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Benefits of HVDC & FACTS for Sustainability and Security of Power Supply
Dietmar Retzmann*, Karl Uecker
Siemens, Germany
ABSTRACT Deregulation and privatization are posing new challenges to high voltage transmission and distributions systems. System components are loaded up to their thermal limits, and power trading with fast varying load patterns is leading to an increasing congestion.
In addition to this, the dramatic global climate developments call for changes in the way electricity is supplied [1, 2].
Innovative solutions with HVDC (High Voltage Direct Current) and FACTS (Flexible AC Transmission Systems) have the potential to cope with the new challenges.
1. INTRODUCTION The vision and enhancement strategy for the future electricity networks is depicted in the program of “SmartGrids”, which was developed within the European Technology Platform (ETP) of the EU in its preparation of the 7th Frame Work Program.
Features of a future “SmartGrid” of this kind can be outlined as follows [1]:
• Flexible: fulfilling customers’ needs whilst responding to the changes and challenges ahead
• Accessible: granting connection access to all network users, particularly to renewable energy sources (RES) and highly efficient local generation with zero or low carbon emissions
• Reliable: assuring and improving security and quality of supply
• Economic: providing best value through innovation, efficient energy management and ‘level playing field’ competition and regulation
It is worthwhile mentioning that the “SmartGrid” vision is in the same way applicable to the system developments in other parts of the world. Smart Grids will help achieve a sustainable development.
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2. TRENDS IN POWER MARKETS On the one hand, a dramatic growth of population is to be seen in developing and emerging countries. On the other hand, the number of population in highly developed countries is stagnating. Despite these differences, the expectancy of life increases everywhere. This increase in population (the number of elderly people in particular) poses great challenges to the worldwide infrastructure.
This development goes hand in hand with a continuous reduction in non-renewable energy resources. The resources of conventional as well as non-conventional oil are gradually coming to an end. Other energy sources are also running short.
So, the challenge is as follows: for the needs of a dramatically growing world population, coupled with the simultaneous reduction in fossil power resources, a proper way must be found to provide reliable and clean power. This must be done in the most economical way, for a lot of economies, in the emerging regions in particular, cannot afford expensive environmentally compatible technologies. Consequently, we have to deal with an area of conflicts between security of supply, environmental sustainability as well as economic efficiency. The combination of these three tasks can be solved with the help of ideas, intelligent solutions as well as innovative technologies, which is the today’s and tomorrow’s challenge for the planning engineers worldwide.
Emerging countries face a dramatic growth of power demand. Enormous power blocks must be transmitted to large industrial regions, partly over long distances, that is, from large hydro power plants upcountry to coastal regions. In the view of the increasing demand for power and for security and sustainability of power supply, high investments are required. Moreover, higher voltage levels are called for, as well as long-distance transmission by means of HVDC and FACTS.
During the transition, the newly industrialized countries require energy automation and life-time extension of the system components, such as transformers and substations. More investments in distribution systems are essential as well. Decentralized power supplies, e.g. wind farms, are coming up.
Fig. 1: CO2 Increase due to Human Influence is much higher than Natural Fluctuation
Sources: Wikipedia, Siemens PTD TI, 2006
Carbon Dioxide Variations in the Air400
350
300
250
200
0 100 200 300 400
Thousandsof Years ago
Würm / Weichsel Riß / SaaleIce Age Mindel / Elster
CO2Concentration (ppmv)
A crucial Global Issue: to achieve CO2 Reduction
A crucial Global Issue: to achieve CO2 Reduction
The Industrial Revolution has caused a dramatic Rise in CO2
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Industrialized countries in their turn have to struggle against transmission bottlenecks, caused, among other factors, by increase in power trading. At the same time, the demand for high security of power supply, high power quality and, last but not least, clean energy increases in these countries. In spite of all the different requirements, one challenge remains the same for all: sustainability of power supply must be provided. Our resources on the Earth are limited, and the global climate is very sensitive to environmental influences. The global industrialization with its ongoing CO2 production is causing dramatic changes in the climate development, ref. to Fig. 1.
There is no ready-made solution to this problem. The situation in different countries and regions is too complex. An appropriate approach is, however, obvious: power generation, transmission, distribution and consumption must be organized efficiently.
The approach of the EU’s “SmartGrid” vision is an important step in the direction of environmental sustainability of power supply, and new transmission technologies can effectively contribute to reduction in losses and CO2 emissions.
In the future of liberalized power markets, the following advantages will become even more important: pooling large power generation stations, sharing spinning reserve, using the most economical energy resources and also taking into account ecological constraints, such as the use of large nuclear and hydro-power stations at suitable locations, solar energy from desert areas and embedding of big offshore wind farms.
3. PROSPECTS OF POWER SYSTEM DEVELOPMENT Due to an increased demand for energy and the construction of new generation plants, first built close and then at remote locations from the load centers, the size and complexity of power systems all over the world have grown.
Power systems have been extended by applying interconnections to the neighboring systems in order to achieve technical and economical advantages. Large systems, covering parts of or even whole continents came into existence, to gain the following advantages: the possibility to use larger and more economical power plants, reduction in reserve capacity within the systems, utilization of the most efficient energy resources, as well as an increase in the system security.
Fig. 2: Trends in High Voltage Transmission Systems
* SCC = Short-Circuit Current** Example UCTE: 400 kV is in fact too low** Example UCTE: 400 kV is in fact too low
PrivatisationGlobalisation/Liberalisation
Deregulation - Privatization: Opening of the Markets, Independent Transmission Companies ITCs, Regional Transmission Organisations RTOs
PrivatisationGlobalisation/Liberalisation
Deregulation - Privatization: Opening of the Markets, Independent Transmission Companies ITCs, Regional Transmission Organisations RTOs
Problem of uncontrolled Loop FlowsOverloading & Excess of allowed SCC* LevelsSystem Instabilities & Outages
Problem of uncontrolled Loop FlowsOverloading & Excess of allowed SCC* LevelsSystem Instabilities & Outages
System Enhancement & Interconnections:Higher Voltage Levels **New Transmission TechnologiesRenewable Energies for CO2 Reductionfor CO2 Reduction
PrivatisationBottlenecks inTransmissionPrivatisationBottlenecks inTransmission
Investments inPower SystemsInvestments inPower Systems
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However, it is a crucial issue that with an increasing size of the interconnected systems, the advantages diminish. There are both technical and economical limitations in the interconnections if the energy has to be transmitted over extremely long distances through the interconnected synchronous AC systems. These limitations are related to problems with low frequency inter-area oscillations [11], voltage quality and load flow. This is, for example, the case in the UCTE system, where the 400 kV voltage level is in fact too low for the large cross-border and inter-area power exchange [13, 17]. Bottlenecks are already spotted, and to increase the power transfer, advanced solutions need to be applied. These problems are even deepened by the deregulation of the electrical power markets, where contractual power flows do not follow the design criteria of the existing network configuration, ref. to Fig. 2.
3.1 Lessons learned from the Blackouts The electric power supply is essential for life of a society, like the blood in the body, ref. to Fig. 3.
Without power supply, there are devastating consequences for daily life: breakdown of public transportation systems, traffic jams, computer outages as well as a standstill in factories, shopping malls, hospitals etc.
Based on the global experience with large blackouts [13], strategies for the development of power systems go clearly in the direction of more system interconnections with enhanced transmission.
Power electronics is to be used to control load flow, to reduce transmission losses and to avoid congestion, loop flows and overloading of transmission lines.
Fig. 4 outlines the tasks and solutions.
Fig. 3: Security of Supply – highly important for the Society
Electrical Energy is the Backbone of our Society …
… today and in Future
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3.2 Integration of Renewable Energy Sources – a Big Challenge
Environmental constraints will play an important role in the system development [14]. Specific problems are expected when renewable energies, such as large wind farms, have to be integrated into the system, particularly when the connecting AC links are weak and when sufficient reserve capacity in the neighboring systems is not available. In the future, an increasing part of the installed capacity will, however, be connected to the distribution levels (dispersed generation), which poses additional challenges to the planning and safe operation of the systems, ref. to Fig. 5. In these cases, power electronics can clearly enhance the power systems and improve their performance.
Renewable Energy Resources at favorableLocations *Renewable Energy Resources at favorableLocations *
Transmission of large Power Blocks over long Distances (Hydro, Wind * and Solar Energy)Transmission of large Power Blocks over long Distances (Hydro, Wind * and Solar Energy)
Increased Power Exchange among the Interconnected SystemsIncreased Power Exchange among the Interconnected Systems
Extensions of Interconnected SystemsExtensions of Interconnected Systems
* A big Issue for the Grid Developments – in all Countries* A big Issue for the Grid Developments – in all Countries
Fig. 4: Power System Development – and Sustainability of Supply
Fig. 5: Regenerative Energy Sources and Dispersed Generation – Impact on the whole T&D Grid Structure
G
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Today: Tomorrow:
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Today: Tomorrow:
Use of Dispersed GenerationUse of Dispersed Generation Load Flow will be “fuzzy”
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Power output of wind generation can vary fast in a wide range [12], depending on the weather conditions. Therefore, a sufficiently large amount of controlling power from the network is required to substitute the positive or negative deviation of actual wind power infeed to the scheduled wind power amount. 3.3 Examples of HVDC and FACTS Solutions for Security and Sustainability of Supply
After the 2003 blackout in the United States, new projects are gradually coming up in order to enhance the system security.
One example is the Neptune HVDC project. Siemens was awarded a contract by Neptune Regional Transmission System LLC (RTS) in Fairfield, Connecticut, to construct an HVDC transmission link between Sayreville, New Jersey and Long Island, New York. Neptune RTS was established to develop and commercially operate power supply projects in the United States. By delivering a complete package of supply, installation, service and operation from one single source, Siemens is providing a seamless coverage of the customer’s needs. The availability of this combined expertise fulfills the prerequisites for financing these kinds of complex supply projects through the free investment market.
Siemens and Neptune RTS were developing the Neptune HVDC project over three years to prepare it for implementation. In addition to providing technological expertise, studies, and engineering services, Siemens also supported its customer in the project’s approval process.
In Fig. 6, highlights of this innovative project are depicted.
Fig. 6: Highlights of Neptune HVDC Project – USA
Customer:
End User:
Location:
Project
Development:
Supplier:
Transmission:
Power Rating:
Transmission Dist.:
Neptune RTS
Long Island Power Authority
(LIPA)
New Jersey: Sayreville
Long Island: Duffy Avenue
NTP-Date: 07/2005
PAC: 07/2007
Consortium
Siemens / Prysmian
Sea Cable – 500 kV
600/660 MW monopolar
82 km DC Sea Cable
23 km Land Cable
Customer:
End User:
Location:
Project
Development:
Supplier:
Transmission:
Power Rating:
Transmission Dist.:
Neptune RTS
Long Island Power Authority
(LIPA)
New Jersey: Sayreville
Long Island: Duffy Avenue
NTP-Date: 07/2005
PAC: 07/2007
Consortium
Siemens / Prysmian
Sea Cable – 500 kV
600/660 MW monopolar
82 km DC Sea Cable
23 km Land Cable
Ed Stern, President of Neptune RTS: “High Voltage Direct Current Transmission will play an increasingly important Role, especially as itbecomes necessary to tap Energy Reserves whose Sources are far away from the Point of Consumption”
Ed Stern, President of Neptune RTS: “High Voltage Direct Current Transmission will play an increasingly important Role, especially as itbecomes necessary to tap Energy Reserves whose Sources are far away from the Point of Consumption”
Atlantic Ocean
Safe and reliable Power Supply for the Megacities –“Blackout Prevention”
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As new overhead lines can not be built in this densely populated area, power should be brought directly to Long Island by HVDC cable transmission, by-passing the AC sub-transmission network. For various reasons, environmental protection in particular, it was decided not to build a new power plant on Long Island near the city in order to cover the power demand of Long Island with its districts Queens and Brooklyn which is particularly high in summer. The Neptune HVDC interconnection is an environmentally compatible, cost-effective solution which will help meet these future needs. The low-loss power transmission provides access to various energy resources, including renewables. The interconnection is carried out via a combination of submarine and subterranean cable directly to the network of Nassau County which borders on the city area of New York.
During trial operation, 2 weeks ahead of schedule, Neptune HVDC proved its Blackout prevention capability in a very impressive way. On June 27, 2007, a Blackout occurred in New York City. Over 380,000 people were without electricity in Manhattan and Bronx for up to one hour, subway came to standstill and traffic lights were out of operation.
In this situation, Neptune HVDC successfully supported the power supply of Long Island and due to this, 700,000 households could be saved there.
In June 2007, Siemens received the order from China Southern Power Grid Company, Guangzhou, to construct a high-voltage DC transmission (HVDC) system between the province of Yunnan in the southwest of China and the province of Guangdong on the south coast of the country together with the Chinese partners. The system will be the first in the world to transmit electricity at a DC voltage of +/- 800 kV. At the same time, this project with a power transmission capacity of 5000 MW will be the long distance HVDC link with the world’s highest power capacity which has ever been achieved. The additional electric power from Yunnan is intended to supply the rapidly growing industrial region of the Pearl River delta in the province of Guangdong and the megacities of Guangzhou and Shenzhen. In the future, the electricity generated by several hydro-electric power plants will be transported from Yunnan via 1,400 km to Guangzhou over the long distance HVDC link. This HVDC link will save the CO2 emissions of more than 30 million tons a year. This corresponds to the amount of harmful gases which would be produced otherwise, for example through the construction of additional conventional fossil power plants in the province of Guangdong to serve the regional grid. Fig. 7 gives an overview of this world’s biggest HVDC project.
Yunnan-GuangdongYunnan-Guangdong
5,000 MW1,418 Km
+/- 800 kV DC
Commercial Operation:2009 – Pole 12010 – Pole 2
Commercial Operation:2009 – Pole 12010 – Pole 2
Reduction in CO2 versus local Power Supply with Energy-Mix
32.9 m tons p.a. - by using Hydro Energy and HVDC for Transmission
Fig. 7: World’s first 800 kV HVDC – in China Southern Power Grid
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In South-Africa, India and Brazil there is also a huge demand for further system interconnections, both within the national grid and to the neighboring countries. The reasons are as follows: strong increase in regenerative energy sources, as well as creating new import and export capabilities to the neighbors to meet the booming energy demand in the regions. Bulk Power UHV DC transmission will be the preferred solution to keep the transmission losses low.
A particular advantage of the HVDC transmission technology when compared to constructing a new power plant or building a new conventional three-phase AC transmission line is the fact that the short-circuit power of the network does not rise; that is, no complex measures to upgrade the existing short-circuit capacity of the grid are required.
In addition, fast acting control functions enable HVDC systems to help stabilize the connected networks, which constitutes a decisive benefit of this technology in the event of outages and blackouts in the system. A further significant benefit of the HVDC (in comparison with FACTS) is its incorporated ability of fault-current blocking, which serves as an automatic firewall for Blackout prevention in case of cascading events which is not possible with synchronous AC links.
In Fig. 8, an example for an innovative FACTS application in Germany with SVC in combination with an HVDC is shown. This project is in fact the first high voltage FACTS controller application in the German network, see the photo Fig. 9. The reason for the SVC installation at Siems substation nearby the landing point of the Baltic Cable HVDC were unforeseen right of way restrictions in the neighboring area, where an initially planned new tie-line to the strong 400 kV network for connection of the HVDC was denied. Therefore, with the existing reduced network voltage of 110 kV, only a limited power transfer (350 MW) of the DC link was possible since its commissioning in 1994 in order to avoid repetitive HVDC commutation failures and voltage problems in the grid. In an initial first step for grid access improvement, an additional transformer to connect the 400 kV HVDC AC bus with the 110 kV bus was installed. Finally, in 2004, with the new SVC equipped with a fast coordinated control, the HVDC could fully increase its transmission capacity up to the design rating of 600 MW. In addition to this measure, a new cable to the 220 kV grid was installed to increase the system strength with regard to performance improvement of the HVDC control.
Application of SVC –
Return on Investmentswithin a few Months only!
more Hydro Power fromNORDEL to Germany
HVDC: Power Increase – from 450 MW to 600 MWHVDC: Power Increase – from 450 MW to 600 MW
SVC for Grid Reinforcement and full Power Operation of the Baltic Cable HVDC Link
SVC for Grid Reinforcement and full Power Operation of the Baltic Cable HVDC Link
HVDC and FACTS in parallel OperationHVDC and FACTS in parallel Operation
Fig. 8: SVC Siems, Germany - Support of HVDC Baltic Cable
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4. SMARTGRID SOLUTIONS WITH POWER ELECTRONICS
4.1 HVDC and FACTS Converter Technologies HVDC systems and FACTS controllers based on line-commutated converter technology have a long and successful history. Thyristors were the key components of this converter topology and have reached a high degree of maturity due to their robust technology and their high reliability.
In the second half of the last century, high power HVDC transmission technology was introduced, offering new dimensions for long distance transmission.
This development started with the transmission of power in a range of a few hundred MW and was continuously increased. Transmission distances over 1,000 to 2,000 km or even more are possible with overhead lines. Transmission rating of 3 GW over large distances with only one bipolar DC line is state-of-the-art in many grids today. World’s first 800 kV DC project in China has a transmission capacity of 5 GW (ref. to section 3.3) and further projects with 6 GW or even higher are at the planning stage. As a multiterminal system, HVDC can also be connected at several points with the surrounding AC networks [3, 17]. In general, for transmission distances above 700 km, DC transmission is more economical than AC transmission (≥ 1000 MW). Power transmission of up to 600 - 800 MW over distances of about 300 km has already been achieved with submarine cables, and cable transmission lengths of up to about 1,000 km are at the planning stage. Due to these developments, HVDC became a mature and reliable technology [18, 19]. FACTS and HVDC use power electronic components and conventional equipment which can be combined in different configurations to switch or control reactive power and to convert the active power. Conventional equipment (e.g. breakers, tap-changer transformers) has very low losses, the switching speed is, however, relatively low. Power electronics can provide high switching frequencies of up to several kHz, however, with an increase in losses.
Fig. 10 indicates the typical losses depending on the switching frequency and Fig. 11 gives an overview of today’s power electronic solutions with HVDC in high voltage transmission systems.
Fig. 9: The Solution – the first HV SVC in the German Grid at Siems Substation
Problem – no Right-of-Way for 400 kV ACGrid Access of Baltic Cable HVDCProblem – no Right-of-Way for 400 kV ACGrid Access of Baltic Cable HVDC
The SolutionThe Solution
2004
SVC - Essential forenhanced Grid Access of the HVDCSVC - Essential forenhanced Grid Access of the HVDC
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From Fig. 10, it can be seen that due to the low losses, line-commuted Thyristor technology is the preferred solution for bulk power transmission, today and in the future.
More Dynamics for better Power Quality:Use of Power Electronic Circuits for Controlling P, V & QParallel and/or Series Connection of ConvertersFast AC/DC and DC/AC Conversion
ThyristorThyristor
50/60 Hz
ThyristorThyristor
50/60 Hz
GTOGTO
< 500 Hz
GTOGTO
< 500 Hz
IGBT / IGCT
Losses
> 1000 Hz
IGBT / IGCT
LossesLosses
> 1000 Hz
Transition from “slow” to “fast”
Switching Frequency
On-Off Transition 20 - 80 ms
Transition from “slow” to “fast”Transition from “slow” to “fast”
Switching Frequency
On-Off Transition 20 - 80 ms
1-2 %1-2 %
The Solution for Bulk Power Transmission The Solution for Bulk Power Transmission
Depending on SolutionDepending on Solution
2-4 %2-4 %
Fig. 10: Power Electronics for HVDC & FACTS – Transient Performance and Losses
Fig. 11: Transmission Solutions with HVDC
HVDC - High Voltage DC Transmission: It makes P flow
HVDC-LDT - Long Distance Transmission
HVDC “Classic” with LT Thyristors* (Line-commutated Converter)
HVDC “Bulk” with 800 kV – for 5,000 MW to > 7,000 MW
HVDC PLUS (Voltage-Sourced Converter – VSC)
HVDC can be combined with FACTS
V-Control included
B2B - The Short Link
Back-to-Back Station
AC AC
DC Cable
AC AC
Submarine Cable Transmission
800 kV for minimal LineTransmission Losses
Long Distance OHL Transmission
DC Line
AC AC
* LTT = Light-Triggered Thyristor with integrated Break-over Protection
HVDC - High Voltage DC Transmission: It makes P flow
HVDC-LDT - Long Distance Transmission
HVDC “Classic” with LT Thyristors* (Line-commutated Converter)
HVDC “Bulk” with 800 kV – for 5,000 MW to > 7,000 MW
HVDC PLUS (Voltage-Sourced Converter – VSC)
HVDC can be combined with FACTS
V-Control included
B2B - The Short Link
Back-to-Back Station
AC AC
B2B - The Short Link
Back-to-Back Station
AC AC
Back-to-Back Station
ACAC ACAC
DC Cable
AC AC
Submarine Cable Transmission
DC Cable
ACAC ACAC
Submarine Cable Transmission
800 kV for minimal LineTransmission Losses
Long Distance OHL Transmission
DC Line
AC AC
Long Distance OHL Transmission
DC Line
ACAC ACAC
* LTT = Light-Triggered Thyristor with integrated Break-over Protection
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Fig. 12 shows the basic configurations of FACTS controllers. Since the 60s, FACTS have been being developed to a mature and reliable technology with high power ratings [5, 20, 21].
FACTS, based on power electronics, was developed to improve the performance of weak AC Systems and to make long distance AC transmission feasible [4, 5]. Moreover, FACTS can help to solve technical problems in the interconnected power systems. FACTS are applicable in a parallel connection (SVC, Static VAR Compensator - STATCOM, Static Synchronous Compensator), in a series connection (FSC, Fixed Series Compensation - TCSC/TPSC, Thyristor Controlled/Protected Series Compensation, S³C - Solid-State Series Compensator), or as a combination of both (UPFC, Unified Power Flow Controller, CSC - Convertible Synchronous Compensator [15]) to control load flow and to improve dynamic conditions. GPFC is a special DC back-to-back link which is designed for fast power and voltage control at both terminals [16]. In this manner, GPFC is a “FACTS B2B” which is less complex and less expensive than the UPFC.
Rating of SVCs can go up to 800 MVAr, series FACTS devices are installed on 550 and 735 kV levels to increase the line transmission capacity up to several GW. A large number of different FACTS controllers were put into operation either as commercial projects or prototypes. Recent developments are the TPSC (Thyristor Protected Series Compensation) and the Short-Circuit Current Limiter (SCCL), both innovative solutions which use special high power thyristor technology [16].
The world’s biggest FACTS Project with Series Compensation (TCSC/FSC) is at Purnea and Gorakhpur in India with a total rating of 1.7 GVAr [21].
It is, however, necessary to mention that line-commutated converters have some technical restrictions. Particularly the fact that the commutation within the converter is driven by the AC voltages requires proper conditions of the connected AC system, such as a minimum short-circuit power of the surrounding AC systems.
4.2 Voltage-Sourced Converters Power electronics with self-commutated converters can cope with the limitations mentioned above and provide additional technical features. In DC transmission, an independent control of active and
Fig. 12: Transmission Solutions with FACTS
FACTS - Flexible AC Transmission Systems: Support of Power FlowSVC – Static Var Compensator* (The Standard of Shunt Compensation)SVC PLUS (= STATCOM - Static Synchr. Compensator, with VSC) FSC – Fixed Series Compensation TCSC – Thyristor Controlled Series Compensation*TPSC – Thyristor Protected Series Compensation**GPFC – Grid Power Flow Controller* (FACTS-B2B)UPFC – Unified Power Flow Controller (with VSC)
TCSC/TPSC
FSC
ACAC
/ TPSC
/ STATCOMSVC
ACAC
GPFC/UPFC
AC AC
/ UPFC
* with LT Thyristors** with special High
Power LT Thyristors
and SCCL **for Short-Circuit Current Limitation
FACTS - Flexible AC Transmission Systems: Support of Power FlowSVC – Static Var Compensator* (The Standard of Shunt Compensation)SVC PLUS (= STATCOM - Static Synchr. Compensator, with VSC) FSC – Fixed Series Compensation TCSC – Thyristor Controlled Series Compensation*TPSC – Thyristor Protected Series Compensation**GPFC – Grid Power Flow Controller* (FACTS-B2B)UPFC – Unified Power Flow Controller (with VSC)
TCSC/TPSC
FSC
ACAC ACACACAC
/ TPSC
/ STATCOMSVC
ACAC
SVC
ACAC ACAC ACACACAC
GPFC/UPFC
AC AC
GPFC/UPFC
AC ACACAC ACAC
/ UPFC
* with LT Thyristors* with LT Thyristors** with special High
Power LT Thyristors** with special High
Power LT Thyristors
and SCCL **for Short-Circuit Current Limitation
and SCCL **for Short-Circuit Current Limitation
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reactive power, the capability to supply weak or even passive networks and lower space requirements are some of the advantages. In many applications, the VSC has become a standard of self-commutated converters and will be used increasingly more often in transmission and distribution systems in the future. Voltage-sourced converters do not require any “driving” system voltage - they can build up a 3-phase AC voltage via the DC voltage. This kind of converter uses power semiconductors with turn-off capability such as IGBTs (Insulated Gate Bipolar Transistors). Up to now, the implemented VSC converters for HVDC applications have been based on two or three-level technology which enables switching two or three different voltage levels to the AC terminal of the converter. To make high voltages in HVDC transmission applications controllable by semiconductors with a blocking ability of a few kilovolts, multiple semiconductors are connected in series – up to several hundred per converter leg, depending on the DC voltage. To ensure uniform voltage distribution not only statically but also dynamically all devices connected in series in one converter leg have to switch simultaneously with the accuracy in the microsecond range. As a result, high and steep voltage steps are applied at the AC converter terminals which require extensive filtering measures. In Fig. 13, the principle of two-level converter technology is depicted. From the figure, it can be seen that the converter voltage, created by PWM (Pulse-Width Modulation) pulse packages, is far away from the desired “green” voltage, it needs extensive filtering to approach a clean sinus waveform.
4.3 The Modular Multilevel Converter (MMC) Approach Both the size of voltage steps and the related voltage gradients can be reduced or minimized if the AC voltage generated by the converter can be selected in smaller increments than at two or three levels only. The finer this gradation, the smaller is the proportion of harmonics and the lower is the emitted high-frequency radiation. Converters with this capability are called multilevel converters. Furthermore, the switching frequency of individual semiconductors can be reduced. Since each switching event creates losses in the semiconductors, converter losses can also be effectively reduced.
Different multilevel topologies [6, 7, 9, 10], such as diode clamped converters or converters with what is termed “flying capacitors” were proposed in the past and have been discussed in many publications.
In Fig. 14, a comparison of two, three and multilevel technology is depicted. A new and different multilevel approach is the modular multilevel converter (MMC) technology [8, 10].
The principle design of conventional multilevel converter and advanced MMC is shown in Fig. 15.
Fig. 13: VSC Technology – a Look back
High harmonic Distortion
High Stresses resulting in HF Noise
)
Desired voltage Realized voltage
- Vd /2
0
+Vd /2
)
Desired voltage Realized voltage
- Vd /2
0
+Vd /2
VConv.
Vd /2
Vd /2VConv.
Vd /2
Vd /2
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Power Electronic Devices:
Topologies: Two-Level Three-Level Multilevel
IGBT in PP IGBT ModuleGTO / IGCT
Fig. 14: The Evolution of VSC and HVDC PLUS Technology
Fig. 15: The Multilevel Approach a) Conventional Solution b) Advanced MMC Solution c) Sinus Approximation – and Benefits
Vd /2
Vd /2VConv.
Vd /2
Vd /2
Vd /2
Vd /2VConv.VConv.
a)
Vd
VConv.
Vd
VConv.VConv.
b)
Small Converter AC Voltage Steps
Small Rate of Rise of Voltage
Low Generation of Harmonics
Low HF Noise
Low Switching Lossesc)
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Fig. 16 depicts the HVDC PLUS MMC solution in detail.
Fig. 16: HVDC PLUS with MMC – Basic Scheme
Submodule (SM)
Vd
Fig. 17: The Result – MMC, a perfect Voltage Generation
VConv.
- Vd /2
0
+Vd /2
AC and DC Voltages controlled by Converter Leg Voltages:
VAC
VConv.
- Vd /2
0
+Vd /2
AC and DC Voltages controlled by Converter Leg Voltages:
VAC
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Fig. 18: a) Features and Benefits of MMC Topology b) Space Saving in Comparison with HVDC “Classic”
High Modularity in Hardware and Software
Low Generation of Harmonics
Low Switching Frequency of Semiconductors
Use of well-proven Standard Components
Sinus shaped AC Voltage Waveforms
Easy Scalability
Reduced Number of Primary Components
Low Rate of Rise of Currents even during Faults
High Flexibility, economical from low to high Power Ratings
Only small or even no Filters required
Low Converter Losses
High Availability of State-of-the-Art Components Use of standard AC
TransformersLow Engineering Efforts,
Power Range up to 1000 MWHigh Reliability, low
Maintenance Requirements
Robust System
a)
HVDC PLUSHVDC PLUS
HVDC “Classic”
HVDC “Classic”
Example 400 MWExample 400 MW
Space Saving
b)
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Fig. 17 shows the advanced principle of AC voltage generation with MMC. It can be seen that there is almost no or – in the worst case – very small demand for AC voltage filtering to achieve a clean “green” voltage, in comparison with the two-level circuit with PWM in Fig. 13.
Thanks to its modular construction, the HVDC PLUS converter is extremely well scalable, i.e. conveniently adaptable to any required power and voltage ratings.
Figs. 18-20 summarize the advantages of HVDC PLUS in a comprehensive way. Added to these are the aforementioned advantages that ensue from the use of VSC technology in general. More details of the PLUS control and protection strategies are explained in [10].
Due to these features, HVDC PLUS is ideally suitable for the following DC systems (Fig. 21): - Cable transmission systems. Here, the use of modern extruded cables, i.e. XLPE, is possible, since the voltage polarity in the cable remains the same irrespective of the direction of current flow - Overhead transmission lines, due to the capability to manage DC side short-circuits and prompt resumption of system operation - Back-to-back arrangement, i.e. rectifier and inverter in one station - The implementation of multiterminal systems is relatively simple with HVDC PLUS. In these systems, more than two converter stations are linked to a DC connection. It is even possible to configure complete DC networks with branches and ring structures. The future use for systems such as these was addressed in the development of HVDC PLUS by pre-engineering the control strategies required for them - It goes without saying that the converters can also be used as STATCOMS, e.g. when the transmission line or cable is out of service during maintenance or faults. STATCOM with PLUS technology is also useful in unbalanced networks, for instance in the presence of large single-phase loads. Symmetry of the three-phase system can to some extent be restored by using load unbalance control.
Clean Energy to Platforms & Islands …Clean Energy to Platforms & Islands …
Low Switching Frequency
Reduction in Losses
Less Stresses
In Comparison with 2 and 3-Level Converter Technologies
In Comparison with 2 and 3-Level Converter Technologies
… with Advanced VSC Technology… with Advanced VSC Technology
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Fig. 19: HVDC PLUS – The Power Link Universal System
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This multitude of possibilities in combination with the performance of HVDC PLUS opens up a wide range of applications for this technology:
- DC connections for a power range of up to 1,000 megawatts, in which presently only line- commutated converters are used
- Grid access to very weak systems or islanded grids
- Grid access of renewable energy sources, such as offshore wind farms, via HVDC PLUS. This can substantially help reduce CO2 emissions. And vice versa, oil platforms can be supplied from the coast via HVDC PLUS, so that gas turbines or other local power generation on the platform can be avoided.
Compact Modular Design
Less Space Requirements
Advanced VSC Technology
HVDC PLUS – One Step ahead
Compact Modular Design
Less Space Requirements
Advanced VSC Technology
HVDC PLUS – One Step ahead
Fig. 20: HVDC PLUS – The Smart Way
DC Cable TransmissionDC Cable Transmission
DC Overhead Line TransmissionDC Overhead Line Transmission
Back-to-Back SystemsBack-to-Back Systems
Multiterminal SystemsMultiterminal Systems
STATCOM Features includedSTATCOM Features included
Fig. 21: Applications and Features of HVDC PLUS
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5. SECURITY, SUSTAINABILITY AND MARKET ASPECTS OF HVDC AND FACTS Aspects of security of power supply are summarized in Figs. 22-23. Technical benefits of each technology are depicted with regard to transmission system performance, in both steady-state and transient network conditions. Fig. 22 is focused on interconnection tasks, and Fig. 23 depicts the “stand-alone” features of each application. As it can be seen in both figures, these evaluations are thoroughly considered, based on a long experience of study and project applications. All technology developments are focused on reliability, synergies and modularity of the different applications with regard to cost optimization and minimization of transmission losses.
The aspects of market and sustainability, which are the technology drivers, are depicted in Fig. 24.
Fig. 25 schematically shows a generalized solution for a large interconnected system, applying the above discussed possibilities of system enhancement: a hybrid solution, with AC interconnection as well as HVDC and FACTS.
Power exchange between the neighboring areas of interconnected systems can be achieved by an AC interconnection. FACTS controllers should be applied to support the AC interconnections and to improve the dynamic conditions in the system. Back-to-back HVDCs between the subsystems act as firewalls to separate parts of the interconnected systems to avoid the spread of large disturbances throughout the whole system (Blackout prevention). Transmission of large power blocks over long distances should, however, be achieved by HVDC point-to-point transmission directly between generation and the locations of power demand. Furthermore, the HVDC transmissions can support the AC interconnections, in order to avoid possible dynamic problems which exist in these complex interconnections.
Fig. 22: Impact of HVDC and FACTS on System Performance in Interconnection Applications
* SCP = Short-Circuit Power (System MVA)
SystemA
SystemA
SystemB
SystemB
SystemC
SystemC
SystemE
SystemE
SystemF
SystemF
SystemD
SystemD
Risk of Spread of Voltage CollapseRisk of Spread of Voltage CollapseRiskRisk BarrierBarrierBarrierBarrier
HVDC LDT **HVDC LDT **
B2B ** – GPFCB2B ** – GPFC
FACTSFACTS TCSC SVC** SVC**
VoltagePowerFlow
Control of Limitation of
Faults SCP *
PowerSwingDamping
Spread ofVoltage Collapse
SCCL Risk
Barrier
Barrier
** “Classic” or PLUS
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HVDC - B2B, LDT
UPFC (Unified Power Flow Controller)
MSC/R(Mechanically Switched Capacitor / Reactor)SVC(Static Var Compensator)STATCOM ** (Static Synchronous Compensator)
Load-Flow Control
Voltage Control: Shunt Compensation
FSC (Fixed Series Compensation)TPSC (Thyristor Protected Series Compensation)TCSC (Thyristor Controlled Series Compensation)
Variation of the Line Impedance: Series Compensation
Voltage QualityStabilityLoad Flow
SchemeDevicesPrincipleImpact on System Performance
HVDC PLUS - VSC
HVDC - B2B, LDT
UPFC (Unified Power Flow Controller)
MSC/R(Mechanically Switched Capacitor / Reactor)SVC(Static Var Compensator)STATCOM ** (Static Synchronous Compensator)
Load-Flow Control
Voltage Control: Shunt Compensation
FSC (Fixed Series Compensation)TPSC (Thyristor Protected Series Compensation)TCSC (Thyristor Controlled Series Compensation)
Variation of the Line Impedance: Series Compensation
Voltage QualityStabilityLoad Flow
SchemeDevicesPrincipleImpact on System Performance
HVDC PLUS - VSC
Influence: *
no or lowsmallmediumstrong
* Based on Studies & practical Experience
** = SVC PLUS
Fig. 23: Impact of HVDC and FACTS on System Performance in Stand-Alone Applications
Fig. 24: Market and Sustainability Aspects for HVDC and FACTS
Excellent Market
Emerging Market
Small Market
Tiny Market
UPFC
TCSC / TPSC
FSC
STATCOM
SVC
Series Compensation
Shunt Compensation
Combined Device
DC Power Transmission &System Interconnection
MSC/R
SVC PLUS
HVDC “Classic”
HVDC “Bulk” - UHV
HVDC PLUS - VSC
focused onGreen Energy and CO2 Reduction
Future Molding Technologies,
Excellent Market
Emerging Market
Small Market
Tiny Market
UPFC
TCSC / TPSC
FSC
STATCOM
SVC
Series Compensation
Shunt Compensation
Combined Device
DC Power Transmission &System Interconnection
MSC/R
SVC PLUS
HVDC “Classic”
HVDC “Bulk” - UHV
HVDC PLUS - VSC
focused onGreen Energy and CO2 Reduction
Future Molding Technologies,
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6. SUMMARY, CONCLUSIONS AND VISIONS
Deregulation and privatization is posing new requirements to the flexibility and performance of high voltage transmission systems. System components are going to be loaded up to their thermal limits, and wide-area power trading with fast varying load patterns will lead to an increasing congestion.
Environmental constraints, such as loss minimization and CO2 reduction, will play an increasingly important role [1, 2]. The loading of existing power systems will further increase, leading to bottlenecks and reliability problems. As a consequence of “lessons learned” from the large blackouts in 2003 [13], advanced transmission technologies will be essential to the system developments, leading to Smart Grids with better controllability of power flows [1, 10].
HVDC and FACTS provide the necessary features to avoid technical problems in the power systems; they increase the transmission capacity and system stability very efficiently, and they assist in prevention of cascading disturbances. They effectively support the grid access of renewable energy resources and they reduce the transmission losses by optimization of power flows.
Bulk power UHV AC and DC transmission technology will be applied in emerging regions and countries to serve their booming energy demands in an efficient way.
The Smart Grid vision could also be applied globally. The idea would be a “Global Link”, which uses both AC and DC Bulk Power transmissions. This vision has been in discussion since long [22], the technology however has just become available, as depicted in the previous sections.
Fig. 26 shows examples of possible interconnections and promotes the benefits of this idea.
SystemA
SystemA
SystemC
SystemC
SystemE
SystemE
SystemF
SystemF
SystemB
SystemB System
DSystem
D
SystemG
SystemG
Large System Interconnections, with HVDC…Large System Interconnections, with HVDC…
High Voltage- via AC Lines
HVDC - Long Distance DC TransmissionHVDC - Long Distance DC TransmissionHVDC B2B
AC Transmission & FACTS
and FACTSLarge System Interconnections
DC – the Stability Booster and“Firewall” against “Blackout”DC – the Stability Booster and
“Firewall” against “Blackout”
Solutions for a “Smart” & Strong GridSolutions for a “Smart” & Strong Grid
The Result
“Countermeasures”against large Blackouts
“Countermeasures”against large Blackouts
Fig. 25: Lessons learned - Benefits of hybrid System Interconnections with HVDC and FACTS
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7. REFERENCES [1] European Technology Platform SmartGrids – Vision and Strategy for Europe’s Electricity Networks of the Future; 2006, Luxembourg, Belgium
[2] DENA Study Part 1: „Energiewirtschaftliche Planung für die Netzintegration von Windenergie in Deutschland an Land und Offshore bis zum Jahr 2020“; February 24, 2005, Cologne, Germany
[3] Economic Assessment of HVDC Links; CIGRE Brochure Nr.186 (Final Report of WG 14-20)
[4] N.G. Hingorani: “Flexible AC Transmission”; IEEE Spectrum, pp. 40-45, April 1993
[5] “FACTS Overview”; IEEE and CIGRE, Catalog Nr. 95 TP 108
[6] Working Group B4-WG 37 CIGRE: “VSC Transmission”, May 2004
Fig. 26: Global Link – Benefits for Security and Sustainability of Power Supply
Benefits of a “Global” Solution for System Interconnection:
Solving local Problems of Energy Resources by worldwide Energy TradingImproving Frequency Stability in weak Systems by Support through strong Systems Chance to use remote Regenerative and clean Energy Sources:
Solar Fields in DesertsOffshore Wind FarmsHydro Energy
Independent from the Time Zones
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[7] F. Schettler, H. Huang, N. Christl: “HVDC transmission systems using voltage-sourced converters – design and applications”; IEEE Power Engineering Society Summer Meeting, July 2000
[8] R. Marquardt, A. Lesnicar: “New Concept for High Voltage – Modular Multilevel Converter”; PESC 2004 Conference, Aachen, Germany [9] S. Bernet, T. Meynard, R. Jakob, T. Brückner, B. McGrath: “Tutorial Multi-Level Converters”; Proc. IEEE-PESC Tutorials, 2004, Aachen, Germany
[10] J. M. Pérez de Andrés, J. Dorn, D. Retzmann, D. Soerangr, A. Zenkner: “Prospects of VSC Converters for Transmission System Enhancement”; PowerGrid Europe, June 26-28, 2007, Madrid, Spain
[11] A. Menze, H. Ross, H. Borgen, B. Ek, W. Winter, R. Witzmann, H. Breulmann, T. Ham- merschmidt, W. L. Kling, F. J. C. M. Spaan, H. Knudsen, H. Ring: “New HVDC power links between UCTE and NORDEL – Analysis of AC/DC interactions in the time and frequency domains”; Report 38-207, CIGRE Session 2002, Paris
[12] M. Luther, U. Radtke: “Betrieb und Planung von Netzen mit hoher Windenergieeinspeisung”; ETG Kongress, October 23-24, 2001, Nuremberg, Germany
[13] G. Beck, D. Povh, D. Retzmann, E. Teltsch: “Global Blackouts – Lessons Learned”; Power-Gen Europe, June 28-30, 2005, Milan, Italy
[14] U. W. Niehage: “Future Developments in Power Industry”; Key-Note Address at AESIEAP, September 28-05, 2005, New Delhi, India
[15] L. Kirschner, D. Retzmann, G. Thumm, “Benefits of FACTS for Power System Enhancement”; IEEE/PES T&D Conference, August 14-18, 2005, Dalian, China
[16] U. Armonies, M. Häusler, D. Retzmann: “Technology Issues for Bulk Power EHV and UHV Transmission”; HVDC 2006 Congress – Meeting the Power Challenges of the Future using HVDC Technology Solutions, July 12-14, 2006, Durban, Republic of South Africa
[17] D. Povh, D. Retzmann, E. Teltsch, U. Kerin, R. Mihalic: “Advantages of Large AC/DC System Interconnections”; Report B4-304, CIGRE Session 2006, Paris
[18] W. Breuer, D. Povh, D. Retzmann, E. Teltsch: “Trends for future HVDC Applications”; 16th CEPSI, November 6-10, 2006, Mumbai, India
[19] W. Breuer, M. Lemes, D. Retzmann, “Perspectives of HVDC and FACTS for System Interconnection and Grid Enhancement”; Brazil-China-India Summit Meeting on HVDC and Hybrid Systems – Planning and Engineering Issues, July 16-18, 2006, Rio de Janeiro, Brazil
[20] V. Ramaswami, D. Retzmann, K. Ücker: “Prospects of Bulk Power EHV and UHV Transmission”; GRIDTECH 2007 – New Technologies in Transmission, Distribution, Load Dispatch & Communication, February 5-6, New Dehli, India
[21] K. Braun, A. Krummholz, D. Retzmann, U. Rohr, G. Thumm: “The worldwide Biggest FACTS Project with Series Compensation – the Purnea and Gorakhpur TCSC/FSC in India”; ELECTRA, N° 229 - December 2006
[22] Global Link – Interkontinentaler Energieverbund; VDI Berichte 1129, October 1994, Germany
