36 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 1, NO. 1, MARCH 2015

An Integrated Control and Protection System forPhotovoltaic Microgrids

Laijun Chen, Member, IEEE and Shengwei Mei, Fellow, IEEE

Abstract—The microgrid has shown to be a promising solutionfor the integration and management of intermittent renewableenergy generation. This paper looks at critical issues surroundingmicrogrid control and protection. It proposes an integratedcontrol and protection system with a hierarchical coordinationcontrol strategy consisting of a stand-alone operation mode, agrid-connected operation mode, and transitions between thesetwo modes for a microgrid. To enhance the fault ride-throughcapability of the system, a comprehensive three-layer hierarchicalprotection system is also proposed, which fully adopts differentprotection schemes, such as relay protection, a hybrid energystorage system (HESS) regulation, and an emergency control.The effectiveness, feasibility, and practicality of the proposedsystems are validated on a practical photovoltaic (PV) microgrid.This study is expected to provide some theoretical guidance andengineering construction experience for microgrids in general.

Index Terms—Control strategies, integrated protection, micro-grid, operation modes.

I. INTRODUCTION

PHOTOVOLTAIC (PV) technologies have receivedwidespread attention in recent years owing to their

ability to reduce fossil energy use and provide positiveimpacts to the environment. Photovoltaic generation in theform of distributed photovoltaic microgrids that are integratedinto the power system rely on efficient use of solar energy[1], [2]. When compared to traditional distribution networks,photovoltaic microgrids are distinctly different in termsof their control strategies and protection methods [3], [4].Specifically, when PV microgrids are being operated inisolated mode, improving peer-to-peer control strategiesare considered as critical factors for supporting islandedmicrogrid operations [5]. In [6], the authors have presenteda coordinated voltage/frequency (V/F) and active powerand reactive power (PQ) control system for both islandedand grid-connected mode in a PV microgrid [6] that showseffective coordination between inverter V/F (or PQ) controls.

Manuscript received November 25, 2014; revised January 25, 2015 andFebruary 9, 2015; accepted February 13, 2015. Date of publication March30, 2015; date of current version March 4, 2015. This work was supportedby the National High Technology Research and Development of China863 Program under Grant 2012AA050204, China, and the National NaturalScience Foundation of China under Grant 51321005, 51207076.

L. J. Chen is with State Key Laboratory of Power System, Department ofElectrical Engineering, Tsinghua University, Beijing 100084, China (e-mail:chenlaijun@tsinghua.edu.cn).

S. W. Mei is with State Key Laboratory of Power System, Department ofElectrical Engineering, Tsinghua University, Beijing 100084, China (e-mail:meishengwei@mail.tsinghua.edu.cn).

Digital Object Identifier 10.17775/CSEEJPES.2015.00005

Several other strategies have been proposed for seamlesstransfer between different microgrid operation modes [7]–[9]. They include a seamless control methodology for aPV-diesel generator microgrid that can operate both in thegrid-connected and islanded modes, and at the same timedoes not require any islanding detection mechanism [7].Similarly, in [8], a control strategy has been proposed thatcontains the control state/reference compensation algorithm toeffectively reduce the impact caused by microgrid operationmode transitions on critical loads and distributed generators(DGs).

The control strategies mentioned above provide excellentsolutions for microgrid operational control. However, thesestrategies are relatively independent having poor flexibilityand weak expansibility, which may lead to collapse when themicrogrid contains multiple distributed generators. Therefore,there is a need to integrate microgrid operational controltechnologies at steady and transient states in more practicalways.

In addition to advanced control technologies, microgridsalso require effective protection systems [10]–[12]. Themaximum short-circuit current in a microgrid is generallylimited to less than two times the rated current because ofa large number of DGs configured with power electronicinterface devices [13]. Power flow and short-circuit capacitychanges are significant under different microgrid operationmodes. As a consequence, conventional protection methodsin large-scale power grids are not able to effectively meet theneeds of an inverter-dominated microgrid.

A range of advanced methodologies is available in theliterature for microgrid protection. They include a simplethree-phase four-wire system with differential current andzero sequence current used to detect faults in a microgrid[13]; a protection scheme that uses both modes of operationfor optimally setting direction over current relays [14]; as wellas other protection schemes with voltage restraint algorithmsor inverse time characteristics [15], [16]. All these systemscan be adapted to address the frequent changes in a microgrid.However, the main focus of these aforementioned methodsis on relay protection, and they tend to neglect regulationresources and means available in the microgrid, e.g., energystorage systems. As a result, there is a need to further exploreand develop integrated microgrid protection systems.

Many recently built microgrid laboratory systems have beenbased on the above mentioned control strategies and protection

2096-0042 c© 2015 CSEE

CHEN et al.: AN INTEGRATED CONTROL AND PROTECTION SYSTEM FOR PHOTOVOLTAIC MICROGRIDS 37

schemes [17]–[19]. However, microgrids that are integratedwith advanced control and protection systems have receivedrelatively less attention.

This paper addresses key issues of PV microgrid controland protection. The rest of the paper is organized as follows:Section II presents the hierarchical coordination control sys-tem. The integrated and layered protection system is describedin detail in Section III. The field applications and test resultsare demonstrated in Section IV, and the conclusion is given inSection V.

II. HIERARCHICAL CONTROL SYSTEM

A. System Structure

Grid-connected and stand-alone operations are the twotypical operation modes in a microgrid. The requirementsof PV microgrid operation modes include: 1) The microgridvoltage and frequency should be stable and the power flowshould be balanced, so as to realize the independent operationin different modes; 2) The two modes can transfer smoothlyfrom one to the other, which can help avoid transient surge inthe microgrid. The proposed hierarchical coordination controlarchitecture is shown in Fig. 1.

Fig. 1. Control structure of PV microgrid.

Fig. 1 shows the three-layered hierarchical control archi-tecture of a PV microgrid in which the microgrid operationmodes are interchangeable based on the control strategiesand some basic techniques. The microgrid control strategiesmainly include two parts: the system-level control modesand the device-level control strategies. The first part includes

the peer-to-peer and the master-slave controls. The secondpart mainly refers to the idiographic control techniques, usedin the local controllers, such as V/F control, PQ control,and droop control. The proposed structure is flexible so thatdifferent control strategies and basic techniques can be appliedto realize different microgrid operation modes. This papermainly discusses the master-slave control combined with V/Fcontrol and PQ control, which is demonstrated in the followingsections.

B. Stand-alone Operation

When the microgrid operates in stand-alone mode, the Li-battery energy storage system (BESS) is the main powersource for providing stable voltage and frequency with theV/F control [20]. To improve the practical application of thissystem, the proposed V/F control block diagram is shown inFig. 2.

PLL

abc

to

dq

SVPWM

generator

Magnitude

and phase

angle

calculation

PI

drefv

qv

qrefv

PI

v

dvbv

brefv

crefv

+

+arefv

av cv

PWM pulse

drv

qrv

Fig. 2. VF control block diagram.

In Fig. 2, varef , vbref , and vcref denote the reference ofthe three-phase output voltage of BESS. vd and vq representthe d axis and q axis component of the measured threephase voltage (va, vb, and vc) based on the dq coordinatetransform, respectively. vdref and vqref are the d axis and q axiscomponent of the reference voltage (varef , vbref , and vcref ),respectively.

The proposed V/F control mainly corresponds to layer2 in Fig. 1, which is based on the coordinate transformand proportion integration (PI) regulation with some basictechniques, such as magnitude and phase angle calculation andspace vector pulse width modulation (SVPWM). The proposedV/F control has good dynamic response though it only adoptsvoltage loops. Moreover, only two PI regulators are includedin the V/F control, which is more effective than the traditionalV/F control.

C. Grid-connected Operation

The BESS is controlled as a power buffer to provide powerflow with PQ control when the microgrid operates in grid-connected mode.

Based on the fact that the current can be obtained frompower and voltage, a simplified PQ control is proposed as inFig. 3.

In Fig. 3, the quantities of current reference are obtained byutilizing the instantaneous power theory, and the current loopsare used to regulate the output value with PI regulators. Inaddition, the SVPWM technique is also adopted in generating

38 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 1, NO. 1, MARCH 2015

SVPWM

generatorto

dq αv

βv

PWM pulse

+- di

drv

+-

qi

qrv

++

dv

T( + )L L

qv

-

+

++

PI

PI

dri

qri

refP

refQ

refdr

d

refqr

d

=1.5

-=

1.5

Pi

v K

Qi

v K

αβT( + )L L

Fig. 3. PQ control block diagram.

PWM signals.The proposed PQ control strategy, as shown in Fig. 3, with

two PI control units reduced and the decoupling control ofactive and reactive power realized, has more applicability inengineering and is equivalent to the traditional control methodsin both power loops and current loops.

D. Operation Modes Transition

1) Transition from Stand-alone Mode to Grid-connectedMode: Self-synchronization control is adopted to realize theseamless transfer from stand-alone mode to grid-connectedmode. The detailed stages are demonstrated as follows.

Stage 1: Preparation—voltage regulation.The microgrid voltage is regulated through V/F control with

the calculated voltage reference. Take the voltage magnitudecontrol scheme as an example; the control block diagram isshown in Fig. 4.

V/Fcontrol

+- iV

++

refiV

PgridiV

refiV

Fig. 4. The voltage magnitude regulation control.

In Fig. 4, Vi−grid represents the power grid voltagemagnitude of phase A/B/C, and Vi is the voltage of phaseA/B/C of the microgrid, respectively. In the control block,only proportion control is utilized due to the fact that thevoltage is of AC sinusoidal quantity. The regulated voltage(V

′

i ) is applied to V/F control.Then, the voltage magnitude, phase angle, and frequency

are adjusted consistently with those of the main grid, asdescribed in (1). |fmic − fgrid| ≤ flimit

((|Vgrid| − |Vmic|)/|Vgrid|)× 100% ≤ Vlimit

|θmic − θgrid| ≤ θlimit

(1)

where f , V , and θ represent the frequency, voltage magnitude,and voltage phase, respectively. The subscript “mic” and“grid” denote the microgrid and power distribution grid. Thesubscript “limit” means the threshold of those variables.

Stage 2: Contactor states transition.After the voltage is synchronized with the main grid, the

grid-connected contactor should be turned off by the BESS,in which an I/O output signal for turning off the contactorinstalled on the point of common coupling (PCC) is generated.

Stage 3: Control modes transition.The V/F control mode is immediately switched to PQ

control as the contactor is turned off. At this point, thetransition from stand-alone mode to grid-connected mode isachieved.

2) Transition from Grid-connected Mode to Stand-aloneMode: In general, self-isolation control is utilized to ensurethe smooth transfer from grid-connected mode to stand-alonemode. However, to describe briefly, this paper mainly takes theintentional islanding as an example. The stages are describedas follows.

Stage 1: Preparation.The power flow regulation is the first stage, which can be

achieved by adjusting the power reference of the BESS. Thecorresponding control block diagram is depicted as in Fig. 5.

Fig. 5. The power flow regulation control.

In Fig. 5, PPCC−ref and QPCC−ref means the expectedactive power and reactive power on the tie line, which arepreset as zero. PPCC and QPCC are the measured power onthe tie line. The reference power values (P

′

ref , Q′

ref ) put intothe PQ control module are obtained from proportion controland difference calculation, which are described in Fig. 5.

Then, the power flow on the tie-line should be regulated tosatisfy (2). {

Ppcc ≤ Plimit

Qpcc ≤ Qlimit(2)

where Plimit and Qlimit denote the threshold of the activepower and reactive power, respectively.

Stage 2: Contactor states transition.After performing the power flow regulation control, the

BESS can rapidly turn on the grid-connected contactor bydelivering an I/O input signal to open the PCC contactor.The power interaction between the microgrid and the powerdistribution grid is so little that there is very little transientsurge when the contactor opens.

Stage 3: Control modes transition.The PQ control mode should be quickly changed to the V/F

control mode as the contactor is turned on. The transition fromgrid-connected mode to stand-alone mode is then completed.

III. HIERARCHICAL PROTECTION SYSTEM

A. System Structure

The microgrid has many advantages when compared tothe bulk power grid because of its smaller scale and fewerfeeders. More information can be acquired through reasonableconfiguration of measurements and establishment of a data

CHEN et al.: AN INTEGRATED CONTROL AND PROTECTION SYSTEM FOR PHOTOVOLTAIC MICROGRIDS 39

center, which provide possibilities for designing a compre-hensive protection system. A hierarchical protection system isproposed as shown in Fig. 6.

The proposed hierarchical protection system, consisting ofrelay protection, HESS regulation, and emergency control,is aimed at improving microgrid security. Compared to atraditional protection scheme having only relay protection, thehierarchical protection system not only clears the fault, but alsodeals with adjustments of HESS and load shedding. Therefore,it has many advantages. First, it is straightforward with threelayers of hierarchical structure; second, many techniques, suchas relay protection, hybrid energy storage system (HESS)regulation, and emergency control are integrated to improvethe reliability of the PV microgrid. The details of the three-layer hierarchical protection system is depicted in Section III,B–D.

Fig. 6. Structure of the hierarchical protection system.

B. Relay ProtectionRelay protection in PV microgrids can be classified into

three categories: system protection, feeder protection, andelement protection. Among these, feeder protection is a re-search priority for PV microgrid protection. In general, themicrogrid feeders can be divided into three types where thefeeder contains only either the source or the load, or thefeeder contains both source and load. Different relay protectionschemes should be applied on different feeders due to theirdifferent characteristics.

1) Feeder Only Contains Source: For this kind of feeder,the power flows from the feeder to the bus. It could bedetermined that the short circuit fault occurs in the feeder onlywhen the bus voltage and feeder current meet the followinginequality [21]:

cos(argUk

Ik+ α) < 0 (3)

where α is interior angle of the transformer. The directionalpower protection (DPP) method can be configured on this typeof feeder that only contains source.

2) Feeder Only Contains Load: A sequence componentprotection (SCP) method is proposed for this issue. The mainprinciple of SCP is described as follows.

When a fault occurs in a certain load feeder, an equiv-alent negative/zero sequence source is added to the faultlocation, and the negative/zero sequence component spreadsfrom the fault location to other lines of the network. Thus,the negative/zero sequence component of the fault line isrelatively larger than other lines. Therefore, the fault line couldbe located by searching for the largest component amongdifferent feeders, which is expressed as (4).

Lfault = Lk|{(Ink = max(Izi, Ini))or(Izk = max(Izi, Ini))}i = 1, 2, . . . , n

(4)where Lfault denotes the fault feeder and Li represents theith feeder. Ini and Izi are the negative-sequence current andzero-sequence current of the ith feeder, respectively.

Another key point of the SCP is to determine when tostart the searching procedure as described in (4). When A-B fault, A-G fault or A-B-G fault happens, there is anobvious rise in the negative/zero sequence voltage, which canbe used as the trigger signal. Moreover, back-up protection,such as over-current protection can be configured to clearthe symmetric fault, which does not produce a negative/zerosequence component.

3) Feeder Contains both Source and Load: Inside thefeeder, the DPP method can be applied in the branches withsources; SCP may be utilized in the branches with loads, andthe delayed SCP (DSCP) can be installed in the terminal ofthe feeder. The DSCP means that if the negative sequencevoltage and current is greater than the thresholds in short delayduration, the breaker on the main feeder will trip. In this way,the DSCP method can avoid the risk of the whole feeder beingcut when the fault occurs in the branches with source or thebranches with load.

C. HESS Regulation

The hybrid energy storage system (HESS) that consists ofenergy density unit (such as a battery) and the power densityunit (such as a super capacitor (SC)) is widely used in amicrogrid. To make full use of the HESS, an HESS regulationscheme is designed as shown in Fig. 7.

In Fig. 7, the battery is utilized as the main power thatis a response to the power vacancy with low frequency; thesuper capacitor is responsible for the power vacancy withhigh frequency. The cooperation of the HESS power outputregulation can effectively enhance the stability of the entiremicrogrid.

D. Emergency Control

The emergency control is set as the last defender in aPV microgrid. It should start working when the HESS isunable to maintain the stability of the voltage and frequencyof the microgrid. The flow chart of the emergency control isdescribed as in Fig. 8.

40 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 1, NO. 1, MARCH 2015

Obtain power vacancy

End

Power balanced?

Y

N

Start

Calculate high frequency component(Pref_h) and low

frequency component (Pref_l)

Battery regulation

SC regulation

Pref_l Pref_h

Fig. 7. The HESS scheme.

Recover loads or cut off PV units

Y

End

N

Y

Start

Obtain power vacancy( )

Pcut>0?

Recover PV units or cut off loads

N

Calculate the power to be cut off(Pcut)

beyond HESS regulating abilitity？

Obtain the new power vacancy( )'P

'P

P

Fig. 8. The emergency control scheme.

As shown in Fig. 8, the microgrid power vacancy (∆P ) iscalculated using the data obtained from the data center, andthen the equivalent power to be cut off (Pcut) can be computedby (5).

Pcut = |∆P | − |Pmax − Pact| (5)

where Pact and Pmax represent the actual and the max-imum output power of the HESS, respectively. Moreover,the executing procedure of the emergency control can becomprehensively determined due to some influence factors,such as the state of charge (SOC) of the HESS, the quantitiesof PV units that can be restored, and the priority level of theloads.

IV. MICROGRID PROJECT AND FIELD TESTS

A. Description About the Project

A PV microgrid integrated with the proposed control andprotection systems is built under a research project with thefund of the State Grid Corporation of China (SGCC). Thetopology of the project is shown in Fig. 9.

DC/AC

Li-

Battery

Load 1

40 kW

PV units (100 kW)

DC/AC

HESS

Load 2

40 kW

Controllable loads

10 kV bus

0.4 kV/10 kV

AC bus

PCC contactor

SC

50 kWh 0.33 kWh30 kW 10 kW

Load 2

40 kW

DC/ACDC/AC

30 kW 30 kW

Feeder 1 Feeder 2 Feeder 3 Feeder 4

PV1 PV2 PV3 PV4

Feeder 5

Fig. 9. Topology of the PV microgrid.

The details of the PV microgrid configuration are depictedas follows: 1) renewable sources: 60 kW PV shed and 40kW PV roofs; 2) HESS: 50 kWh Li-batteries and 0.33 kWhsuper capacitors; 3) loads: 120 kW controllable loads withthree different priority levels. It should be noted that differenttypes of inverters, including single-phase inverter, three-phaseinverter, and micro-inverter are installed in the PV microgrid.

B. Field Applications and Tests

To validate the proposed control strategies and protectionschemes, many tests have been undertaken on the PV micro-grid.

1) Stand-alone Operation: In this test, to realize a smoothstart, the output voltage reference is raised from 0 to 220 V(RMS) and the frequency is set as 50 Hz. The test result isshown in Fig. 10.

In Fig. 10, it can be seen that the RMS of the microgridvoltage is about 223 V/50 Hz with little harmonic distortion,which can provide stable voltage and frequency for the loadsin islanded mode.

2) Grid-connected Operation: Under this condition, theexpected output and input active power are both set as 10 kW.The voltage and current waveform of the tie line is shown inFig. 11.

Test results show that the power interaction between themicrogrid and power grid is adjusted from 10 kW to -10 kW,which well meets the demands of the grid-connected mode.

3) Operation Modes Transition: In this case, the outputpower of PV units is about 15 kW, the load is 5 kW, and thetie-line power flow is controlled at 10 kW when the microgridoperates on grid-connected mode.

CHEN et al.: AN INTEGRATED CONTROL AND PROTECTION SYSTEM FOR PHOTOVOLTAIC MICROGRIDS 41

Fig. 10. The output voltage in stand-alone operation.

Fig. 11. The power flow in grid-connection operation.

From Figs. 12 and 13, it can be seen that the transition be-tween stand-alone mode and grid-connected mode is smooth.The transient state of the transition is within 4 ms, whichmeans that the seamless transfer between those two modes isachieved.

Fig. 12. Transition from stand-alone mode to grid-connected mode.

Fig. 13. Transition from grid-connected mode to stand-alone mode.

4) Protection Tests: The proposed protection system hasbeen integrated into the built microgrid, and the simulationsare carried out to validate the effectiveness and feasibility ofthe protection schemes.

The PV microgrid is isolated from the power grid with

the PCC contactor opened. Meanwhile, the Li-battery and SCoutput power range are 50 kW and the states of the microgridbefore the fault are shown in Table I.

A fault occurs at feeder 1 (in Fig. 9) and then 25 kW PVis lost. The effectiveness of three different protection schemesare compared in terms of load shedding under the same fault.Test results are shown in Table II.

TABLE IPOWER STATES OF PV MICROGRID (IN KW)

PV1 PV2 PV3 Load 1 Load 2 Load 3 Battery SC

25 25 20 40 40 37 +50 0

TABLE IIEFFECTS OF DIFFERENT PROTECTION SCHEMES

Fault Location Protection Scheme Load Lost (kW)

¬ 117

Feeder 1 ¬+® 37

¬+­+® 0

Protection schemes: ¬ Layer 1: relay protection; ­ Layer 2: HESS regulation;® Layer 3: emergency control.

As shown in Table II, the proposed three-layered hierarchi-cal protection system is the most effective and stable schemewith the least load shedding.

Moreover, the bus voltage in the microgrid are comparedamong the three protection schemes as shown in Table II.When fault occurs at feeder 1, Figs. 14, 15, and 16 give theresults as shown.

Fig. 14. Bus voltage with protection ¬.

From Fig. 14, it can be seen the system eventually collapseswhen only relay protection is utilized. When the microgrid isapplied with relay protection and load shedding, the systemloses 37 kW load, shown in Fig. 15. In Fig. 16, the systemfinally is restored to stable state and loses no load when thehierarchical protection system is adopted.

42 CSEE JOURNAL OF POWER AND ENERGY SYSTEMS, VOL. 1, NO. 1, MARCH 2015

Fig. 15. Bus voltage with proposed protection ¬+®.

Fig. 16. Bus voltage with protection ¬+­+®.

V. CONCLUSION

In this paper, an integrated protection and control systemwith a hierarchical structure is proposed and a 100 kWpphotovoltaic microgrid is built to validate the effectivenessand feasibility of the proposed strategy. Test results showthat stable and flexible transition between different operationmodes of the PV microgrid are achieved and the viability ofthe micro-grid under severe fault is greatly improved.

REFERENCES

[1] N. Hatziargyriou, H. Asano, R. Iravani, and C. Marney, “Microgrids,”IEEE Power Energy Magzine, vol. 5, pp. 78–94, 2007.

[2] M. Ding, Y. Xue, M. Mao, and L. Chang, “Research and development ofdistributed generation in China,” in Proc. 2009 IEEE Power & EnergySociety General Meeting, July 2009, pp. 1–5.

[3] C. Wang, Y. Li, K. Peng, B. Hong, Z. Wu, and C. Sun, “Coordinatedoptimal design of inverter controllers in a micro-grid with multipledistributed generation units,” IEEE Transactions on Power Systems,vol. 28, no. 3, pp. 2679–2687, Aug. 2013.

[4] M. Ding, Y. Zhang, and M. Mao, “Key technologies for microgrids beingresearched,” Power System Technology, vol. 33, no. 11, pp. 6–11, 2009.

[5] J. Peas Lopes, C. Moreira, and A. Madureira, “Defining control strate-gies for microgrids islanded operation,” IEEE Transactions on PowerSystems, vol. 21, no. 2, pp. 916–924, May 2006.

[6] S. Adhikari and F. Li, “Coordinated V-f and P-Q control of solarphotovoltaic generators with mppt and battery storage in microgrids,”IEEE Transactions on Smart Grid, vol. 5, no. 3, pp. 1270–1281, May2014.

[7] S. Mishra, D. Ramasubramanian, and P. Sekhar, “A seamless controlmethodology for a grid connected and isolated PV-diesel microgrid,”IEEE Transactions on Power Systems, vol. 28, no. 4, pp. 4393–4404,Nov. 2013.

[8] W. Zhang, D. Xu, X. Li, R. Xie, H. Li, D. Dong, C. Sun, and M. Chen,“Seamless transfer control strategy for fuel cell uninterruptible powersupply system,” IEEE Transactions on Power Electronics, vol. 28, no. 2,pp. 717–729, Feb. 2013.

[9] C. Wang, X. Li, L. Guo, and Y. Li, “A seamless operation modetransition control strategy for a microgrid based on master-slave control,”in Proc. 31st Chinese Control Conference (CCC), 2012, pp. 6768–6775.

[10] Z. Zhang, Y. Li, and W. Chen, “Research on the microgrid protectionrelay,” in Proc. 2012 Innovative Smart Grid Technologies-Asia Confer-ence, 2012, pp. 1–6.

[11] M. A. Redfern and H. Al-Nasseri, “Protection of micro-grids dominatedby distributed generation using solid state converters,” in Proc. IET 9thInternational Conference, 2008, pp. 670–674.

[12] P. P. Barker and R. W. de Mello, “Determining the impact of distributedgeneration on power systems,” in Proc. 2000 IEEE Power EngineeringSociety Summer Meeting, 2000, pp. 1645–1656.

[13] H. Nikkhajoei and R. H. Lasseter, “Microgrid protection,” in Proc. 2007IEEE Power Engineering Society General Meeting, 2007, pp. 1–6.

[14] W. Najy, H. Zeineldin, and W. Woon, “Optimal protection coordina-tion for microgrids with grid-connected and islanded capability,” IEEETransactions on Industrial Electronics, vol. 60, no. 4, pp. 1668–1677,April 2013.

[15] T. Loix, T. Wijnhoven, and G. Deconinck, “Protection of microgridswith a high penetration of inverter-coupled energy sources,” in Proc.2009 CIGRE/IEEE PES Joint Symposium: Integration of Wide-ScaleRenewable Resources into the Power Delivery System, July 2009, pp.1–6.

[16] M. Dewadasa, R. Majumder, A. Ghosh, and G. Ledwich, “Control andprotection of a microgrid with converter interfaced micro sources,” inProc. 2009 IEEE International Conference on Power Systems, 2009, pp.1–6.

[17] C. Wang, Z. Yang, and K. Cheng, “Construction, operation and controlof a laboratory-scale microgrid,” in Proc. 3rd International Conferenceon Power Electronics Systems and Applications, May 2009, pp. 1–5.

[18] R. Lasseter, J. Eto, B. Schenkman, J. Stevens, H. Vollkommer, D. Klapp,E. Linton, H. Hurtado, and J. Roy, “Certs microgrid laboratory test bed,”IEEE Transactions on Power Delivery, vol. 26, no. 1, pp. 325–332, Jan.2011.

[19] B. Zhao, X. Zhang, and J. Chen, “Integrated microgrid laboratorysystem,” IEEE Transactions on Power Systems, vol. 27, no. 4, pp. 2175–2185, Nov. 2012.

[20] C. Hou, X. Hu, and D. Hui, “Design and implementation of grid-connected converter for lithium battery energy storage system,” PowerSystem Technology, vol. 36, no. 3, pp. 246–251, 2012.

[21] A. Sallam and O. Malik, Protection of Electric Distribution System.New York: Wiley, 2011.

Laijun Chen (M’12) received the B.Eng. and Ph.D.degree, both in electrical engineering, from TsinghuaUniversity, Beijing, China in 2006 and 2011, respec-tively.

He is currently an associate professor in the De-partment of Electrical Engineering, Tsinghua Uni-versity. His research concerns power system analysisand control, renewable energy integration.

Shengwei Mei (SM’06-F’14) received the B.Sc. degree in mathematics fromXinjiang University, Urumqi, China, the M.Sc. degree in operations researchfrom Tsinghua University, Beijing, China, and the Ph.D. degree in automaticcontrol from Chinese Academy of Sciences, Beijing, China, in 1984, 1989,and 1996, respectively.

He is currently a professor at Tsinghua University. His research interestsinclude power system analysis and control, and the application of game theoryin power systems.