Development of a laboratory platform for testing newsolutions to integrate renewable energy sources in power

systems

Eduardo Prieto-Araujo, Marc Cheah-Mane, Roberto Villafafila-Robles, Oriol Gomis-Bellmunt1

CITCEA-UPCC. Comte d’Urgell, 187

08036 BarcelonaBarcleona, Spain

Email: eduardo.prieto-araujo@citcea.upc.eduURL: http://www.citcea.upc.edu

Adria Junyent-Ferre2

Control & Power Group, Imperial College LondonSouth Kensington Campus, London SW7 2AZ, United Kingdom

AcknowledgmentsThis work is supported by the Ministerio de Ciencia e Innovacion under Project IPT-2011-1892-920000

Keywords<<Smart grids>>, <<Smart microgrids>>, <<Renewable energy systems>>, <<Test bench>>.

AbstractThe article deals with the design and implementation of a reduced scale laboratory platform consisting ofa number of programmable emulators. By connecting these devices into the same grid, different micro-grid configurations can be emulated, allowing to perform many experiments related to its management,control and protection. Here, the proposed platform layout is described and its performance is shownthrough experimental results.

IntroductionThe number of distributed generation (DG) facilities is increasing worldwide. DG enhances the reliabilityand flexibility of the system while reducing its dependance from the large centralized power plants [1].In order to perform a proper grid integration of these resources, microgrid systems are created. Thesefacilities can be defined as a set of generators and loads operated as a single controllable system, whichis able to deliver electrical and heating power in a local scale [2]. However, this concept is constantlyevolving, thus other definitions include elements as storage resources [3] or advanced capabilities as theislanded operation [4]. Actually, microgrids could gather many different devices connected to the samesystem, therefore issues regarding its control, operation and protection must be analyzed in detail.In order to address these issues, important experimental research projects are being developed today[5]. For example, the CERTS (Consortium for Electrical Reliability Technology Solutions) Microgridlaboratory Project [6], aims to demonstrate the ease of integrating distributed energy sources into amicrogrid, performing real field tests. Many other centers are working in this field, the PSERC (PowerSystem Engineering Research Center), the BCIT (British Columbia Institute of Technology) in Canada

1Oriol Gomis is also with IREC Catalonia Institute for Energy Research2Adria Junyent-Ferre was with CITCEA-UPC at the time this paper was written.

or the NEDO (New Energy and Industrial Technology Development Organization) in Japan, defining thespecifications of the future microgrids [7]. In a reduced scale, the microgrid proposed by IREC (CataloniaInstitute for Energy Research) [8] shows a scheme focused on the control of the entire microgrid usingcommunications based on the IEC 61850 standard. As it can be seen, many different projects are ongoingdue to the importance of performing field tests to validate the current developments.This paper proposes a microgrid laboratory platform that allows performing experimental research testsin a reduced power scale. The main objective of this test bench is to create a complete system whereinterdisciplinary experiments could be performed as controls validation, communication tests, protectionanalysis, energy managers system trials, among others. Therefore, the system must be flexible, secure,scalable, inexpensive, expandable and its dimensions must be reduced in order to fit it inside a laboratory.These requirements suggest that the use of programmable emulators to represent the different resourcescould be an interesting option, because they allow to represent many different resources and scenarioswith the same hardware. As an example, this work describes the implementation of a testing microgridbased on emulators, compounded by three different nodes connected to the conventional single phaseAC grid.

The microgrid conceptFigure 1 shows a generic microgrid layout. It aims to represent one the most ambitious definitionsof the microgrid concept [3], considering electrical and thermal loads, storage systems and distributedgeneration resources, connected to the same grid.

PCC /

+ -

CB1

CB2 CB3

CB4

CB5

CB6

LV AC Grid

PV Power

Batteries

Energy managementsystem

Electric

vehicles

Loads

Loads

Loads

Wind

power

H2

Fuel cellLoads

Loads

Loads

Converter Converter

Converter

Converter

Converter

CHP

Cogeneration

Loads

Thermal

Loads

Figure 1: Generic microgrid layout

Next, the different elements of the microgrid are described in more detail:

• Loads. This term gathers all the possible consumptions connected to the microgrid. These couldbe electrical loads, as lights and appliances, or thermal loads, as air conditioning systems or waterheaters.

• Generators. Microgrids usually include renewable generation as wind power and PV panels, butalso co-generation systems could be incorporated.

• Storage systems. This term describes the different elements which are able to store energy, includ-ing batteries, flywheels, among others.

• Circuit breakers (CB). These elements allow the connection or the disconnection of the differentparts of the microgrid. The breaker located at the point of common coupling (PCC) allows theislanding operation of the microgrid, if it is needed.

• Energy management system. It is in charge of the operation and control of the complete system.Different manager versions can be implemented, with different objectives.

• Converters. These elements are in charge of performing a proper grid integration of the distributedgeneration and the storage systems.

Many different microgrid types can be built. Those facilities which include distributed generation andstorage systems could be considered a microgrid. A simple house or larger facilities as for exampleoffices, universities, councils, can be examples of microgrids.

Microgrid scenarioMicrogrid facilities present a different energetic scenario, in comparison with the conventional instal-lations. Elements as generators and storage systems could be connected to the grid, therefore severalconsiderations must be taken into account:

• Microgrids are not a passive consumer inside the grid. By means of their own generation systems,they are capable to feed part, or even all their consumptions.

• The classic protections could not be sufficient once the storage and generation systems are incor-porated in microgrid facilities. Some changes might be performed in the electrical installation.

• It should be considered that the system could operate disconnected from the grid. If this mode isincluded, storage systems are needed in order to feed properly the loads without outages.

• Related to the previous point, during islanded operation the combined power of the generation andthe storage systems could not be enough to feed all the connected loads. Therefore low priorityloads must be disconnected in order to maintain energized the most critical ones. On the otherhand, the reconnection of these loads should be allowed when conditions become more favorable.

• The system could be capable of performing a black-start, without being connected to the grid.

• The energy produced by the sources connected to the microgrid must be integrated to the grid.Converters might be required in most of the applications to adapt the energy produced.

• An energy management system must be included to control the entire microgrid. The generation,the storage and the loads must be controlled and coordinated to ensure a correct operation of thesystem.

• Renewable energy sources as PVs and wind power vary with weather conditions. Therefore, thesystem needs an energy management system, which can operate the entire microgrid consideringthese variations.

• The connection of different active systems to the same grid may produce some electrical interac-tions between them, that must be taken into account.

• Other issues as fault ride-through capability, voltage and frequency support to the grid could berequired in the near future, in order to help the grid performance during disturbances.

In order to address the mentioned issues, experimental platforms allow to perform real field tests whichare extremely useful to evaluate the impact of the previous considerations. These platforms may also beused to test new developments in the microgrid fields, as for instance new energy management systems,new protections designs, among others. In this article, the experimental platform is combined with theemulator concept to increase the system testing capabilities, thus being able to reproduce many differ-ent scenario configurations without changing the platform hardware. In the next section, the emulatorconcept is accurately described.

The emulator conceptAn emulator is a programmable power electronics device that represents the electrical behavior of a realsystem. Figure 2 shows an example of an emulator representing a PV cell. Its main properties are:

• Flexibility. Through the same software, different resources could be emulated.

• Costs. The costs of the emulator hardware are usually lower than the costs of the real resource.

• Dimensions. Their dimensions are reduced in comparison with the real resources ones.

• Availability. The system is always ready for testing if it is needed.

• Security. Different tests can be performed without damaging real systems.

Power

electronicsPower

Supply

Power

electronicsPower

Software

Real system Emulator

Figure 2: Real PV installation - PV emulator

• Emulation levels. An emulator can represent for example a simple PV cell or the set of houseappliances.

The microgrid laboratory platform described in this article includes several of these devices behaving asdifferent real resources. Hence, many different scenario configurations could be represented consideringonly a hardware implementation. Next chapter details the layout of the platform and the specific emulatorcharacterization.

Laboratory platform

Platform descriptionThe microgrid laboratory platform is compounded by three different elements (Figure 3), a PV installa-tion, a load and a battery storage system connected to the main AC single-phase grid. This system hasbeen designed to include different nature elements as generation, storage and loads, in order to build asimple microgrid. This configuration considers that the loads are connected directly to the grid. Whereas,the battery and the PV installation need power converters to be integrated to the grid due to its direct cur-rent nature. This platform layout would allow to perform many different experiments. However, thereplacement of these elements by emulators is proposed to increase the experimental possibilities.

LV Grid

+ -

LV Grid

Emulators

+ -

Battery AC Loads

Real AC

systems

PV

Power

supplyBattery PV AC Loads

Emulated

microgrid

Platform

setup

Real systems

Figure 3: Emulated system and experimental platform layout

The three different elements are represented by emulators, which are also connected to the AC grid. Interms of hardware, these emulators are AC/DC H-bridge single phase Voltage Source Converters (VSCs)(Figure 4), based on Insulated-Gate Bipolar Transistors (IGBT). Each emulator is connected on one sideto the AC grid and in the other side to a power supply which allows them to absorb or to inject power tothe grid. In order to simplify the platform and to reduce the costs, the three emulators are built using thesame hardware.

dcV

1q

2q

4q

3q

lv

lzi

zv

Idcm

CL

L

Figure 4: Electrical scheme of the emulator

Emulator controlThe emulators included in the platform behave as power emulators. In other words, they consume orinject to the grid, the equivalent power that the real resource would be exchanging with the grid, under thesame conditions. Hence, the device must be able to regulate active and also reactive power to representproperly any kind of resource. Figure 5 shows the power control scheme based on a resonant currentcontroller R [9]. The converter is synchronized to the grid through a single phase Phase-locked loop(PLL) [10]. Once the grid angle θ and voltage Vz are obtained, the emulator is able to regulate activeP and reactive power Q at the same time. In a higher emulation level, the active P∗ and reactive Q∗

power set-points are calculated based on the resource that is being emulated and, through these values,the converter control creates the current reference ilz∗ input for the current loop control. This currentcontroller R is in charge of achieving the desired amount of grid flowing current ilz, through applying theproper voltages vl using a three-level Pulse Width Modulation (PWM) [11]. The control is performed bya Digital Signal Processor (DSP) and the communications with the higher emulation level are carried outthrough CAN bus.

=C

L

*P

*Q

µ

µ

*

µ

Reference calculation

Current regulator

x..

zVzV

x

x

cos

sin

+ lzi lv

PLL

zv

l

Ll /2

PWM

=

lzi

+-

+++

zvlzi

R

x..

/2

Grid filters

AC grid

Figure 5: Control scheme of the emulator

This control scheme is implemented in each emulator of the platform. Therefore, to develop the emula-tion of an specific resource, only the active and reactive power references must be sent to the emulator.

Power supplyThe emulator operation always require a power supply which provides or absorbs the amount of en-ergy that the emulator needs to represent the real resource. In this case, the system is required to bebidirectional, because different nature elements could be represented, as generation and loads. Then, abidirectional AC/DC converter connected to the single-phase AC grid has been chosen to operate as apower supply element. This supply selection implies that there is no waste of energy during the tests,because the energy used for the emulation is injected again to the grid, without considering the losses,obviously.

Platform implementation and emulator descriptionThree different emulators and the power supply complete the test platform as it is shown in Figure 6.The emulators are supplied by the same converter, therefore they are connected to the grid throughtransformers in order to avoid possible circulating currents. Figure 7 shows the real implementation ofthe setup. Only the emulators are shown, in order to make the illustration more understandable.Next, the emulators characterization is described:

• The PV generation. It is emulated considering that the system is always working on its MaximumPower Point (MPP). Based on the current irradiance, the temperature and the solar cell character-istics, it is possible to obtain an expression of the power generated PMPP by each panel [12]:

PMPP =VMPP · IMPP

VMPP =VMPP0 ·ln(G)

ln(G0)· (1+ kv · (Tp −Tp,0))

IMPP = IMPP0 ·GG0

· (1+ ki · (Tp −Tp,0))

(1)

Real

load

PGPQ

S

S

PV2

AC loads3

PQPS

PSPS

Grid

Power supply

GridFilter

1 BatteryGridFilter

GridFilter

PQ

D

D

Grid Filter

+ -

P QTT

Emulators

G

Figure 6: Scheme and physical implementation of the platform

Figure 7: Setup of the emulation platform

Where, G and Tp are the irradiance and the panel temperature, while VMPP0, IMPP0, G0 and Tp,0 arethe solar panel parameters on the Standard Test Conditions (STC). Also, it must be mentioned thatthe emulated PV inverter is considered to be ideal. However, its efficiency can be straightforwardlyincluded.

Figure 8 shows the steady state operation of the PV emulator. It can be observed that the gridvoltage (blue line) and the current (green line) are in phase. Therefore, it can be stated that theemulator is injecting power (red line) with a unity power factor. In order to understand better thepower curve, it can be useful to analyse the single phase power expression:

p(t) = P+S · cos(2ωt +φ) (2)

The function average value is the active power P and the amplitude value of its oscillating part isthe apparent power S. Thereby, when the emulator is only injecting or absorbing active power, theapparent power is equal to the active power. Therefore, the instantaneous power must be eitherpositive or negative. In this case, as the emulator is set to inject only reactive power, it can beobserved that the power is always negative, accomplishing the theory.

• Load emulator. The emulator can be programmed to consume the desired amount of active andreactive power from the grid, within its rated limits. Figure 9 shows the steady state operationof the load emulator consuming different power levels. It can be seen that there is a phase shiftbetween the grid voltage (blue line) and the grid current (magenta line), because the emulatorpower references are settled to absorb active power with a certain power factor (PF). This factcould also be seen in the power waveform (red line) which is positive and negative during theoperation, due to the active and reactive power flow.

• Battery emulator. The emulator can be controlled to absorb or inject the desired amount of powerwithin its limits. As in the PV case, the battery inverter is considered to be ideal. However, it isstraightforward to include the converter efficiency in the emulation.

Regarding the power levels, the emulators and the power supply rated powers are 1.5 and 5 kW respec-tively. However, it must be mentioned that the system results could be scaled to higher power levelseasily. In the following section, two different scenarios are described to show the emulation possibilitiesof the presented platform.

Figure 8: Stationary results for the photovoltaic emulator injecting PG=1000 W

(a) PD=500 W, PFD=0.9 (b) PD=1200 W, PFD=0.9

Figure 9: Stationary results for the load emulator

Emulators and scenario descriptionThis section describes two different scenarios for the emulation platform presented previously (Figure6). The emulator configurations for both case studies are described below.

Scenario 1: Microgrid without storageThis scenario is performed to show behavior of a microgrid connected to the low voltage AC grid withouta storage system. The emulation scenario configuration is:

• Load emulator. It starts consuming 500 W from the grid with a 0.9 power factor and changes to1200 W with the same power factor.

• PV emulator. It is injecting constantly 900 W due to an irradiance of 600 W/m2.

• Battery emulator. It is not considered in this case.

Figure 10 shows an oscilloscope capture of the system currents. The currents measured are the PV current(green), the load current (magenta) and the current flowing to the main grid (yellow) (see Figure 6). It canbe seen that the PV current remains constant around 5 A, whereas the load current is increased during thetest from around 3 A to 8 A, as expected. Before the load step change, the microgrid is injecting powerto the grid because the power input of the PV generation is higher than the load consumption. However,when the load increases its demand, it can be observed that the current exchanged with the grid becomesnegative, so the microgrid needs to consume power from the grid to feed the load. The mentioned changeof the grid current phase can not be clearly observed due to the effect of the power factor of the load.

Figure 10: Scenario 1 currents - PV (green) - Load (magenta) - Grid (yellow)

This scenario shows a proper behavior of the emulated microgrid when injecting or consuming powerfrom the grid, without any problem. Regarding the test, it can be stated that microgrids without storagecapability, must absorb power from the grid when the generation is not enough for feeding the loads, asexpected. This configuration, could cause evident supply problems during islanding operation, becauseof the high variability of the generation.

Scenario 2: Microgrid with storageThe second scenario is performed to show the capabilities of the microgrids systems which incorporatestorage systems. The emulation scenario is:

• Load emulator. It starts consuming 500 W from the grid with a 0.9 power factor and changes to1200 W with the same power factor.

• PV emulator. It is injecting constantly 900 W due to an irradiance of 600 W/m2.

• Battery emulator. It is operated with the aim of achieving a zero active and reactive power exchangebetween the grid and the microgrid during the test.

Figure 11 shows an oscilloscope capture of the system currents. The currents measured are the PVcurrent (green), the load current (magenta), the main grid current (yellow) and the battery current (blue)(see Figure 6). It can be observed that during the first part of the test the microgrid is no exchangingenergy with the grid (the current flowing to the grid is almost zero). This is because the PV generation isenough to feed the load, so the excess of power is being stored in the battery. When the load changes itsvalue to a higher one, transitorily, it can be seen that the grid feeds it because the PV installation does notproduce enough power to do it. However, a few milliseconds later, the battery starts to inject the extraamount of active and reactive power to compensate the extra load consumption, in order to maintain theobjective of no exchanging active and reactive power with the grid.

Figure 11: Scenario 1 currents - PV (green) - Load (magenta) - Battery (Blue) - Grid (yellow)

In this case, the power references for the battery operation were calculated offline and changed manuallyduring the test, acting as the energy manager would need to do, to maintain the power exchange with thegrid fixed at zero. In further experiments, a real energy management system is expected to be included tocontrol the state of charge of the battery, to achieve the same goal, autonomously. Of course, this is notthe only objective that could be implemented in the microgrid energy management system. However, itis useful to test the system behavior and extract some conclusions of the system operation.In summary, this experiment aims to demonstrate the possibilities that the storage devices show duringthe microgrid operation. Considering a proper energy manager, the microgrid could reduce significantlyits consumption from the grid as it has been shown. Regarding the emulation platform, it shows a properbehavior during the experiments, both in steady state and during transients. Besides, the emulatorsbehave as their equivalent real systems as expected. As a comment, the number of nodes could beincreased if it is needed to perform other experiments and also the emulators could be reprogrammed tobehave as other systems.

ConclusionsA reduced scale laboratory platform implemented for testing different microgrid developments is de-scribed. The platform performance is demonstrated through two different experiments. The first config-uration represents a common microgrid which includes loads and a PV generation system. The secondone adds a storage system to the previous configuration, in order to show the possibilities of these sys-tems along with a proper energy management system. From these experiments, the laboratory platformoperation is validated and ready for other experiments regarding the operation, control and protectionof microgids. This platform is the first step towards a bigger implementation which aims to combinedifferent real resources with emulators to increase the experimentation possibilities.

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