Integrated Microgrid Laboratory System

IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 27, NO. 4, NOVEMBER 2012 2175

Integrated Microgrid Laboratory SystemBo Zhao, Xuesong Zhang, and Jian Chen

Abstract—The paper presents an integrated microgrid labora-tory system with a flexible and reliable multimicrogrid structure;it contains multiple distributed generation systems and energystorage systems and integrates with a diesel generator that servesas a back-up power source and flywheel energy storage for fastbalancing to provide uninterruptible power-supply services incooperation with the diesel generator. The microgrid system,by adopting the master–slave control strategy, can be transitedflexibly between grid-connected and islanded modes and can bedisconnected from the utility when a fault occurs or the powerquality falls below specified standards. The developed bi-direc-tional inverter which is applied in the system plays an importantrole. The small microgrids of this system are intended to operateseparately or in the form of one large microgrid with a certainswitch status. Furthermore, experiments on control, protection,and other technologies have been carried out. The results showthat the operation conditions meet the related IEEE Standard1547 and power quality requirements. The integrated microgridlaboratory system is able to operate stably and reliably underdifferent conditions, including mode transition and fault events.

Index Terms—Bi-directional inverter, diesel generator, flywheelenergy storage, master–slave control, microgrid.

I. INTRODUCTION

T HE increase in the penetration depth of distributed gen-erations (DGs) and the presence of multiple DGs in the

electrical proximity to one another have brought about the con-cept of the microgrid, which can provide more technical bene-fits and control flexibilities to the utility gird [1]–[5]. Researchon microgrid technologies has received increasingly widespreadattention recently. Many microgrid technologies such as en-ergy management, control strategies, protection methods, powerquality, laboratory systems, and field tests have been studiedin particular [6]–[12]. As the carrier of microgrid technologies,the microgrid laboratory system is designed to provide a verifi-cation platform for the researches. The development of labora-tory-scale microgrid systems with the DGs and energy storagesystems has become one of the key technology problems thatneed to be solved in microgrid research.

Many microgrid systems have been built in recent yearsinternationally. In North America, the CERTS Microgrid Lab-oratory Test Bed [13] is one of the most authoritative microgrid

Manuscript received October 06, 2011; revised February 01, 2012; acceptedMarch 13, 2012. Date of publication May 01, 2012; date of current versionOctober 17, 2012. This work was supported by the Hi-Tech Research and De-velopment Program of China (863) under Contract 2011AA05A107. Paper no.TPWRS-00940-2011.

B. Zhao and X. S. Zhang are with the Zhejiang Electric Power Test and Re-search Institute, Hangzhou, Zhejiang, 310014, China (e-mail: [email protected]; [email protected]).

J. Chen is with the School of Electrical Engineering and Automation, TianjinUniversity, Tianjin, 300072, China (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRS.2012.2192140

systems and plays an important role in microgrid research.FortZED (Zero Energy District) [14] is a community-driveninitiative to create one of the world’s largest net zero energydistricts in the downtown area and the main campus of Col-orado State University. The 1-MW FREEDM System [15]demonstration laboratory of North Carolina State Universitynot only demonstrates the Center-developed technologies, butit is also used to showcase the third-party renewable energytechnologies, such as solar, wind, fuel cell, battery storage,flywheel storage, and plug-in vehicles. Perfect Power at theIllinois Institute of Technology (IIT) is setting an examplefor smart grid projects throughout the country. Launched in2006, the smart microgrid-based system is a ground-breakingapproach to electricity distribution and management, creating areliable power system [16], [17]. Many works have been donein Europe as well. The Association of European DistributedEnergy Resources Laboratories (DERlab) [18] aims to clusterthe best European DER laboratories from each EU memberstate, including IWES, KEMA, AIT, NTUA, CRES, and soon. Each member laboratory of DERlab is strong in specificDER-related areas, and together they cover the whole field ofdistributed generation and smart grids. The DERlab associationoffers an access point to the testing capabilities. Testing of thequalifications of system components and products can be per-formed according to standards or customer specifications. TheMicrogrids Consortium [19] comprises major European man-ufacturers, power utilities, and potential microgrid operatorsand research teams with complementary high-quality expertise.The microgrid systems include the Kythnos microgrid, theManheim microgrid, CESI, the Bornholm microgrid, the Kozufmicrogrid, and so on. European institutes play an importantrole in promoting microgrid systems. In Asia, a small-scalemicrogrid pilot plant has been designed at the Korea Elec-trotechnology Research Institute (KERI) [20]. However, partsof the distributed power sources in this system are emulators,including the wind turbine (WT) simulator and photovoltaic(PV) simulator. Many microgrid projects have been constructedin Japan, including the Aichi microgrid project [21], Kyotangoproject, Hachinohe project, CRIEPI [22], and so on. The mi-crogrid testbed constructed at Hefei University of Technology(HFUT) [23] is a small laboratory-scale microgrid includingdifferent distributed power sources in China. At present, mi-crogrid laboratory and project systems are increasingly underconstruction. Microgrid systems which integrate more advan-tages are the development trend of the future.

Recently, the Chinese government has paid more and moreattention to microgrids that can help fulfill the targets of energyconservation and emission reduction. In this paper, we presentthe integrated microgrid laboratory system that was formulatedat Zhejiang Electric Power Test and Research Institute in 2010as a cluster of multiple DGs and energy storage systems with theability to operate stably and reliably under different conditions.

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2176 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 27, NO. 4, NOVEMBER 2012

Fig. 1. Configuration of the integrated microgrid laboratory system.

The DGs in the system are of multiple types, including a windturbine (WT) system, photovoltaic (PV) system and Double-fedInduction Generator (DFIG) system. A battery energy storagesystem is used in the system and a diesel generator serves as aback-up power source. Moreover, flywheel energy storage hasbeen used in this microgrid system, which is applicable for fastbalancing and can provide uninterruptible power-supply ser-vices in cooperation with the diesel generator. The system struc-ture is so flexible that several different topological structures areavailable in accordance with different research requirements.

The objective of the integrated microgrid laboratory systemis to carry out experimental and theoretical studies on DGsand microgrids, including: 1) studies on the operation controlstrategy and development of microgrid related equipment;2) studies on the impacts of microgrid on power systems;3) analyses of the economic efficiency and social benefitsof microgrids; and 4) formulation of guidelines regardingmicrogrids and implementation of microgrid security accessto the utility grid. The project accomplishes this objective bydeveloping and demonstrating advanced technologies in thefirst stage, including: 1) control methods including islanded op-eration control and automatic and seamless transitions betweengrid-connected and islanded modes; 2) dynamic simulation ofthe distribution network with DGs and microgrids; and 3) anapproach to microgrid protection considering the features andimpacts of DGs.

Section II introduces the physical configuration and controlstructure of the integrated microgrid laboratory system, andSection III describes the proposed control strategy in detail.The results of experiments and discussions are reported inSection IV. The conclusion is given in Section V.

II. MICROGRID LABORATORY SYSTEM

A. Physical System Configuration

Fig. 1 presents the configuration of the integrated microgridlaboratory system. The microgrid system, which is connected to

the external grid through an isolation transformer, is composedof two small microgrids: microgrid A and microgrid B.

The rules for naming switches are as follows: The switchesin microgrid A are denoted by names beginning with the letter“A,” and the switches in microgrid B by names beginning with“B.” Meanwhile, switches which are connected to equipmentshave names beginning with “F.” The two switches that can beautomatically controlled by the mode controller have names be-ginning with “M,” and the two interconnection switches havenames beginning with “L.”

Microgrid A includes three busbars: LM3, LM4, and LM5.It is composed of the PV system, WT system, DFIG simulationsystem, the Battery Energy Storage System (BESS), and twoload banks. The 30-kW DFIG simulation system is a simulatorwith an ABB frequency converter and a real generator to emu-late the real system. However, the 33-kW PV system and 10-kWWT system are real systems. The three-phase output power ofthe 33-kW PV system passes through a three-phase PV inverter,and the BESS near to the point of common coupling (PCC)will be utilized while microgrid A is being transferred. The loadbanks are composed of RLC (resistance, inductance, capacity)branches that can be changed by remote control device. Whenmicrogrid A operates in islanded mode, the battery units providethe reference voltage and frequency. Microgrid A is connectedto the external grid through M1.

Microgrid B includes three busbars: LM7, LM8, and LM9.There are a PV system, flywheel energy storage, diesel gen-erator, and two load banks. However, differently from micro-grid A, in microgrid B, the three-phase output power of the30-kW PV system passes through three single-phase inverters.Therefore, the PV system can be tested with different connec-tion forms, and the flywheel energy storage is utilized for fastbalancing to supply uninterruptible power to important loadsin the short-term during mode transition and other fault events.Then, the diesel generator starts to provide the long-term refer-ence voltage and frequency to microgrid B. The interconnectionswitch L2 between LM8 and LM9 allows microgrid B to have

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Fig. 2. Hierarchical control structure.

different topological structures with certain switch statuses. Mi-crogrid B is connected to the external grid through M2.

Details of the components of the system are as follows:1) System configuration:

a) renewable sources: PV, WT, and DFIG;b) energy storage system: BESS and flywheel energy

storage;c) controllable source: diesel generator;

2) Capacity of microsources:a) PV: 63 kW;b) WT: 10 kW;c) DFIG: 30 kW;d) diesel generator: 250 kW;

3) Capacity of energy storage systems:a) BESS: 168 kWh;b) Bi-directional inverter of BESS: 100 kW;c) Flywheel energy storage: 250 kVA;

4) Loads:a) Load 1: 30 kW 24 kVar 24 kVar;b) Load 2: 60 kW 45 kVar 45 kVar;c) Loads 3 and 4: 10 kW 8 kVar 8 kVar.

Interconnection switch L1 is designed between LM5 andLM8. When L1 is opened, microgrids A and B operate sepa-rately. When L1 is closed, microgrids A and B are combinedinto one large microgrid. In addition, the diesel generator orbattery units provide the reference voltage and frequency whenthe large microgrid operates in islanded mode. Otherwise, eachDG can be directly connected to the external grid when thereare no tests. For example, when F22 is opened with F72 closed,the output power of the 33-kW PV system flows into LM1directly.

Furthermore, in order to simulate real transmission lines,there are simulated lines with a certain resistance betweendifferent busbars. There are also five simulated fault pointsin the system, which are connected to ground only througha small resistance, to simulate different fault events. Faults 1and 4 are designed to simulate the faults outside the microgrid.On the other hand, faults 3 and 5 are designed to simulate the

faults inside the microgrid, while fault 2 is designed to simulatefaults in PCC. Through the above five simulated fault points,different fault events can be simulated easily, allowing methodsof protection of the microgrid to be studied further in thissystem.

B. Control System Structure

The microgrid system has a hierarchical control structure, asshown in Fig. 2. There are four control layers: the main sta-tion, coordinated control, the Feeder Terminal Unit (FTU) andprotection device, and microgrid control. The main station layerimplements the functions such as graphical display, monitoring,operation, management and application of historical data, re-mote and other communications (embedded), configuration, andmodification of control logic. In the coordinated control layer,the controls of the bi-directional inverter and microsources inboth grid-connected and islanded modes are managed by themode controller. Without the mode controller, the microgridsystem cannot be transited between grid-connected and islandedmodes flexibly and automatically. The FTU and protection de-vice layer is composed of the devices which are responsible forprotection and distribution terminal control. The microgrid con-trol layer, as the local controller, receives commands from thecontrol logic and implements different functions. The designedhierarchical control structure contributes to DG control, coordi-nated control, and the whole control process.

III. CONTROL STRATEGY

A. Overview

The microgrid control system is responsible for the overallcontrol and coordination of operation, including mode transi-tion, frequency control, voltage control, stability control, blackstart, and so on. In general, microgrid control includes two mainparts: a local distributed power control strategy and a system-level control mode.

The major power sources of most microgrids are the inverter-type DGs that are based on power electronic inverters which


decide the stability of the microgrid. At present, there are threeinverter-based DG control strategies: 1) constant power control,that is, PQ control; 2) constant voltage and frequency control,that is, V/F control; and 3) droop control [24]–[26].

• PQ control: The purpose of PQ control is to enable theoutput power of the DGs to equal the reference value. PQcontrol generally adjusts the decoupled active power andreactive power, respectively.

• V/F control: Regardless of the power changes of dis-tributed generation, the purpose of V/F control is to keepthe voltage and frequency of the inverter connected bussystem unchanged.

• Droop control: Droop control simulates the power-fre-quency static characteristics of the generator, which canprovide voltage and frequency support. The units adoptingdroop control can provide voltage and frequency supportseparately or combined with other support units.

Also, there are three main system-level control modes.• Master–slave control: master–slave control refers to the

operation mode in which only one DG adopts V/F con-trol to provide the reference voltage and frequency, whilethe other DGs adopt PQ control. The master control unit isusually stable output energy power.

• Peer-to-peer control: peer-to-peer control refers to the op-eration mode in which the controls of all the DGs have thesame status. The master–slave relationship does not exist.Each DG system is controlled based on the local voltageand frequency. The strategy of the DG controller is criticalfor this control mode, while a method that is currently ofinterest is the droop control mentioned previously.

• Hierarchical control: the hierarchical control generallyhas a central controller to dispatch control information.The central controller is responsible for work includingforecasting power generation and load demand, devel-oping appropriate operation plans, collecting voltage,current, power, and other status information, adjusting thereal-time operation plan, and controlling the start-stop ofDGs, load, and energy storage devices to ensure that thevoltage and frequency are stable and also provide relatedprotection.

B. Master–Slave Control

Master–slave control is adopted in the integrated microgridlaboratory system where a master control unit is needed for op-eration. A DG or energy storage system which adopts constantV/F control can serve as the master control unit, providing thereference voltage and frequency to other DGs.

Obviously, there is no need to adjust the frequency when themicrogrid operates in grid-connected mode as the external gridcan stabilize the frequency. Thus, all of the DGs in the micro-grid, adopting PQ control, only output a certain active power andreactive power. Conversely, the master control unit, adoptingV/F control, has to stabilize the voltage and frequency whenthe microgrid operates in islanded mode. The control strategymethod is shown in Fig. 3.

In this system, the bi-directional inverter is the key part inmicrogrid operation, especially when battery units serve as themaster control unit. Fig. 4 shows the portion of the structurein microgrid A. The microgrid control system is responsible for

Fig. 3. Control method of master control unit.

Fig. 4. Part structure of microgrid A.

managing the components’ control units. As shown in Fig. 4, thebi-directional inverter is composed of dc–dc and dc–ac circuits.The bi-directional inverter can operate in either grid-connectedmode or islanded mode. When the bi-directional inverter oper-ates in grid-connected mode, the external grid provides the refer-ence voltage and frequency to it, and the bi-directional inverter,adopting PQ control, controls the power flow in the dc–dc anddc–ac circuits by adjusting the dc bus voltage. When the bi-di-rectional inverter operates in islanded mode, battery units pro-vide the reference voltage and frequency to the system, and thebi-directional inverter, adopting V/F control, controls the outputvoltage and frequency by adjusting the dc bus voltage.

Therefore, two kinds of control strategies are used to operatethe bi-directional inverter. The inverter model differs accordingto the following control strategies.

1) PQ Inverter Control: The PQ controlled inverter operatesby injecting into the grid the power available at its input. Thereactive power injected corresponds to a prespecified value, de-fined locally (using a local control loop) or centrally from themicrogrid control center. As shown in Fig. 5, when operatingin grid-connected mode, the dc–dc module adopts constant cur-rent control and the dc–ac module adopts the control structurein which the dc bus voltage is the outer loop and the ac side cur-rent is the inner loop. The control’s target is to keep the dc busvoltage steady.

2) V/F Inverter Control: The V/F controlled inverter emu-lates the behavior of a synchronous machine, thus controllingthe voltage and frequency on the ac system. The inverter acts asa voltage source, with the magnitude and frequency of the outputvoltage being controlled. As shown in Fig. 6, when operating in


Fig. 5. Power control structure of grid-connected mode.

Fig. 6. Voltage and frequency control structure of islanded mode.

islanded mode, the dc–dc module adopts the double loop con-trol to keep the dc bus voltage steady, while the role of the dc–acmodule is to control the voltage and frequency, keeping themsteady. When the microgrid operates in islanded mode, if theoutput power of the DGs is smaller than the load, the bi-direc-tional inverter operates in islanded discharge mode at this time.Load that cannot be satisfied by the DGs is supplied by batteryunits. If the output power of the DGs is bigger than the load, thebi-directional inverter operates in islanded charge mode. Part ofthe output power of the DGs flows into the battery units.

C. Mode Transition Control

1) Microgrid A: The mode controller implements mode tran-sition of microgrid A by detecting voltage changes in LM2 whenmicrogrid A and microgrid B operate separately.

Case 1: External fault occurs—fault 1: Fig. 7 shows the con-trol flow chart of microgrid A when fault 1 occurs. In this case,PV and WT will disconnect from the grid when the voltage inmicrogrid A droops because of islanding protection, which al-lows harm due to non-synchronization closing to be avoided.The mode controller will open M1 when the voltage of LM2falls below the set value and remains so for a certain duration(such as 3 s, to avoid the transient effect). Then the bi-direc-tional inverter shifts to islanded discharge mode and serves asthe master control unit of microgrid A. The PV and WT will re-connect to the grid when the bi-directional inverter provides a

Fig. 7. Control flow chart of microgrid A.

steady voltage and frequency to microgrid A. Finally, microgridA operates in islanded mode.

In islanded mode, the bi-directional inverter first shifts tostandby mode when the voltage of LM2 (external grid voltage)is normal and remains so for a certain duration (such as 3 s). PVand WT disconnect from the grid because of voltage droops.The mode controller will close M1 when LM3 has no voltage.After M1 has closed, the external grid supplies microgrid Aagain. Then the bi-directional inverter changes to grid-con-nected mode, and PV and WT reconnect to the grid. MicrogridA operates in grid-connected mode again.

Case 2: Internal fault occurs—fault 2 and fault 3: When fault2 occurs in LM3, the busbar differential protection will openA2, A3, and F12. In this case, electricity failure inevitably oc-curs in microgrid A. The mode controller does not issue a modetransition instruction.

In another case, when fault 3 occurs in the feeder, the voltageof LM2 droops for a moment and the voltages in microgrid Asoon droop. PV and WT disconnect from the grid because ofislanding protection, and the bi-directional inverter changes tostandby mode. The voltage of LM2 will be established againafter the pilot protection clears the fault. In this case, the modecontroller does not issue a mode transition instruction becausethe voltage of LM2 returns to normal 3 s after the fault is cleared.Then DGs reconnect to the grid and the bi-directional inverterreverts to grid-connected mode.

2) Microgrid B: The mode controller implements mode tran-sition of microgrid B by detecting voltage changes in LM6 whenmicrogrid A and microgrid B operate separately. As microgridB has different topological structures, the situation where mi-crogrid B is operating under the condition where L2 is closedand B3 is open will be discussed.

Case 1: External fault occurs—fault 4: Fig. 8 shows the con-trol flow chart of microgrid B when fault 4 occurs. In this case,the flywheel is applicable as an energy storage device only for


Fig. 8. Control flow chart of microgrid B.

fast balancing during fault events. The mode controller will openM2 when the voltage of LM6 falls below the set value and re-mains so for a certain duration (such as 3 s). At the same time,the diesel generator starts instantly to provide voltage and fre-quency to microgrid B. After the diesel generator starts, theflywheel goes to charge mode again when the voltage of LM7is normal. The diesel generator supplies microgrid B indepen-dently at this time. Finally, microgrid B operates in islandedmode.

In islanded mode, the diesel generator first stops when thevoltage of LM6 (external grid voltage) is normal and remainsso for a certain duration (such as 3 s). Then the mode controllercloses M2. Microgrid B reconnects to the grid and operates ingrid-connected mode at this time.

Case 2: Internal fault occurs—fault 5: When fault 5 occursin microgrid B, directional pilot relaying will start to clear thefault, which leads to the exit of the flywheel. In this case, it doesnot make sense to start the diesel generator because there areno loads connected to it. Thus, microgrid B will not operate inislanded mode when fault 5 occurs.

3) One Large Microgrid: In one-large microgrid mode, L1is closed, and only one switch between M1 and M2 can beclosed at the same time. The mode controller implements modetransition by detecting voltage changes of LM2 or LM6. Thediesel generator serves as the master control unit and the energystorage system serves as the regulating unit when the microgridoperates in islanded mode as one large microgrid. In this case,regardless of the microgrid operation mode, the bi-directionalinverter operates in grid-connected mode all the time. The con-trol strategy is similar to that of microgrids A and B.

4) Black Start: In microgrid A, the battery bi-directional in-verter is the startup power source in the black start process. Afterthe battery bi-directional inverter begins operating, other DGs

connect to microgrid A and begin to work normally. In micro-grid B, the diesel generator is the startup power source in theblack start process. After the diesel generator begins operating,other DGs connect to microgrid B and begin to work normally.For one large microgrid, the battery bi-directional inverter ordiesel generator can serve as the startup power source in theblack start process.

IV. EXPERIMENTAL STUDY

Fig. 9 shows the major components of the integrated mi-crogrid laboratory system which is composed of the PVsystem, WT system, DFIG simulation system, BESS, flywheel,diesel generator, isolation transformer, bi-directional inverter,electricity meters, loads, primary equipments, and secondaryequipment. These components communicate with the manage-ment software, which is installed on a remote PC, by networkcommunication. Fig. 10 shows the supervisory software devel-oped for management of the microgrid system. It monitors anddisplays the system’s current states including power output,voltage, current, state of charge of battery storage, state offlywheel and diesel generator, and so on. Relying on thismicrogrid system, different classes of tests were performed.

As is well known, a microgrid is a small autonomous powersystem that can operate in grid-connected mode or islandedmode, and the transition between the two modes is an impor-tant issue of microgrid control. Since the microgrid laboratoryis composed of different DGs and energy storage systems, it issignificant to study and implement the transition process in thissystem. Thus, experiments carried out with regard to this topicare mainly discussed next.

A. Microgrid A

1) Mode Transition of Microgrid A: In this experimentalcase, the control performance of mode transition of microgridA is evaluated. Fig. 11 shows the dynamics of transition of mi-crogrid A from grid-connected to islanded mode. When the gridside switch opens at T1 because of protection or maintenance,the voltage of LM2 droops immediately. When this situationlasts for 3 s, the mode controller opens M1 at T2 and ordersthe bi-directional inverter to shift from grid-connected chargeto islanded discharge mode. The voltage is established within2.5 s and microgrid A operates in islanded mode at this time.Fig. 12 shows the dynamics of transition of microgrid A fromislanded to grid-connected mode. The voltage of LM2 returnsto normal at T1. The mode controller orders the bi-directionalinverter to switch from islanded discharge to standby mode afterensuring the normal state, and then closes M1 at T2 when LM3has no voltage. Microgrid A is supplied by the external grid andthe bi-directional inverter shifts to grid-connected charge mode.Microgrid A operates in grid-connected mode again. PV andWT reconnect to the grid later as it takes several seconds for thePV and WT inverters to connect to the grid and work again.

At this stage, the battery bi-directional inverter does not havethe ability to achieve seamless transition. Thus, the voltage ofthe battery bi-directional inverter becomes zero during modetransition. After the voltage becomes zero, the DGs disconnectfrom the microgrid because of islanding protection, which al-lows harm due to non-synchronization closing to be avoided.


Fig. 9. Components in the microgrid (site photographs).

In the transition of microgrid A from grid-connected mode toislanded mode, there is actually a black start process with bi-di-rectional inverter when the battery bi-directional inverter startsas master control unit which adopts V/F control to provide ref-erence voltage and frequency. After the bi-directional inverterprovides reference voltage and frequency, PV and WT recon-nect to microgrid A again and then work normally. Thus, wecan see the starting transients and voltage conditions of a blackstart process with power electronic devices in Fig. 11 that thestartup waveforms are smooth, without violent fluctuations inthe microgrid system.

It must be noted that the time scales of dynamic events shownin Figs. 11–18 are not uniform. The sampling rate of the faultrecorder changes in different situations so that the sampling rateis low in the steady state and high in the transient state, as canbe seen from Figs. 11 –18. In these figures, “m: s: ms” in thehorizontal coordinate means “minute: second: millisecond.”

2) Startup Characteristics: The startup characteristics of thebattery storage system are shown in Fig. 13. In this test condi-tion, the 100-kW bi-directional inverter charges the battery at23.9 kW in grid-connected mode and its startup time is approx-imately 5 ms. As we can see, the startup waveforms of the bi-di-rectional inverter are smooth, without violent fluctuations.

B. Microgrid B

1) Mode Transition of Microgrid B: In this experimentalcase, the control performance of mode transition of microgridB is evaluated. The dynamics of transition of microgrid B fromgrid-connected to islanded mode are shown in Fig. 14. Whenthe grid side switch opens at T1 because of protection or mainte-nance, the voltage of LM6 droops immediately. When this situa-tion lasts for 3 s, the mode controller of microgrid B opens M2 at

T2 and sends the command to start the diesel generator and thencloses F61 after a short time delay. Microgrid B soon shifts toislanded mode, and the flywheel for fast balancing provides un-interruptible power-supply services to the important load in co-operation with the diesel generator during the transient process.Fig. 15 shows the dynamics of transition of microgrid B fromislanded to grid-connected mode. The voltage of LM6 returnsto normal at T1. The mode controller sends a command to stopthe diesel generator after ensuring the normal state, and thencloses M2 when LM7 has no voltage. Microgrid B operates ingrid-connected mode, supplied by the external grid again.

2) Startup Characteristics: The startup characteristics of fly-wheel energy storage are shown in Fig. 16. The startup timeof the 250-kW flywheel energy storage is 15.25 s. The startupwaveforms of flywheel energy storage are smooth.

These experimental results in part A and B indicate that thecontrol strategy can implement the mode transition successfullyand the microgrid system is able to operate stably and reliablyunder different conditions.

C. Protection Tests

1) Fault Events in Grid-Connected Mode: The protectionperformances are also evaluated in this system. Fig. 17 showsthe dynamics of fault events of microgrid A in grid-connectedmode with the initial condition that the bi-directional inverteroperates in discharge mode. Meanwhile, the battery dischargepower is 25.7 kW, the PV system output power is 5.3 kW, theresistive load is 30 kW, and the inductive load is 5 kVar. TheBC-phase ground fault is simulated at the location of fault 1 andthe fault current is 102 A. In order to isolate the fault, feederautomation opens A1 and A2 after detecting it. The islandingprotection of PV and WT then starts. The isolation setting timesare both 1 s while the actual breaking times are 1.40 and 1.71 s.


Fig. 10. Developing supervisory software (only the Chinese version exists so far).

Fig. 11. Dynamics of transition of microgrid A from grid-connected to islandedmode.

Fig. 12. Dynamics of transition of microgrid A from islanded to grid-connectedmode.

2) Fault Events in Islanded Mode: The dynamics of faultevents of microgrid A in islanded mode are shown in Fig. 18,with the initial condition that the battery discharge power is

Fig. 13. Dynamics of startup of battery storage system.

Fig. 14. Dynamics of transition of microgrid B from grid-connected to islandedmode.

9 kW, the PV system output power is 10.5 kW, the resistive loadis 20 kW, and the inductive load is 5 kVar. The A-phase groundfault is simulated in the location of fault 3, which is located at


Fig. 15. Dynamics of transition of microgrid B from islanded to grid-connectedmode.

Fig. 16. Dynamics of startup of 250 kW flywheel storage system.

Fig. 17. Dynamics of fault events of microgrid A in grid-connected mode.

the midpoint of line A3–A4, and the fault current is 112 A. Thefeeder automation opens A3 and A4 to isolate the fault afterdetecting it. The islanding protection of PV and WT then starts.The isolation setting times are both 1 s while the actual breakingtimes are 1.5 s and 1.52 s.

These experimental results indicate that the protection systemcan clear the faults accurately and immediately, which plays animportant role in improving the system control performance.

Fig. 18. Dynamics of fault events of microgrid A in islanded mode.

Fig. 19. Harmonic voltage spectrum of microgrid A.

D. Power Quality Test

The power quality tests are carried out in normal operationand test conditions. Figs. 19 and 20 show the harmonic voltagespectrum and harmonic current spectrum of the grid-connectedpoint M1 of microgrid A. The tests are carried out from 21:30to 05:00 the next day. The average statistical method is adopted,and the results are shown in Table I. The results show that mi-crogrid A supplies high-quality power. The harmonic voltagespectrum and harmonic current spectrum of the grid-connectedpoint M2 of microgrid B are shown in Figs. 21 and 22, respec-tively. The tests are carried out from 00:00 to 14:00, and theresults, obtained using the same statistical method as those ofTable I, are shown in Table II. From the results, we can see thatmicrogrid B has good quality power.

Following further analysis, some differences are found in thepower quality of microgrids A and B. First, in order to satisfy theobjective of the test, the converters installed in microgrids A andB are produced by different manufacturers, and thus inevitablyhave different performances. What is more, in microgrid A, thethree-phase PV is connected through one three-phase converter,while in microgrid B, the three-phase PV is connected throughthree single-phase converters, as has been discussed above. Thisfactor also contributes to the differences in power quality be-tween the two small microgrids. During the power quality test,the influence of WT is insignificant because of the poor wind re-sources. So the harmonics are mainly decided by PV converters,and the differences in the PV converters mentioned above even-tually cause the differences in power quality.


Fig. 20. Harmonic current spectrum of microgrid A.

Fig. 21. Harmonic voltage spectrum of microgrid B.

Fig. 22. Harmonic current spectrum of microgrid B.

TABLE IPOWER QUALITY TEST RESULT OF MICROGRID A

Power quality tests of one large microgrid have also beencarried out. The results of the power quality tests in normalconditions show that the power quality meets all the nationalstandards. The microgrid system has the ability to provide ahigh-quality power supply to system loads.

TABLE IIPOWER QUALITY TEST RESULT OF MICROGRID B

V. CONCLUSION

An integrated microgrid laboratory system with multipleDGs and energy storage systems was developed and tested.The system structure was flexible as the microgrid systemhad several different topological structures according to dif-ferent requirements. Control strategies for application duringgrid-connected mode, islanded mode, and mode transition wereproposed. Many tests of the control strategy, protection, andpower quality were carried out to obtain the optimal controlmethod and operating conditions. The tests demonstrated theflexibility of mode transition, stable behavior under differentconditions, and high quality of power supply. All the resultsshow that the microgrid system performs as expected and has ahigh level of robustness.

As most components of the microgrid laboratory are real, itcan also serve as a verification platform for engineering applica-tions. Since microgrid projects are often constructed in remoteareas, such as islands, it is difficult to validate the functions ofequipments due to the adverse power situation. Thus, debuggingand validation of equipment, such as converters, can be carriedout with the aid of the microgrid laboratory. Because of the flexi-bility and controllability of the microgrid laboratory, the controlsystems of the project can be debugged and validated more con-veniently and easily by simulating different working conditions.We can adopt a verified method in the microgrid laboratory andthen transfer it to projects. In this way, the microgrid laboratoryhas played an important role in our construction of an islandmicrogrid project, Dongfu island wind-solar-diesel-battery-sea-water desalination project. Thus, the microgrid laboratory alsohas a vital role to play in the construction of microgrid projects.Relevant information about the project will be presented in de-tail in the future.

In the second phase of this integrated microgrid laboratorysystem, a monitoring system based on the IEC 61850 standardwill be imported. The whole microgrid system will be connectedto a distribution automation system. Additionally, more typesof energy storage, such as compressed air, lithium batteries,sodium–sulphur batteries, super-capacitors, and electric vehicledevices will be employed. The integrated microgrid laboratorysystem will be further improved. Furthermore, continuing workwill be carried out in the near future in order to conduct furtherresearch on key issues.

ACKNOWLEDGMENT

The authors would like to thank all research teams of the mi-crogrid project for valuable discussions.


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Bo Zhao was born in Guizhou, China, in 1977. He received the Ph.D. degreefrom the Department of Electrical Engineering, Zhejiang University, Hangzhou,China, in 2005.

He is currently an Engineer with the Research Center, Zhejiang ElectricPower Test and Research Institute, Zhejiang, China. His research interestsinclude distributed generation and microgrids.

Xuesong Zhang was born in Hebei, China, in 1979. He received the Ph.D.degree from the Department of Electrical Engineering, Zhejiang University,Hangzhou, China, in 2006.

He is currently an Engineer with the Research Center, Zhejiang ElectricPower Test and Research Institute, Zhejiang, China. His research interestsinclude relay protection and microgrids.

Jian Chen was born in Shandong, China, in 1986. He is currently working to-ward the Ph.D. degree at the School of Electrical Engineering and Automation,Tianjin University, Tianjin, China.

His research interests include design and operation optimization issues inmicrogrids.

Integrated Microgrid Laboratory System

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