such an optimization between ESSs and genera-tional sources would be more effective and necessary with the integration of onboard RESs in the near future.

Control and Coordination of the Microgrid-Based Power System

Hierarchical Control: The Future Smart Power System’s InterfaceDespite the benefits offered by the dc-based IPS, it is still a challenging task to simultaneously achieve voltage

regulation in a vessel’s highly dynamic load condition (especially in dynamic positioning operation) and real-time optimization of fuel economy. According to IEEE Standard 1709–2010, the shipboard dc power system needs to fulfill the following control objectives:

power system stability: the ability to maintain autono-mous equilibrium in normal conditions and regain a state of operating equilibrium after being subjected to a physical disturbancepower quality: the ability to maintain or restore the common dc buses at their nominal voltage with acceptable voltage tolerancepower management: the ability to optimize systemic efficiency by intentional scheduling or intervention without affecting the maximization of the power sup-ply to the demand side.

In terrestrial applications, dc microgrids also face similar challenges. According to the IEEE Standard 1547 series standards, microgrids should be able to

Figure 5. A schematic diagram of the fuel efficiency characteristic of diesel generation (at a fixed speed). SFOC: specific fuel oil consumption.

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Figure 6. The power and energy densities of different energy storage devices. FW: flywheel; NiCd: nickel-cadmium; LMO: lithium manga-nese oxide; LPF: lithium ferrophosphate; LCO: lithium cobalt oxide; and LTO: lithium titanate.

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operate both in grid-connected and islanded modes. Power flows are also expected to be managed at the same t ime. With the act ive research and development in recent years, a series of advanced control and coordination tech-niques have been investigated for dc microgrids. One of the most rep-resentative ones is the hierarchical control scheme, which is an adap-tation of the International Society of Automation ISA-95 grid opera-tion standard in microgrid control. Generally, to effectively achieve different control functions, the hierarchical control scheme is pro-posed, with the following typically defined levels:

Level 0 (inner control loops): the fundamental control loops to regulate the output voltage and/or current within each power electronic converter connected to the microgrid.Level 1 (primary control): the control methods to emu-late the physical behaviors that make the system sta-ble and more damped power sharing.Level 2 (secondary control): the control methods to ensure that the major variables of the system are within the required values.

Level 3 (initial tertiary control): the control methods to manage and control the power flow among the upper-layer grid and/or other microgrids.Upper levels (extended tertiary control): the control and decision-making methods to achieve extra targets (such as practical economic benefits).

Figure 7 shows a typical scheme of hierarchical con-trol. At present, mature power electronic converters are designed precisely to ensure that they remain stable and controllable under the worst working conditions. For this reason, hierarchical control of the microgrid is allowed, concentrating on system-level control, refer-ences as primary, secondary, and tertiary control. Gener-ally, the primary control performs the local control of output voltage and current of the power electronic inter-faces, following the setting points of the upper control levels. The secondary control that appears above the pri-mary control deals with voltage or frequency restoration and the management of power quality. Additionally, the secondary control is in charge of power exchange with the external grids in the same layer (e.g., other microgrids). The tertiary control is conventionally issued with the task of man-aging the power exchange between the microgrid and its upper-layer grid. In recent studies, there is a trend to inte-grate the upper control levels, which are initially issued to achieve extra targets in the tertiary control. To this end, the tertiary control is to introduce intelligence to the microgrid

and optimize the microgrid opera-tion based on specific interests—normally efficiency and economics.

Figure 8 shows a typical control architecture applying hierarchical control in a generalized dc microgrid. Droop control can be installed as the primary control method for active power sharing purposes. In recent studies, either output power or output current could be selected as the feed-back signal in the droop control. The droop coefficient can be regarded as a virtual internal resistance. In this case, the droop control consists of the physical connection of dc sources, and it therefore simplifies the design of the parallel converter systems in the

dc microgrid. A small voltage deviation will be introduced by droop-based primary control. Therefore, a secondary control is introduced to compensate for the voltage devia-tion. In most cases, a straightforward proportional-integral controller can be employed to meet the need of tracking nominal voltage reference. However, adaptive droop con-trol that uses adaptively changing droop coefficients instead of fixed ones has also been introduced to some high-requirement systems using decentralized coordina-tion. It differs from primary and secondary control in that the tertiary control is providing optimization functions.

Figure 7. Different levels in hierarchical control.

Upper-Level Operators(Interfaces to Intentional Operation)

Tertiary LevelEconomic Dispatching and Optimization

Supervisory Optimizing Generation Scheduling

Secondary LevelPower Quality Control

VoltageRestoration

Mode Selection

Primary LevelPower Sharing Control

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Physical Level of Microgrid

For large-scale dc microgrids, hierarchical control is often a preferred choice since itoffers decoupled behavior between different control layers.

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Thus, not only the controller itself but also decision-mak-ing methods have been proposed to achieve specific opti-mization objectives.

Centralized, Decentralized, or Distributed Coordination: Scenario-Based ChoicesFor large-scale dc microgrids, hierarchical control is often a preferred choice since it offers decoupled behavior between different control layers. However, hierarchical control is achieved by simultaneously using local control of the power electronic interfaces and the coordinated control of all these components. The secondary and tertia-ry control levels rely on the cooperation of several or all local controllers. For this reason, the coordination in the microgrid will also impact on system stability, reliability, and performance. According to their different communi-cation modes, coordination methods can be divided into three categories: centralized, decentralized, and distribut-ed. Figure 9 shows the different operating principles of these three coordination methods.

Centralized coordination control can be implemented in dc microgrids by employing a central controller and a

communication network, as shown in Figure 9(a). In small-scale dc microgrids, each unit can be directly regulated by the central controller via high-bandwidth communication using the master/slave method. It should be noted that centralized control provides the best foundation for the advanced control functionalities and system-level optimi-zation, since all relevant data can be collected and pro-cessed within a single controller. However, the cost and difficulty of implementing centralized control increases nonlinearly with the increasing number of accessed com-ponents. Moreover, the most obvious drawback is that the control architecture has to face the potential failure of the central controller and/or key communication links, which may block the transmission of the commands and result in a systemic failure. In addition, the emerging issue of cyber-attack also needs to be considered, especially for some mission-oriented applications.

Decentralized coordination control is achieved exclu-sively by the local controllers, as shown in Figure 9(b). The obvious advantage of decentralized coordination is its inde-pendence from the communication and central controller, allowing this architecture to offer higher flexibility and

Figure 8. Hiearchical control in a practical dc microgrid.

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exemption from single-point failure. In recent studies, decentralized coordination can be achieved in several ways, such as by the dc bus signal (DBS) and power line signal (PLS) methods. These methods exploit the information-car-rying potential of global variables (i.e., dc bus voltage) to achieve coordinated operation. Meanwhile, master/slave control and multimode control strategies are commonly used to coordinate the energy sources to achieve compara-ble performance. However, decentralized coordination methods have their own drawbacks, the most important being the lack of global information awareness, which will

result in inherent performance limitation, especially when performing optimization. In addition, the major methods of decentralized coordination are based on the response to specific global variables; the accuracy of measurement thus impacts the effectiveness of the entire system.

Therefore, instead of centralized coordination control or decentralized coordination control, distributed coordi-nation control can be seen as a good compromise between both approaches, where a central controller does not exist but where local controllers are able to communicate with each other. The most important dis-tributed coordination method is the multiagent system (MAS), in which each local controller could be regarded as an intelligent agent, with all agents together compos-ing the MAS. By applying a consensus algorithm, it could achieve information awareness comparable to that of centralized control and offer the possibility of applying wider functionalities than decentralized control. Mean-while, it maintains a reliability comparable to decentral-ized control. The MAS is also considered to be an effective way to achieve global optimization objectives (e.g., overall efficiency improvement). However, it requires a complex interaction network among the local controllers, and its main limitation is the complexity of the analytical performance analysis, especially in noni-deal environments (e.g., communication time delays and measurement errors).

Smart Grid Technologies, the Key to the Smart Onboard Power SystemThe common trend of power systems is moving toward higher intelligence and efficiency. As one of the major objectives of the future smart grid, the concepts of intelli-gent management (e.g., supervisory energy management) and smart protection (e.g., adaptive reconfiguration) have been introduced to the microgrid as an extension of the conventional hierarchical control architecture. These con-cepts can also be introduced to the IPS to achieve an effi-cient and reliable shipboard power system for the future smart AES, and therefore contribute to the further improvement of fuel efficiency, limitation of greenhouse gas emissions, and fault-tolerant character of the ship-board power system.

Smart Coordinated Management for Lower Cost and Reduced EmissionsUnder normal conditions, the voyage or mission of a ves-sel can be divided into several operating scenarios, such as docking, acceleration, deceleration, and cruise. These sce-narios will not transfer in random order—for example, the vessel will not dock directly after acceleration. Based on this important fact, preplanned onboard energy manage-ment and its optimization would be applicable to coordi-nate the onboard generation and ESSs for optimal fuel efficiency. In recent years, the International Maritime Organization has promoted the Ship Energy Efficiency

Figure 9. The operating principles of different coordination methods.

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Management Plan (SEEMP) to encour-age emissions limitation, and it may be required for each vessel in the near future. To design a SEEMP, it is possi-ble to employ advanced offline opti-mization algorithms to improve the fuel-saving effect with reasonable assumptions (i.e., the fuel efficiency is affected only by engine speed and load). However, the practical operation of a vessel may be influenced by innu-merable contingencies (e.g., unexpect-ed travel distance due to wind and waves), which make the offline prede-signed SEEMP result in suboptimal fuel efficiency. To maximize fuel effi-ciency and/or minimize emissions, a potential method is to combine scenario-based multimode control and real-time optimization, in which real-time optimizing could be done within the constraints given by the tertiary level of hierarchical control (i.e., the energy management level) and according to the detailed system status (i.e., the overall state of charge information from the ESSs and the operation mode).

To implement a SEEMP, joint management, on both the generation side and demand side, is required. From the perspective of generation, a dc distribution system allows each prime mover to operate independently in a variable-speed mode without the limitation of synchronization. Fig-ure 10 shows an experimental result of the specific fuel oil consumption (SFOC) in g/kWh under the full operating range of a typical shipboard diesel generator. It indicates that fuel consumption is a nonlinear function of the engine speed and load condition and has a high-efficiency area. Generally, the generation-side management tends to keep the onboard generators either working in their high-efficiency area or working in idle speed. In this way, the SFOC is maintained at its lowest point. However, the onboard generation is not stand-alone; it always depends on the power demand.

The traditional demand-side management method in power systems is based on load shedding methods. How-ever, the onboard loads are usually mission oriented, and the major energy consumer will be the electric propulsion system in the future AES. Thus, conventional load shed-ding will result in unwanted performance degradation in mission-oriented function or the propulsion system, which makes the methods unsuitable for such coordinat-ed management. With the help of ESSs, the dynamic active power balance can be achieved by properly and bidirectionally managing the power flow between ESSs and the dc bus. Thus, an equivalent demand-side man-agement can be achieved in this way, which allows highly flexible operation of the other onboard electrical equip-ment. At the same time, the major optimization objec-tives, such as maximum fuel efficiency and support of

emerging pulsed mission-oriented equipment, can also be achieved.

The role of ESSs in the SEEMP is extremely important due to their invaluable bidirectional characteristic. The presence of ESSs breaks the con-ventional dependency between the generation side and demand side, thus significantly improving the flexi-bility of the SEEMP. In addition, it is noteworthy that the electric propul-sion could also act as generation while doing regenerative braking. The traditional method is not able to deal with such bidirectional loads, and the excess energy has to be dissipated on dumping resistors to maintain the

stability of the power system. With the help of ESSs, this part of the energy can be partly or fully stored, thus help-ing to reduce the overall cost. ESSs are also able to take the role of the primary energy resource during short-term voyages (e.g., in-port moving) or emergency conditions (i.e., auxiliary generation), which may significantly reduce the environmental impact and enhance reliability.

Smart Protection and Reconfiguration for Fault-Tolerant and Highly Reliable SystemsThe protection of dc power systems, especially those with complex dc ZEDS configurations, is a challenging task requiring the development of SSCBs suitable for MVdc solutions and complex coordination between power con-verters and protective functions. Moreover, compared with conventional transformers, the instantaneous overcurrent capability of power electronic converters must be limited to avoid equipment damage, whereas conventional trans-formers inherently carry a reserve inertia to sudden tran-sient electrical events. As a result, an adequate shipboard IPS, which delivers power through power electronic con-verters, usually leads to overdesign of the power electronic equipment, which is a problem when considering space

Figure 10. The SFOC of a typical diesel engine at variable speedand torque.

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In small-scale dc microgrids, each unit can be directly regulated by the central controllervia high-bandwidth communicationusing the master/slave method.

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constraints. Also, considering dc ZEDS, since zones are normally inter-connected, there may be scenarios where a single failure might spread and cause a regional failure or sys-temic crash if the protective architec-ture is not designed to address the potential for such scenarios. Hence, effective fault protection and fault-point isolation are considered as the major challenges for ensuring the safety of the MVdc IPS.

A considerable research and devel-opment effort has been made to enable the protective function by using power electronic converters. However, there are still several challenging points, such as communication delays and measurement failures. In recent studies, measure-ment failures can be overcome by using outliner data detection and reconstruction algorithms. Expert system concepts have also been introduced into dc microgrids to achieve prognosis of fault sections and to guide effective protective activities when faults occur.

The reconfiguration capability is one of the most prom-ising advantages of the MVdc IPS for the future AES, espe-cially for naval applications. However, the nonlinear multiconnectivity and high-dimensionality of the onboard power system make it difficult to achieve fast and efficient reconfiguration. Returning to the dc ZEDS discussion

related to Figure 4, several advanced concepts have been introduced to address the protection dilemma. An essential approach is the self-healing reconstruction method, which first subdivides the power system into sev-eral zonal microgrids and then recon-structs from microgrids when a fault is cleared. The sectionalizing aims at the minimization of the isolated area while at the same time maintaining the power supply to healthy zones. Further, the sectionalized zonal microgrids will attempt to connect with each other and form networked

microgrids, which can improve the operation and reliabili-ty. In this way, the power system will recover from the fault in several steps and isolate the fault location at the same time. Figure 11 shows the process of sectionaliza-tion and reconfiguration based self-healing when three faults occur in different positions.

ConclusionIn this article, we examined dc microgrid-based maritime onboard power systems and outlined the need for and potential benefit of employing both smart grid technolo-gies and the MVdc IPS for the future AES to enhance the controllability and efficiency of shipboard power systems. We introduced a series of technical outcomes from

Figure 11. The process of sectionalization and reconfiguration based on the self-healing method: (a) faults occur, (b) fault 1 clear, (c) fault 2 clear, and (d) fault 3 clear.

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The reconfiguration capability is one of the most promising advantages of the MVdc IPS for the future AES, especially for naval applications.

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research on terrestrial dc microgrids, such as dc power architecture, the application of ESSs, hierarchical control, and different coordination methods. We also presented objective-oriented coordinated management methods and protective functions for future MVdc IPSs, which are to meet the specific need of maritime applications using methodologies from dc microgrids.

In the last decade, there were several prototypes of ships on the low-voltage dc level, while, for the MVdc IPS, there are still technological challenges and de-risk-ing studies to be performed. However, it is foreseeable that the advanced technologies from terrestrial dc microgrids are potentially applicable in the MVdc IPS of the future AES. Thus, such a combination will contribute to the implementation of high-performance MVdc IPSs for both commercial and mission-oriented vessels in the near future.

For Further ReadingJ. M. Guerrero, J. C. Vasquez, J. Matas, L. G. de Vicuña, and M. Castilla, “Hierarchical control of droop-controlled ac and dc microgrids—A general approach toward standardization,” IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 158–172, Jan. 2011.

G. Sulligoi, A. Tessarolo, V. Benucci, A. M. Trapani, M. Baret, and F. Luise, “Shipboard power generation: Design and devel-opment of a medium-voltage dc generation system,” IEEE Industry Applicat. Mag., vol. 19, no. 4, pp. 47–55, July–Aug. 2013.

M. Cupelli, F. Ponci, G. Sulligoi, A. Vicenzutti, C. S. Edrington, T. El-mezyani, and A. Monti, “Power flow control and network stability in an all-electric ship” Proc. IEEE, vol. 103, no. 12, pp. 2355–2380, Dec. 2015.

R. M. Cuzner, D. A. Esmaili, “Fault tolerant shipboard MVDC architectures,” in Proc. Int. Conf. Elect. Syst. Aircraft, Railway, Ship Propulsion, Road Vehicles (ESARS), Aachen, Germany, 2015, pp. 1–6.

B. Zahedi and L. E. Norum, “Modeling and simulation of all-electric ships with low-voltage dc hybrid power systems,” IEEE Trans. Power Electr., vol. 28, no. 10, pp. 4525–4537, Oct. 2013.

N. Remijn and B. Krijgsman, “Advantages of common dc busses on ships,” in Proc. IEEE 2010 Third Inter. Symp., Galati, Romania, pp. 177–182.

V. Staudt, R. Bartelt, and C. Heising, “Short-circuit protec-tion issues in dc ship grids,” in Proc. IEEE 2013 Electric Ship Tech-nologies Symp. (ESTS), Arlington, VA, 2013, pp. 475–479.

J. F. Hansen, J. O. Lindtjørn, and K. Vanska, “Onboard dc grid for enhanced DP operation in ships,” presented at Proc.Dynamic Positioning Conf., Houston, TX, 2011.

T. J. McCoy, “Integrated power systems—An outline of requirements and functionalities for ships,” Proc. IEEE, vol. 103, no. 12, pp. 2276–2284, Dec. 2015.

BiographiesZheming Jin ([email protected]) received his B.S. and M.S. degrees from Beijing Jiaotong University, China, in 2013 and 2015, respectively. He is currently working toward his Ph.D. degree in the Department of Energy Technology, Aal-borg University, Denmark. His current research interests include dc distribution systems, dc microgrid-based power systems for mobile applications, and power and energy management of mobile microgrids.

Giorgio Sulligoi ([email protected]) received his M.S. degree (with honors) in electrical engineering from the University of Trieste, Italy, in 2001 and his Ph.D. degree in electrical engineering from the University of Padua, Italy, in 2005. In 2005, he joined MAI Control Systems, Milan. He first became an assistant professor at the University of Trieste in 2007, where he is now currently an associate professor as well as the founder and director of the grid-connected and marine electric power generation and control lab-oratory in the Department of Engineering and Archi-tecture. His research interests include shipboard power systems, all-electric ships, generator modeling, and voltage control.

Rob Cuzner ([email protected]) is an assistant pro-fessor in the Department of Electrical Engineering, University of Wisconsin, Milwaukee. He was previously a staff systems engineer at DRS Power and Control Technologies, where he worked for 20 years on the development of power conversion equipment for Navy shipboard integrated power systems, electric propul-sion, and shipboard-compatible variable-speed drives.

Lexuan Meng ([email protected]) received his B.S. and M.S. degrees in electrical engineering from the Nanjing University of Aeronautics and Astronautics, China, in 2009 and 2012, respectively. He is currently working as a postdoc in power electronic systems at the Department of Energy Technology, Aalborg University, Denmark, as a member of the Microgrid Research Program. His research interests include energy management systems, secondary and tertiary control for microgrids concerning power-quality regulation, and optimization issues, as well as the applications of distributed control and com-munication algorithms.

Juan C. Vasquez ([email protected]) received his B.S. degree in electronics engineering from the Autonomous University of Manizales, Colombia, in 2004. In 2009, he received his Ph.D. degree in automatic control, robotics, and computer vision from the Technical University of Cat-alonia, Barcelona, Spain. In 2011, he joined the Depart-ment of Energy Technology, Aalborg University, Denmark, where he works as an associate professor and is coleader of the Microgrid Research Program.

Josep M. Guerrero ([email protected]) received his B.S. degree in telecommunications engineering, his M.S. degree in electronics engineering, and his Ph.D. degree in power electronics from the Technical University of Catal-onia, Barcelona, Spain, in 1997, 2000, and 2003, respective-ly. Since 2011, he has been a full professor with the Department of Energy Technology, Aalborg University, Denmark, where he is responsible for the Microgrid Research Program. In 2014 and 2015, he was recognized by Thomson Reuters as a highly cited researcher. He is a Fellow of the IEEE.

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2325-5987/16©2016IEEEIEEE Electr i f icat ion Magazine / JUNE 20165858

INCE THE GREAT DEBATE BETWEEN Thomas Edison and Nikola Tesla, our nation’s power system has operated on alternating current (ac). This was chosen over direct current (dc) because of the need

to increase voltage with ac transformers to a high value using transformers for long-distance power transmission. The system has served its purpose well, but now, many energy sources, such as solar panels, fuel cells, and batter-ies, supply dc voltage. Also, dc/dc power converters are commonly used to transform voltage and to interface these dc sources with a larger system. Because of this, local dc power systems (or microgrids) have become popular topics in research literature. It also turns out that interfac-ing a wind power generator to a dc system is simpler than interfacing it to an ac system because ac/dc conversion is needed for the former and ac/dc/ac conversion is needed for the latter. Although energy sources and power conver-sion are readily available for dc power systems, some high-performance applications require fast-acting dc circuit breakers, which are currently in the experimental phase. This article discusses options for high-performance dc cir-cuit breakers and specifically details the coupled-inductor dc breaker. This breaker is demonstrated for fault protec-tion in a notional dc microgrid.

DC Circuit Breaker TechnologiesMany dc microgrid systems require rapid reconfiguration for survivability. This has led to research into advanced dc circuit breakers. One popular choice is the hybrid dc breaker, which uses a mechanical switch in parallel with a path containing semicon-ductor devices. When the mechanical switch is opened, the current is diverted to the semi-conductor, which is then opened. The current is ultimately diverted to a metal–oxide varistor, which clamps the voltage and allows system inductance to reduce the current. A main advantage of this type of breaker is its low on-state power losses. Another type of dc cir-cuit breaker is the fully solid-state version. There are many types of solid-state breakers. Some use a resonant circuit to cause the semiconductor cur-rent to go to zero, and others divert the current to a free-wheeling diode at the breaker output. The main advantage of solid-state breakers is their extremely rapid operation. The following section describes a coupled-inductor dc breaker, which is a variation of a solid-state breaker that includes automatic fault detection.

The Coupled-Inductor DC BreakerThe coupled-inductor dc circuit breaker is shown in Figure 1. The main conduction path consists of


By Atif Maqsood and Keith Corzine

DC Microgrid ProtectionUsing the coupled-inductor solid-state circuit breaker.

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a silicon-controlled rectifier (SCR) (S1) and coupled inductor. The principle of operation can be described as follows. At start-up, S1 is gated on and the capacitor (C)

is charged from the source through the coupled inductor with the charging resistor (R). At this time, the source supplies the load through the coupled

inductor and S1. Once the steady state is reached, the gate signal is removed from S1. A fault at the load causes the capacitor to discharge through diode (D2)and the secondary side of the coupled inductor. This transient discharge current naturally reflects back to the source by the turns ratio N2/N1. That is, the working aspect of the coupled inductor is such that when the breaker output current (io) rises due to a fault, the source current (is) decreases. These laboratory measure-ments are shown in Figure 2. In this setup, the initial source and load current are 6A when a short-circuit is applied at the load side. As can be seen, the breaker out-put current can go up to nearly 100A. The source cur-

rent rapidly responds by going to zero, and the S1 switches off. The capacitance then resonates with the coupled inductance, but this is stopped by the diode (D1). The

coupled-inductor dc breaker has an auto-matic and rapid response to a fault. Addi-

tionally, the source does not experience the fault current. The amount of transient current that will switch off the breaker can be set using the turns ratio of the coupled inductor. Another mode of operation for the coupled-inductor breaker is as a dc switch. Any time the source is supplying the load, the breaker can be pur-posefully switched off by gating S2. This causes the capacitor to discharge through the secondary of the coupled-inductor and S2. As with the fault, this dis-charge current causes S1 to switch off.

Bidirectional Coupled-Inductor DC BreakerFigure 3 shows a version of the coupled-inductor dc breaker that is capable of bidirectional power flow. It contains a center-tapped transformer as well as

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bidirectional SCRs. SCRs are also added in the negative rail, which was seen as necessary in the microgrid system to prevent circu-latory currents. To use the breaker as a dc switch, current flow direc-tion should be known. If I1, as shown in Figure 3, is positive, then S1 should be gated. This will dis-charge the capacitor on the sec-ondary winding of the transformer such that the primary current will force I1 to zero, thereby switching off the SCRs in the primary conduction path. If I2 is positive, S2 must be gated to use the breaker as a dc switch.

Notional DC MicrogridFigure 4 shows the dc microgrid that will be studied herein. The generators (G) are standard dc power sup-plies with droop control for power sharing. Coupled-inductor breakers are placed to protect the sources, lines, and loads. Two bidirectional coupled breakers (breakers E and F) are placed on one line that can con-duct power in both directions. The line impedance is modeled by small a inductance and resistance. Node 3 represents the bus, with both the sources feeding into it. Nodes 6, 8, and 10 are terminal nodes leading to

loads. An inverter load is connected to node 6, while nodes 8 and 10 feed dc/dc converter loads. In this notional dc microgrid, the dc/dc converters are of the boost topology.

Control OptionsThe dc breaker proposed in this arti-cle is autonomous in that it does not need to detect the fault to switch off. The fault response is completely automatic in the absence of gate sig-

nals for the SCRs; however, in the dc microgrid configura-tion shown in Figure 4, there may be certain cases where external control will be required, such as:

for maintenance or repair, it might be required to divert power from one branch to another (this is an example of where the breakers will be used as a dc switch)for loads with a high starting current, where some control will be required to gate the SCRs until the sys-tem reaches steady statefor reconfiguring the system such that the direction of current through a bidirectional breaker has to be changedfor integrating a branch or load back into the system after a fault has been removed.

In a previous work with z-source dc breakers, the pro-posed control was a central control processor constantly

Figure 4. The dc microgrid test system.

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As can be seen, central control provides the desired result for all of the fault locations, without exception.

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in communication with all of the breakers. In this article, two other control approaches are discussed: local breaker control and paired breaker control. The merits of all three control strategies are presented here and some simulation results are shown in the following section.

Central ControlWith central control, all the breakers receive gate signals from a central processor through a communication protocol. At the same time, all the breakers send a signal corresponding to their input and output current to the central control. The input current is compared to a small threshold to indicate whether the breaker is switched on or off. The output current is compared to a large threshold value that indicates whether the breaker experienced a fault at its output. Using this information from all break-ers, the central control determines the fault location in the grid and ensures that only the minimum required number of breakers are switched off to isolate the fault. The central processor also provides gating signals for start conditions and use of the breakers as dc switches.

Central control has been simulated in previous work and shown to perform well. The advantage of central control is that it can locate and isolate a fault at any location in the grid. Its processing requirement is not excessive, but the dis-advantage of central control is the required communication infrastructure. For example, if a universal asynchronous receiver/transmitter is used as a communication protocol, each breaker must decode an additional device and encode the data into the proper format.

Local Breaker ControlIn ac systems, differential protection is one of the most common schemes employed for bus protection. For this scheme, the output current is compared to the input current. Under normal conditions, the input and output currents are similar, but when a fault occurs, either the input or output current changes rapidly, and the relay uses that as an indication of a fault, signaling the break-er to open.

Each of the breakers can be monitored using the same principle. The one important difference is that the dc breaker does not require a relay to switch off; however, under certain conditions, it may need to switch back on when there is no fault. For example, suppose the inverter load in Figure 4 is much greater than the dc/dc converter loads combined. In that case, the steady-state current flow through the bidirectional breakers would be from nodes 9 to 5. Now, imagine that a fault at node 6 turns off the breaker at node 6 to isolate the fault. This would also isolate the inverter load from the system. Once the

inverter is isolated, the steady-state current through the bidirectional breakers should reverse. Initially, the SCRs in bidirectional breakers are not gated, so the breakers will switch off as the current falls to zero. It is then the responsibility of the local control to identify that the breakers turned off due to a current reversal and not due

to a fault. This is one example of a case where the control must gate the SCRs to switch the breakers back on.

In this case, the local control would be monitoring the output and input currents of the breakers. If the control observes that the input and output current fell to zero without exceeding a threshold set for fault currents, that would indicate that there is no shunt fault at the break-er output, and the breaker must be switched back on. If this control

strategy is employed and the fault occurs at location 6 as described in the aforementioned example, the bidi-rectional breakers will be sent gate signals and will switch back on, allowing them to conduct current in the reverse direction. The advantage of this method is that it does not require an elaborate communication infrastructure. It would be much easier to expand the system if each dc breaker had its own independent local control.

There are some disadvantages associated with this control strategy. If the fault is located behind the break-er, the control would misconstrue that it is a case similar to the example above and would attempt to switch on the device. For example, consider a fault at the output of one of the generators (G). The breaker at that terminal will not see a high current at its output, so the control would gate the SCRs of the breaker. The SCRs will still isolate a shunt-resistive fault because it blocks negative current, but this is risky; if the fault impedance has high inductive element, the fault current might resonate and still interfere with the system. Another disadvantage of this control scheme is that it will not isolate all fault locations. If the fault is at the input end of the breaker, the local control embedded in the breaker will not be able to distinguish it from the case where the breaker must be switched back on. All of these scenarios are summarized in Table 1.

Paired Breaker ControlIt is possible for some breakers to operate with the same gate signals. For example, consider the two breakers on line 4. Either both the breakers conduct the same current or neither of them conducts a cur-rent; it is not possible for one of them to be conducting and the other to be open because they share the same path. Therefore, it is possible for these two breakers to share the same gate signal. The breakers that will be

It would be much easier to expand the system if each dc breaker had its own independent local control.

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paired are listed in Table 1. There will be some commu-nication between the paired breakers. Gate signals will be calculated locally at one of the breakers using the current informa-tion from both breakers. One of the breakers will transmit its current data continuously to the other break-er while receiving the gate signals.

If a fault happens between the two paired breakers, both the break-ers will remain switched off, even though the fault is at the input end for one of them. This is the advan-tage of this control compared to local breaker control. More fault locations can be isolated, and the conditions where breakers need to be switched on can be identified. The simulation results for all locations are

summarized in the results section. One limitation with this control is that the fault at junctions 5 and 9 will be

misconstrued as an example of a case where a breaker goes off with-out a fault. This is because at any given time, either junction 5 or 9 will be at the input end of both the bidirectional breakers. However, this limitation can be tolerated if the breakers at junctions 5 and 9 are located close in space, making the probability of a fault happening at those junctions very low. The communication infrastructure required for this control is not as elaborate as for central control,

which makes it easier to implement. It is a compromise between central control and local control in which abso-lutely no communication is required.

Simulation ResultsThe notional dc microgrid was simulated using the soft-ware PSCAD. For simplicity, the generator, inverter, and converters are modeled using average-value models. Table 2 provides a good summary of the results from the three control strategies discussed in this article. As can be seen, central control provides the desired result for all the fault locations, without exception. The only rea-son other control strategies are considered is to simplify the communication architecture.

For location 7, the local control turns breaker E back on. This is because when the control deduces that there is no fault at the output of breaker E, it will gate the SCRs. Unlike other breakers, the SCRs in E allow current

The only reason other control strategies are considered is to simplify the communicationarchitecture.

Paired Devices Description

Source 1 and breaker A

Breaker A receives output current data from source 1.

Source 2 and breaker B

Breaker B receives output current data from source 2.

Breakers C and D Breaker C transmits current data and receives gate signals.

Breakers J and K Breaker J transmits current data and receives gate signals.

Breakers E and F Breaker E transmits current data and receives gate signals.

TABLE 1. The description of paired devices for control algorithm.

Central Control Local Control Paired Control Ideal Case

Fault Location

Breakers Start Conducting Again

BreakersAre Gated Again


Breakers Are Gated Again


Breakers Should Start Conducting Again

1 None A None None None None

2 None B None None None None

3 None All except A , B None All except A , B None None

4 None D None None None None

5 None C , F , G None G None None

6 C , D , E , F C , D , E , F C , D , E , F C , D , E , F C , D , E , F C , D , E , F

7 None E E None None None

8 None None None None None None

9 None E , F , H , I , J E , F E , F , H , I E , F None

10 None None None None None None

11 E , F , H , I E , F , H , I , K E , F , H , I E , F , H , I E , F , H , I E , F , H , I

TABLE 2. The summary of fault response for all control schemes.

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to flow back into the fault, and the breaker starts con-ducting. One way to counter this problem is to have a system where only breakers E and F have paired control, while other breakers have local control. Howev-er, there are other disadvantages of local control. For example, every time the fault is at the input end of the breaker, the breaker is signaled to turn on the SCRs. This does not necessarily result in the breaker conducting because the SCRs do not allow a path for negative current to flow through, even if they are gated. However, there is a risk—if the fault happens to have resonance and the SCR remains gated, then unwanted current can be injected back into the system, which could cause some other breakers to turn off. Table 2shows results of a simple resistive fault, but it is possi-ble for the fault to resonate current back into the sys-tem if SCRs of adjacent breakers are gated.

As discussed in the section “Paired Breaker Control,” if the faults at junctions 5 and 9 are ignored, which is a reasonable assumption, the paired breaker control scheme provides the ideal response, which is similar to central control without the complex communication architecture. For a fault at location 3, all sources are dis-connected from the system, so it does not matter that paired control sends gate signals to the rest of the breakers. Central control still holds an advantage in terms of coordinating starting gate signals and using the breakers as dc switches to reconfigure the system.

Even with paired control, some variation of centralized control can exist for these operations, but the breakers

will no longer require continuous current data to be sent or continu-ous gate signals to be received.

It would be useful to observe the current through these dc breakers for some of the cases from Table 2.The paired control scheme is employed in the simulation and, for the first case, a fault is created at location 4. Figure 5 shows the input current flowing through breakers C, G, and J, and the output current of breaker C. The first two plots in Fig-ure 5 show the input and output currents for breaker C. Only the out-put current of the breaker C shows the fault current, but the input cur-rent goes to zero instantly. Breaker

G’s current is shown in the third plot. There is some disturbance at the time of the fault, but the steady-state current through breaker G is the same before and after the fault as it is feeding the load. The fourth plot shows the output current through breaker J that rough-ly doubles because branch 11 has to provide current to all the loads now that branch 4 has been isolated from the system.

The next case is for a fault at location 6. Figure 6shows the output current for breakers E, G, and K are shown, as well as the input current for breaker G. The first two plots in Figure 6 show the input and output cur-rents through breaker G, respectively. The fault is at the output of breaker G, so only the output current shows the

Figure 5. The current response to fault at location 4.

10

5

0

600

300

0

20

10

0

20

10

0

i s,G

(A

)i o

,C (

A)

i s,C

(A

)i s

,J (

A)

100 ms

Figure 6. The current response to fault at location 6.

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5

0

400

200

0

2

0

–2

6

4

2

i o,E

(A

)i o

,G (

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i s,G

(A

)i o

,K (

A)

200 ms

After 0.4 s, the breaker turns back on and the output current is now negative, demonstr-ating that the direction of current through breaker E has changed.

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spike from the fault current, but the input current falls instantly to zero. The third plot is the output current for breaker E. The output current does not show any sign of the fault current, so the control sends gate signals to the SCRs to switch the breaker back on. After 0.4 s, the break-er turns back on and the output current is now negative, demonstrating that the direction of current through breaker E has changed. The fourth plot shows the current through breaker K in three distinct regions. After the fault, the current through breaker K drops because the inverter load is removed from the system. The current decreases further after 0.4 s, when lines 4 and 7 are inte-grated back into the system.

The final case is the operation of breaker E as a dc switch to isolate line 7. Input and output currents through breaker E are shown in Figure 7, as well as the output cur-rent through breakers D and K.

The results from Figure 7 show the merit of using a breaker as dc switch. The second and third plots show the input and output currents through breaker E. Both these currents fall instantly to zero without showing any signs of fault current. Once line 7 is isolated, lines 4 and 11 smoothly change their currents to accommodate their new loads, as shown by the first and fourth plots in Figure 7.

ConclusionsThe large-scale implementation of dc microgrids depends on developing reliable dc protection options.

With that in mind, we introduce a coupled-inductor-based dc breaker. The proposed dc-breaker design uses an SCR to instantly interrupt the fault current. A bidirec-tional version of the breaker is also provided so multiple dc breakers can be installed in a notional dc microgrid. The breaker also features a control switch within the design so the breaker can be manually opened without creating a large disturbance in the system. Previously, a central control was proposed to supervise operations of multiple breakers in a microgrid. In this article, two other control strategies are discussed: one involving independent, local control of breakers, and the other involving controlling breakers in pairs. A simulation is carried out with all three control methods and a sum-mary of result is presented. The merits of all three strat-egies are discussed. It can be concluded that, within certain limitations, the paired control method can pro-vide the desired results with a much simpler communi-cation architecture.

For Further ReadingW. Setthapun, S. Srikaew, J. Rakwichian, N. Tantranont, W. Rak-wichian, and R. Singh, “The integration and transition to a dc based community: A case study of the smart community in Chiang Mai world green city,” in Proc. IEEE Int. Conf. DC Microgrids, Atlanta, GA, 2015, pp. 205–209.

D. Ricchiuto, R. A. Mastromauro, M. Liserre, I. Trintis, and S. Munk-Nielsen, “Overview of multi-dc-bus solutions for dc microgrids,” in Proc. IEEE Int. Symp. Power Electronics Distrib. Generation Syst., Rogers, AR, 2013, pp. 1–8.

A. Shukla and G. D. Demetriades, “A survey on hybrid circuit-breaker topologies” IEEE Trans. Power Delivery, vol. 30, no. 2, pp. 627–641, Apr. 2015.

R. Schmerda, R. Cuzner, R. Clark, D. Nowak, and S. Bunzel, “Shipboard solid-state protection: overview and applications,” IEEE Electrification Mag., vol. 1, no. 1, pp. 32–39, Sept. 2013.

BiographiesAtif Maqsood ([email protected]) is currently pursuing his Ph.D. degree at Clemson University, South Carolina. His research interests include power electron-ics, motor drives, power system protection, and electric system modeling.

Keith Corzine ([email protected]) is a professor at Clemson University, South Carolina. He has 20 years of experience working with power electronics, motor drives, naval ship propulsion systems, and electric machinery. He has published more than 50 refereed journal papers, more than 80 refereed international con-ference papers, and holds three U.S. patents related to power conversion.

Figure 7. The response to the dc switch at E turning off.

10

8

6

2

1

02

1

0

6

4

2

i o,E

(A

)i s

,E (

A)

i o,D

(A

)i o

,K (

A)

100 ms100 ms

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D A T E S A H E A D

2016

14–16 JUNEPCIC Europe: Petroleum and Chemical Industry Conference Europe, Berlin, Germany, http://www.pcic-europe.com

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However, the overload capability of a converter-based microgrid is limited by the thermal–electrical capacity of the tiny semiconductor chips inside the converters. The chips typically cannot sustain a fault current of more than two to three times the nominal current for more than a few tens of microseconds, even with some over-rating provisions. If these power or time limits are exceeded, the pow-er converters shown in Figure 1 would either enter into a self-shut-down mode or fail catastrophically.

There is a gap of 10× in fault cur-rent tolerance and a gap of 10,000×in fault time tolerance between tra-

ditional power dis-tribution networks and converter-based microgrids.

State of the Art in Microgrid ProtectionIn the case of a short circuit fault, a power electronic con-

verter will typi cally shut down to pro-tect itself within the first few tens of microseconds, resulting in the col-lapse of its output voltage. A dc microgrid will experience a system-

wide blackout if the main grid-tied ac–dc converter shuts down. In con-trast, after temporarily losing all of its DERs, an ac microgrid might have a chance to ride through the fault because of its direct connec-tion to the grid and can therefore be overlaid onto traditional ac distri-bution system-protective devices.

Figure 2 shows a three-phase ac–dc converter with its dc bus being inadvertently shorted. The fault current is initially provided through the discharging of the dc link capac-itor. Shortly after the collapse of the dc bus voltage, a more sustaining fault current is provided through the antiparallel diodes of the con-verter. This second fault current in

Technology Leaders (continued from page 72)

A dc microgrid

will experience a

system-wide blackout

if the main grid-tied

ac–dc converter

shuts down.

Whether you’re a young professional or a top executive, being a member of the IEEE Power & Energy Society can help you expand your network and enhance your career. Whether it’s chairing a committee, writing articles for our publications, speaking at or attending one of our many conferences, or presenting as part of our monthly webinar series, PES members get involved.

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T E C H N O L O G Y L E A D E R S


the uncontrollable loop cannot be interrupted by turning off the active insulated-gate bipolar transition (IGBT) switches. In such a case, the consequences of a short circuit fault without circuit protection is a loss of renewable power or a system-wide blackout, as well as a catastrophic destruc-tion of electric equip-ment and the risk of fires. Circuit breakers must be used in both ac and dc microgrids at almost every junc-tion, as shown in Figure 1. However, conventional mechanical molded case circuit breakers, although com-monly used in traditional ac power systems, exhibit a long reaction time in the range of tens of milli-seconds, which, for converter-based

renewable power systems, is far too slow.

The problem of slow protective device response can be overcome with solid-state circuit breakers (SSCBs). The most common ap proach is to use normally off IGBT switches

or integrated gate-commuted thyristors. Under normal condi-tions, these devices act as feeders to loads within the sys-tem or as part of a radially distributed

network of switches. Current sensing circuitry with minimal latency is required so that, when a fault occurs, the SSCBs can be commanded to an off state and drive the current into the fault to zero by diverting it into a dissipative parallel circuit, such as a

varistor, surge arrester, or voltage-clamping circuitry. In addition to current sensing, such systems require fast-acting logic circuitry and microprocessor controls for data communication to coordinate with other parts of the system. Aux-iliary power supplies and other sup-port electronic circuitry are also required. These approaches insert additional conduction losses into the system, but, in some cases, these can be unacceptably high. To mitigate this problem, hybrid approaches have been proposed to bypass all or the bulk of the lossy elements during normal, connected, and nonfaulted conditions with electromechanical contacts. Howev-er, this approach slows the fault commutation process and adds con-siderable complexity.

Recent advances in and the avail-ability of wide-bandgap (WBG) power semiconductor devices, such as silicon carbide (SiC) metal–oxide–semiconductor field-effect transis-tors (MOSFETs) and junction field-effect transistors (JFETs), remedy the loss issue. For example, for a voltage rating of 1,200 V, SiC JFETs and MOSFETs exhibit a typical specific RDSON of 2–4 mX/cm2, which is 100× lower than silicon MOSFETs and 10× lower than silicon IGBTs. The normally on or depletion-mode WBG devices, such as JFETs, are generally undesirable for main-stream power converter designs, but they fit into SSCB applications very well, as is demonstrated.

Ultrafast SSCBsA self-powered, autonomous, ultra-fast SSCB design concept was recently proposed. Figure 3 shows a 400-V/40-A SiC JFET-based SSCB prototype modeled after this concept and its short circuit fault-switching waveforms. It has experi-mentally demonstrated repeated interruption of fault currents up to 180 A at a dc bus voltage of 400 V with a response time of 0.8 ns,

Time Scale: 1 μs/div

VGS Going from0 to –20 V (10 V/div)

SSCB Responded in Under 1 μs

VDS (200 V/div)

Short CircuitCurrent: 100 A /div

(b)(a)

Figure 3. (a) A 400-V/40-A SiC JFET-based SSCB prototype and (b) its short circuit-switching waveforms.

C

T1

T4 T6

T3

ec

eb

eaLi

VD1

VD4

T2

T5

VD3

VD6

VD5

VD2

vCvC

iC

iL

R L

+

−

Figure 2. A three-phase ac–dc converter with its dc bus being inadvertently shorted showing 1) an initial fault current loop through discharging of the dc link capacitor and 2) a sustaining fault current loop through the antiparallel diodes of the converter.

There are many

challenges to providing

fault protection in both

ac and dc microgrids.

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which is roughly ten times faster than other SSCBs reported in the lite rature and 10,000 times faster than mechanical circuit breakers. Figure 4 provides a comparison of time–current characteristics among different circuit-breaker technolo-gies, with the thermal–electrical limit of power electronic converters plotted. It can be seen that the new ultrafast SSCBs are well within the overload capacity–tolerance win-dow of power electronic converters. The ultrafast SSCB can limit the peak fault current to a much lower level and dramatically reduce the magnetic and thermal stress on dis-tribution components (e.g., cables and bus bars), leading to a safer and less expensive overall system. The short duration of fault events results in minimal line-voltage dis-turbances to the other loads on the common distribution bus. The abili-ty of other loads in the microgrid to ride through a fault without losing power provides improved system performance without a major hard-ware investment.

Future TrendsThere are many challenges to pro-viding fault protection in both ac and dc microgrids. In traditional ac distribution systems, protec-tive device coordination during faults is achieved by choosing appropriate circuit-breaker current–time characteristics under clear design guidelines and without intercomponent communication. The protection coordination strate-gy of typical SSCB technologies is based mostly on a cluster of net-worked and centrally controlled SSCBs and relies heavily on inter-component communicat ion , ensuring coordination of the fault detection and clearing among mul-tiple devices. The proposed nor-mally on JFET approach shows promise in achieving coordination in radial systems through simple tuning strategies and without

inter communicati-ons. However, any SSCB approach will require additional complexity and capi-tal when it comes to galvanic isolation of the fault, which is typically accomplished with mechanical contactors that open after the SSCB has driven the cur-rent to zero. This fact and the risks asso ciated with achieving device coordination in radial systems that look like ac distribution lead to the conclusion that alternative breaker-less solutions should be explored.

It may be beneficial to take full advantage of the inherent power-limiting and self-shutdown capabili-ty of the power converters in the microgrid, as shown in Figure 5. This would help to eliminate hardware costs and power losses associated with additional circuit breakers.

In this regard, a one-on-one supply-load arrangement, with each power converter connected to a dedi-cated load, is the pre-ferred architecture, as shown in Figure 6.

A large power source can be designed as a cluster of smaller mod-ular power converters, with each

CentrallyControlled

SSCBs

AutonomousSSCBs

ProtectivePower

Converters

AutonomousHybrid CBs

CircuitProtectionStrategy

Figure 5. The circuit protection strategy that combines a diverse set of circuit-breaker technologies. CBs: circuit breakers.

Ultrafast SSCB Technology

Current SSCB Technology

ElectromechanicalCircuit Breakers

Operation Capacityof Power ElectronicConverters

Thermal–Electrical Limit ofPower ElectronicConverters

103

10–1

10–3

10–5

10–7

10

Tim

e (s

)

0.1 1 10

Current (× Normal Rating)

102

Figure 4. A comparison of time-current characteristics among different circuit-breaker technologies and the thermal-electrical limit of power electronic converters.

The proposed normally

on JFET approach

shows promise in

achieving coordination

in radial systems.

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providing power to one load via a designated power cable (Figure 7). The power bus now changes from one thick power cable to a bundle of insu-lated thinner wires, which may have implications for cable cost and weight, as well as the complexity of connecting these wires. One addi-tional benefit of this scheme is the enhanced system redundancy for a particular load if two or more of the modular power source units are con-nected to the power cable through O-ring diodes.

Of course, it may not always be practical to implement this strategy on every load in the microgrid. The

following summary of approaches should also be considered to im prove the robustness of fault- protective strategies:

autonomous SSCBs inserted between dedicated source power converters and loadsnetworked, centrally controlled smart SSCBs judiciously placed at critical nodes of the system to complement the protective capabilities of power convert-ers and autonomous SSCBshybrid circuit breakers with ultrafast contactors that can carry sufficient current to aid in arc extinction.

Human safety, grounding, and economic factors are all consider-ations that cannot be forgotten when the ultimate solution to protection within dc microgrids is pursued.

For Further ReadingZ. J. Shen, G. Sabui , Z. Miao, and Z. Shuai,

“Wide-bandgap solid-state circuit break-

ers for DC power systems: Device and

circuit consi derations,” IEEE Trans. Elec-

tron Devices, vol. 62, no. 2, pp. 294–300,Jan. 2015.

Z. Miao, G. Sabui, A. Chen, Y. Li, Z. J. Shen, J. Wang, Z. Shuai, A. Luo, X.

Yin, and M. Jiang, “A self-powered

ultra-fast DC solid state circuit break-

er using a normally on SiC JFET,” in Proc. 30th Annu. IEEE Applied Power

Electronics Conf. and Exposition (APEC),

Charlotte, NC, 2015, pp. 767–773.Z. J. Shen, Z. Miao, and A. M.

Roshandeh, “Solid state circuit break-

ers for DC microgrids: Current status

and future trends,” in Proc. IEEE First Int.

Conf. DC Microgrids (ICDCM), Atlanta, GA, 2015, pp. 228–233.

Y. Sato, Y. Tanaka, A. Fukui, M.Yamasaki, and H. Ohashi, “SiC-SIT

circuit breakers with controllable

interruption voltage for 400-V DC dis-

tribution systems,” IEEE Trans. Power

Electron., vol. 29, no. 5, pp. 2597–2605,May 2013.

D. P. Urciuoli , V. Veliadis, H. C.

Ha, and V. Lubomirsky, “Demonstra-

tion of a 600-V, 60-A, bidirectional

silicon carbide solid-state circuit

breaker,” in Proc. 26th Annu. IEEE

Applied Power Electronics Conf. and

Exposition (APEC), Fort Worth, TX,

2011, pp. 354–358.

BiographyZ. John Shen ([email protected]) is the Grainger Chair professor of electrical and power engineer-ing at Illinois Institute of Technol-ogy, Chicago. He is a Fellow of the IEEE.

40-kWConverter

4-kWModule

4-kWModule

4-kWModule

4-kWModule

10×

Load

Load

Load

Load

Load Load Load

100-A /400-V Shared Common Bus

10-A /400-V Dedicated Bus

Figure 6. A one-on-one dedicated converter-load power distribution architecture using the power-limiting and self-protection capability of the power converter to provide circuit fault protection.

4-kWModule

4-kWModule

4-kWModule

4-kWModule

3× 10-A Bus

10-A Bus

30-ALoad

Load

Figure 7. A large power source can be designed as a cluster of smaller modular power converters connected in parallel via O-ring diodes. Each modular power converter supplies power to the common load with a designated power cable.

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Ultrafast Solid-State Circuit Breakers

Protecting converter-based ac and dc microgrids against short circuit faults.

By Z. John Shen

E N E WA B L E P OW E R sources and other distrib-uted energy resources

(DERs), such as photovoltaics, wind, and battery storage, feed electricity to the utility grid and/or local loads through interfacing power electronic converters. The safe and uninterrupt-ed operation of these microgrids against fault con ditions is an impor-tant system requirement. Although alternating current (ac) is still the dominant form of electricity, direct current (dc) has been gaining traction in data centers and office buildings because it offers higher system effi-ciency, lower capital and operating expenses, and easier integration of renewable resources and DERs.

Figure 1 conceptually illustrates ac and dc microgrids with renew-able sources and battery storage feeding both local loads, such as plug-in electric vehicles and resi-dences. The power networks are susceptible to short circuit faults when one of the many load branch-es or the common power bus is inadvertently shorted, resulting in a fault current that is much higher than the nominal current.

A traditional ac power distribution system is well protected from short

circuit faults because its power source—comprising large synchro-nous generators and bulky trans-formers—can sustain a very high fault current (e.g., 20–30× the nomi-nal current) for a time period of sev-eral hundreds of milliseconds. This

allows enough time for a slow-responding mechanical circuit break-er in the faulty branch to activate and isolate the fault, leaving the rest of the power system unaffected.

R

Digital Object Identifier 10.1109/MELE.2016.2544058Date of publication: 31 May 2016 (continued on page 67)

Fault Currents

Fault Currents

OverheatedPowerElectronicConverters

OverheatedPowerElectronicConverters

FaultLoad Branch

FaultLoad Branch

ac Bus

dc Bus

CircuitBreaker

CircuitBreaker

(a)

(b)

NormalLoad Branches

NormalLoad Branches

Figure 1. The roles of circuit breakers in (a) ac and (b) dc microgrids with renewable power sources feeding both local loads and the utility grid.

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www.ieee-ecce.orgConference management | [email protected]

ECCE is the foremost IEEE conference in the field of electrical and electromechanical energy conversion. ECCE2016, to be held in Milwaukee, the heartland of the North America energy conversion industry, will provide researchers, engineers and professionals from industry and academia a convivial and innovative atmosphere for interaction and networking. Some highlights of ECCE2016 include:> Stronger than ever industrial support including Gold or Platinum sponsorship

from GM, Rockwell Automation, ABB, Eaton, Danfoss, and Wolong Electric Group.> Keynote speeches will be delivered by prominent leaders from academia

and industry:”Options to Create a Sustainable Energy Future” by Prof. Arun Majumdar, Stanford University and former Director of ARPA-E“Future of the Smartgrid” by Prof. Massoud Amin, Minnesota University.“Intelligent Motor Control in a Connected Enterprise” by Mr. Blake Moret, Senior Vice President, Control Products & Solutions, Rockwell Automation“Optimized Power Management Using Data Analytics”, by Mr. Michael Regelski, SVP and Chief Technology Officer Electrical Sector, Eaton“Very High Power Electronics for HVDC,” by Dr. Guangfu Tang, Vice President of Smartgrid Institute of China State Grid

> A high quality technical program selected from a record setting 1715 digest submissions (10.7% over the previous record).

> 12 tutorials on interesting and relevant technical topics> Over 60 industrial and university exhibitors to showcase the latest technologies

and products> Special sessions, industrial sessions and town hall forums> Vibrant and enjoyable social activities to make you feel at home> WEMPEC Open House event hosted by University of Wisconsin Madison

immediately after the conferenceStudent travel grants available.

Announcement

Important DatesJune 1, 2016Advanced registration available

July 1, 2016Final manuscript submission

August 6, 2016Regular registration available

Milwaukee, WI, USA | SEPTEMBER 18-22, 2016

2016 Milwaukee, WI

Sept. 18–22

Used

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itMilw

auke

e.org

General ChairZ. John ShenIllinois Institute of [email protected]

Technical Program Co-ChairsRobert S. BalogTexas A&M University at Qatar

Avoki OmekandaGeneral Motors – R&D Center

Maryam SaeedifardGeorgia Institute of [email protected]

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Contents | Zoom in | Zoom out Search Issue | Next PageFor ... · the status quo of how power is distributed, the fact is that the number of applications in which dc makes sense is