Research Article Design of a Microgrid with Low-Voltage

Research ArticleDesign of a Microgrid with Low-Voltage Ride-ThroughCapability and Simulation Experiment

Wenchao Fan, Zaijun Wu, Xiaobo Dou, Minqiang Hu, Jieru Wei, and Zhenyu Lv

School of Electrical Engineering, Southeast University, Nanjing 210096, China

Correspondence should be addressed to Zaijun Wu; [email protected]

Received 24 January 2014; Revised 10 May 2014; Accepted 12 May 2014; Published 9 June 2014

Academic Editor: Hongjie Jia

Copyright © 2014 Wenchao Fan et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A microgrid with low-voltage ride-through capability is designed.The designed microgrid avoids operating in unplanned islandedmode during an asymmetric ground fault which occurs in the low voltage distribution network and supports fault recovery fordistribution network. Furthermore, compared with the traditional microgrid topology, the proposedmicrogrid topology also savesa lot of power electronic devices. The simulation results with PSCAD/EMTDC show that the microgrid can keep the distributedgenerations and loads operate normally when an asymmetric ground fault occurs in the low voltage distribution grid. It can alsoincrease the active power output according to the requirement of the distribution network to support the distribution network faultrecovery.

1. Introduction

With the gradual depletion of fuel energy, renewable energy,such as wind power and solar power, is attracting more andmore attention. In order to achieve effective utilization ofrenewable energy and improve the power supply reliability,the microgrid concept is proposed. Microgrid is a smallsystemwhich contains distributed generations (DGs), energystorage devices, loads, protection equipment, and communi-cation equipment. With the microgrid accessing distributionnetwork, the power flow changes from one direction to bidi-rection. So that it had a profound impact on distribution net-work’s load forecasting, plan, protection, and other issues [1].

According to the national regulation codes, microgridshould be disconnected from the distribution networkshortly when a fault occurs in the distribution network.Hence, themicrogrid operationwill be transferred from grid-connected mode to islandedmode in order to ensure that thesensitive loads operate normally.This transition is not planedin advance, so it is called nonplanned islanded mode. Thatmicrogrid operates in nonplanned islandedmodewill cause alot of adverse consequences.When themicrogrid disconnectsfrom the distribution network, power flow through the pointof common coupling (PCC) will reduce to zero. For thedistribution network, it will cause the line voltage’s amplitude

and frequency fluctuate and even expand the failure scopeor reduce the accuracy of protective equipment. At thistime, the microgrid disconnects from network which willlead its internal power to an unbalanced status. Short-timepower imbalance in microgrid may demand loads or DGs todisconnect from distribution network. If the power cannotreach equilibrium in a certain time, power imbalance mayeven lead to the system crash, threatening the loads’ normaloperation. To solve this problem, we should take somemeasurements to make the microgrid operation remain inconnection mode when some fault occurs in the distributionnetwork. Common distribution network asymmetric groundfault mainly consists of two-phase ground fault and single-phase ground fault. They account for more than 90% of thetotal distribution network fault. Sowe focused on these faults,developed microgrid topology and its control strategies inorder to render that the microgrid acquires a low-voltageride-through (LVRT) capability when the above faults occurin distribution network.

2. Design of the Entire System

In order to achieve the above objectives, the microgrid withLVRT capability was designed. The topology is shown in

Hindawi Publishing CorporationJournal of Applied MathematicsVolume 2014, Article ID 324527, 8 pageshttp://dx.doi.org/10.1155/2014/324527

2 Journal of Applied Mathematics

Distributionnetwork

PCC

Windturbines

AC AC

ACAC

loadDC DC

DC

DC

DC

DCDC

AC

AC

DC

DC

AC

DC

AC

DC DC

DC

DC

DC

DC

loadPhotovoltaic

cells load loadSuper

capacitorSuper

capacitor

Hybridenergyetoragesystem

Figure 1: Design of the entire system.

Figure 1. Different from the traditional topology, the designedmicrogrid is mainly composed of two sub-microgrids. Eachsub-microgrid is essentially a DC microgrid, which containsa DG, a storage device, a DC load, and an AC load. Thetwo sub-microgrids’ micropowers are direct-driven windturbine generators and photovoltaic cells, respectively. Theyare named as sub-microgrid A and sub-microgrid B. Eachsub-microgrid’s DC load and AC load connect to the DCbus through converters. Each of the sub-microgrid’s energystorage system is mainly composed of a super capacitor,which connects to the DC bus through DC-DC converter.When the microgrid operates normally, super capacitor cancoordinate sub-microgrid’s internal power. When the sub-microgrid DC bus voltage changes because of an asymmetricground fault in the distribution network, super capacitorabsorbs energy to maintain it. Super capacitor can ensurethat the micropower and load operate normally when anasymmetric ground fault occurs in the distribution network,avoiding the microgrid operating in nonplanned islandedmode.

There is also a hybrid energy storage system connectingin main microgrid feeder. It consists of storage batteries andsuper capacitors. During the microgrid operation in LVRTstate, the energy storage system not only can stabilize powerfluctuations near PCC but also issue an appropriate amountof power according to the distribution grid requirements toprovide support for the distribution network fault recovery.

3. Design of the Sub-Microgrid’s ConverterControl System

For direct-driven wind turbine generators system, distribu-tion network carrying an asymmetric ground fault can cause

idc

Cdc

L isc Rsc

Csc

udc

usc

Figure 2: Circuit structure of super capacitor energy storage system.

its power output to reduce, which will lead its DC bus voltageto drop. This phenomenon can also generate second powerharmonic in power output, which can cause the DC busvoltage fluctuates at a frequency twice of the power frequency,destabilizing the entire system [2].

Similar to direct-driven wind turbine generators, themicrogrid main feeder voltage drop when an asymmetricground fault occurs in the distribution network. So it can alsoresult in the sub-microgrid DC bus voltage increasing andfluctuating. To solve such problems, super capacitor energystorage devices were installed in each sub-microgrid. Theyconnect to the sub-microgridDCbus through the buck-boostDC/DC conversion circuit. This topology’s main advantageis that it can achieve a steady DC bus voltage during themicrogrid operation in LVRT status.

3.1. Design of the Super Capacitor Bidirectional Buck-BoostConverter Control System. Figure 2 is a simplified circuitstructure of super capacitor energy storage system. Thesuper capacitor is simplified for a model, which contains

Journal of Applied Mathematics 3

+−

SawtoothPI Comparator

Buck modeBoost mode

udc

u∗dc

Figure 3: Bidirectional converter control system.

capacitance and resistance in series. The energy storagesystem connects to the sub-microgrid DC bus through thebidirectional buck-boost converter.

Super capacitor bidirectional buck-boost converter con-trol system diagram is shown in Figure 3.

As shown in Figure 3, first of all, by subtracting sub-microgrid DC bus voltage’s actual value from its set-pointvalue, an output signal will be obtained. Secondly, after beingadjusted by the PI regulator, the output signal was comparedwith a preset sawtooth through comparator. Then a resultis obtained, which decides the converter work mode: buckmode or boost mode. The operation mode of the supercapacitor is actually decided by the variety of the exchangepower between sub-microgrid andmicrogridmain feeder [3].The difference between the DG’s exporting power and theload’s wasting power is transported to the microgrid, whichis set as 𝑃out. In this system, we assume 𝑃out is positive whenthe microgrid operates normally. If an asymmetric groundfault occurs in the distribution network, microgrid mainfeeder voltage will drop. Furthermore, due to the fact thatthe current flowing through the grid-connected inverter islimited, the power flowing through it will decline. So there isexcess energy in sub-microgrid which cannot be transferredout, causing the DC bus voltage to rise. In this case, theconverter operates in buckmodel, absorbing excess energy tomaintain the DC bus voltage. If the DC bus voltage decreasesfor some reasons, the converter will operate in boost mode,releasing energy. Since energy canflow in both directions, thiscontrol topology can take some effect to stabilize the DC busvoltage.

Assume that the imbalance power in sub-microgrid isΔ𝑃.According to Figure 2, it can be expressed as

Δ𝑃 = 𝑢dc𝑖dc. (1)

According to Figure 2, the following equations areobtained:

𝐶sc𝑑 (𝑢sc − 𝑖sc𝑅sc)

𝑑𝑡= 𝑖sc, (2)

𝑖dc = 𝐷𝑖sc. (3)

In (3), 𝐷 is the duty cycle. Combine (1), (2), and (3), andthe following is obtained:

𝑢sc =1

𝐷

Δ𝑃

𝐶sc𝑢dc𝑡. (4)

Assume that an initial voltage of the super capacitor is𝑢ini,and themaximum allowable voltage is 𝑢max. According to (4),amaximum low-voltage riding timewhich can bemaintainedby the super capacitor is

𝑇max =𝐷 (𝑢max − 𝑢ini) 𝐶sc𝑢dc

Δ𝑃. (5)

Despite the fact that Δ𝑃 fluctuates when the systemoperates in LVRT status, its average value can stay stable.So formula (5) is reasonable. The super capacitor under thiscontrol method can maintain the DC bus voltage, avoidingadverse effects on theDG andmaking loads operate normallywhen an asymmetrical ground fault occurs in the distributionnetwork.

3.2. Design of Load Side Inverter Control System. In thissystem, load side inverters connect to the sub-microgrid DCbus. As long as the DC bus voltage stays stable, the DC loadcan operate normally and reduce the impact on the AC loadwhen an asymmetric ground fault occurs in the distributionnetwork. The DC load connects to DC bus through DC-DCbuck chopper. The topology’s aim is to maintain the DC loadvoltage stable.

Due to the occurrence of an asymmetric ground fault inthe distribution network, the sub-microgrid’s current outputwill change, which can lead to its DC bus current change.In this case, despite the fact that the DC bus voltage staysstable, the ripple current can still affect the AC load’s normaloperation. Hence, outer power loop and inner current loopcontrol system are designed, which is shown in Figure 4.𝑃Lref and 𝑄Lref are the load rated active and reactive

powers, respectively. As set-points active power value andreactive power value, they minus actual active power valueand reactive power value respectively. After being adjustedby PI regulation, the results are considered the given valueof active component and reactive component of the loadcurrent. Then collect three-phase load current. Active com-ponent 𝑖Ldref and reactive component 𝑖Lqref will be obtainedby coordinate transformation. Then they subtract the givenvalues, respectively. Having been adjusted by PI regulation,the results are considered as the active component andreactive component of load voltage. They are also consideredas the inverter drive signals.

4. Design of Sub-Microgrid Grid-ConnectedInverter Control System

Asmentioned above, the microgrid main feeder voltage dropwill lead the sub-microgrid power output to fluctuate. Inorder to suppress the fluctuations and reduce the impacton the DC bus voltage during the microgrid operation inLVRT status, proportional integral resonant (PIR) regulatoris applied to replace proportional integral (PI) regulator insub-microgrid’s grid-connected inverter control system.

The resonant regulator was proposed according to theprinciple that the control system has no steady error whenit directly controls the DC signal in the two-phase rotatingcoordinate system [4–6].

The transfer function of PIR controller used in the systemis

𝐺 (𝑠) = 𝑘𝑝 +𝑘𝑖

𝑠+

𝑘𝑟𝑠

1 + 2𝜔𝑐𝑠 + (2𝜔𝑠)2. (6)


PLref

QLref

PLd

QLd

regulatoriLdref

iLqref

iLd

iLq

abc

dq0PIregulator

PI

regulatorPI

regulatorPI

+

+

+

+

−

− −

− u∗La

u∗Lb

u∗Lcu

∗Lq

u∗Ld

Figure 4: Load side inverter control system.

Grid-connectedinverter

P∗

P

+ + ++

+

++ + +− −

−

− −Q∗

Q

PIRregulator

PIRregulator

PIRregulator

PIRregulator

i∗d

id

i∗q

i∗q

id

iq

Microgridmain feeder

abc

dq0 abc

dq0

∗𝜔L

∗𝜔L

u∗q

u∗d

ud

ud

uq

ua

ub

uc

udc

La

Lb

Lc

Ra

Rb

Rc

ia

ib

ic

Figure 5: Sub-microgrid inverter control system.

In this formula, 𝜔𝑐 is the resonant regulator’s cutofffrequency, which determines the bandwidth of the resonantregulator. 𝜔𝑠 is the resonant regulator’s angular frequency,which is the samewith the grids for 314 rad/s. For this system,when the signal passes through this function, the sectionwhose angular frequency is 618 rad/s will be amplified, whileother sections will be suppressed. Set the appropriate gain𝑘𝑟. Then the function can compensate for power harmonicswhose frequency is twice of the fundamentals to achievethe purpose of suppressing the sub-microgrid power outputfluctuations.

Constant-power control strategy is utilized in the sub-microgrid grid-connected inverter. The control system con-sists of outer power loop and inner current loop, which isshown in Figure 5.

In outer power loop, set-point power values minus actualones respectively and the results will be obtained. Afterbeing adjusted by PIR regulator, they are considered as

corresponding given current signals for inner current loop.Their expressions are as follows:

𝑖∗

𝑑= (𝑃∗− 𝑃)(𝑘𝑝 +

𝑘𝑖

𝑠+

𝑘𝑟𝑠

𝑠2 + 2𝜔𝑐𝑠 + (2𝜔𝑠)2) ,

𝑖∗

𝑞= (𝑄∗− 𝑄)(𝑘𝑝 +

𝑘𝑖

𝑠+

𝑘𝑟𝑠

𝑠2 + 2𝜔𝑐𝑠 + (2𝜔𝑠)2) .

(7)

As aforementioned, microgrid power output fluctuateswhen themain feeder voltage drops.This phenomenon can beattributed to the current’s DC component fluctuations in 𝑑𝑞coordinate system. Suppressing it actually has a great benefitin inhibiting the power output fluctuations. Hence, the PIcontroller should also be replaced by PIR controller in theinner current loop. Then the system forms a double resonantcontroller structure, which will play better in suppressingthe sub-microgrid power output fluctuations. The voltage


Ps2out

Qs2out

Psc

Qsc

High-passfilter

High-passfilter

Supercapacitor

Storagebatteries

Pb

Qb

++

+

++

+

++

−

−−

−

Psout

Psout

Qsout

Qsout

Pout

Qout

Ps1out

Ps1out

Qs1out

Qs1out

Figure 6: Hybrid energy storage control system.

reference signals generate from inner current loop are asfollows:

𝑈∗

𝑑= (𝑖∗

𝑑− 𝑖𝑑)(𝑘𝑝 +

𝑘𝑖

𝑠+

𝑘𝑟𝑠

𝑠2 + 2𝜔𝑐𝑠 + (2𝜔𝑠)2) − 𝜔𝐿𝑖𝑞 + 𝑈𝑑,

𝑈∗

𝑞= (𝑖∗

𝑞− 𝑖𝑞)(𝑘𝑝 +

𝑘𝑖

𝑠+

𝑘𝑟𝑠

𝑠2 + 2𝜔𝑐𝑠 + (2𝜔𝑠)2) + 𝜔𝐿𝑖𝑑 + 𝑈𝑞.

(8)

5. Design of the Microgrid HybridEnergy Storage System

As mentioned above, when an asymmetric ground faultoccurs in distribution network, its exchange power with themicrogrid will reduce instantly. This phenomenon has someimpact on the distribution network even expands the faultarea in distribution network. Despite the fact that the PIRregulators are utilized in the system to inhibit harmonicpower when the microgrid operates in LVRT status, it isnot completely eliminated. Harmonic power injected intothe distribution network will cause adverse effects for thedistribution network fault recovery. Finally, according tothe energy storage system’s grid-connected inverter poweroutput limitation and the requirement of cooperation withthe distribution network protection devices and others, thepower values, which should be exported from the microgridto distribution network during the system operation in LVRTstatus, are set.

During the microgrid operation in connection mode, thehybrid energy storage system on the microgrid main feederis in floating mode, almost having no action. When themicrogrid operates in LVRT status, the hybrid energy storagesystem has two main roles. First, increase the power outputto compensate for the reduced microgrid’s power output andprovide additional active power for distribution network faultrecovery. Second, stabilize power on the main feeder, makingthe power passing through PCC as smooth as possible to

reduce the harmonic power’s adverse effects on distributionnetwork.

To achieve the above aims, the hybrid energy storagecontrol system is designed as dual-loop control structure,which is shown in Figure 6 [7].

First of all, collect the two sub-microgrids’ power outputs𝑃s1out, 𝑄s1out, 𝑃s2out, and 𝑄s2out. As shown in Figure 6, add𝑃s1out to 𝑃s2out and𝑄s1out to𝑄s2out, respectively.We can obtainthe values of the sub-microgrids’ total active power output𝑃sout and reactive power output𝑄sout. Secondly, design a high-pass filter, and offer 𝑃sout and 𝑄sout to pass through it. Thenwe obtain the high frequency elements of the twomicrogrids’total active power output signal𝑃sc and reactive power outputsignal 𝑄sc. They are utilized as the super capacitor poweroutput command, which can stabilize power in main feeder.At the same time, subtract 𝑃sc from 𝑃sout. We will obtainlow frequency component of the two sub-microgrids’ totalactive power output 𝑃s1out. With the same method, the lowfrequency component of their total reactive power outputcan also be obtained. Finally, according to the energy storagesystem’s grid-connected inverter power output limitation andthe requirement of cooperationwith the distribution networkprotection devices and others, make sure the power values𝑃out and 𝑄out, which should export from the microgrid todistribution network during the system operation in LVRTstatus. Make the difference between 𝑃out and 𝑃s1out and thedifference between 𝑄out and 𝑄s1out, respectively. The resultsare utilized as the storage given power output signals 𝑃𝑏 and𝑄𝑏. As long as the power output tracks the command value, itnot only can keep the power passing PCC stable but also canhelp the distribution network to restore to normal as soon aspossible.

6. Simulation Analysis

At the present time, China has no LVRT requirements forDG connecting in distribution network of 0.4 kV. However,with the large-scale application of microgrid in distributionnetwork, it’s a trend to make microgrid acquire a LVRT


0.900 0.925 0.950 0.975 1.000 1.025 1.050 1.075 1.100

0.000.100.200.300.40

t (s)

−0.10

−0.20

−0.30

−0.40

u(k

V)

(a)

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.200

20

40

60

80

100

t (s)

P (k

W)

Q (k

var)

(b)

0.000.100.200.300.400.500.600.700.800.90

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20t (s)

udc

(kV)

(c)

1214161820222426

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20t (s)

Pa(kW)

Qa(kvar)

(d)

0.0000.0500.1000.1500.2000.2500.3000.350

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20t (s)

uldc(kV)

(e)

Figure 7: (a) Microgrid main feeder voltage waveform under single-phase ground fault in distribution network. (b) Microgrid exportingpower waveform under single-phase ground fault in distribution network. (c) DC bus voltage waveform of sub-microgrid A under single-phase ground fault in distribution network. (d) AC load power waveform of sub-microgrid A under single-phase ground fault in distributionnetwork. (e) DC load voltage waveform of sub-microgrid A under single-phase ground fault in distribution network.

capability in low voltage distribution network to eliminate thethreat of the microgrid non-planned islanded mode on itselfnormal operation and to provide support for the distributionnetwork recovery when an asymmetrical ground fault occursin it.

Based on this, we built a system model in PSCAD/EMTDC simulation software to do simulation experiments.We set themicrogrid’s rated active power output as 85 kWandreactive power output as 25 kvar during the system normaloperation. Sub-microgrid A’s rated active power output is


0.000.100.200.300.40

−0.10

−0.20

−0.30

−0.40

0.900 0.925 0.950 0.975 1.000 1.025 1.050 1.075 1.100t (s)

u(kV)

(a)

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20t (s)

020406080

100

P (k

W)

Q (k

var)

(b)

0.000.100.200.300.400.500.600.700.800.90

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20t (s)

udc

(kV)

(c)

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20t (s)

1214161820222426

Pa(kW)

Qa(kvar)

(d)

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20t (s)

0.0000.0500.1000.1500.2000.2500.3000.350

uldc(kV)

(e)

Figure 8: (a)Microgridmain feeder voltage waveform under two-phase ground fault in distribution network. (b)Microgrid exporting powerwaveform under two-phase ground fault in distribution network. (c) DC bus voltage waveform of sub-microgrid A under two-phase groundfault in distribution network. (d) AC load power waveform of sub-microgrid A under two-phase ground fault in distribution network. (e)DC load voltage waveform of sub-microgrid A under two-phase ground fault in distribution network.

50 kW and reactive power output is 15 kvar. Sub-microgridB’s rated active power output is 35 kW and reactive poweroutput is 10 kvar. Take the sub-microgrid A, for example,analyzed of the effect of sub-microgrid control system. SetDC bus voltage-stabilized capacitor as 6000 𝜇F, DC busrated voltage as 0.8 kV, DC load rated voltage as 0.3 kV, DCload rated power as 90 kW. Set DC-DC converter controlsystem’s PI regulator parameters 𝑃 = 15 and 𝐼 = 0.02.AC loads rated line voltage is 0.38 kV, rated active poweris 25 kW, and rated reactive power is 15 kvar. Active powercontrol component of the AC load control system has thesame PI regulator parameters with the reactive power controlcomponent. Current inner loop PI regulator parameters are𝑃 = 10 and 𝐼 = 0.1, and power outer loop PI regulatorparameters are 𝑃 = 10 and 𝐼 = 0.1. Grid-connected inverterfilter capacitor is 𝐶 = 12 𝜇F, and filter inductance is 𝐿 =

0.2mH. The grid-connected inverter control system’s activepower outer control loop PIR regulator parameters are 𝑃 =10, 𝐼 = 0.01, and 𝑅 = 5; current inner control loop PIRregulator parameters are 𝑃 = 10, 𝐼 = 0.1, and 𝑅 = 5.The system’s reactive power outer control loop PIR regulatorparameters are 𝑃 = 10, 𝐼 = 0.01, and 𝑅 = 5; current innercontrol loop PIR regulator parameters are 𝑃 = 10, 𝐼 = 0.1,and 𝑅 = 5.

Assumed that when an asymmetric ground fault occursin the distribution network, the microgrid should provideadditional 15 kW active power. Now, since China has norequirements for DG possessing LVRT ability in low voltagedistribution network, we refer to “LVRT requirements of thephotovoltaic power plants in medium voltage distributionnetwork” [8]. We conducted a simulation experiment whensingle-phase ground fault or two-phase ground fault occurs


in the distribution network, respectively. Set that the faultsoccurred in the distribution network in 1 second and thevoltage drop down to 20% for 1 second.

Figures 7 and 8 are simulationwaveforms for single-phaseground fault and two-phase ground fault in distributionnetwork respectively. From the simulation waveform we canlearn that during the microgrid operation in LVRT status,its reactive power output for distribution network remainedstable. Due to the energy storage system’s active power outputincrease, microgrid’s power output increased to 100 kW,providing support for distribution network fault recovery. Assoon as the fault in the distribution network is eliminated, thepower passing through PCC returned to normal. Althoughthe microgrid main feeder voltage drop caused the sub-microgrid A’s power output to decrease, this energy wasabsorbed by its internal super capacitor. Hence power insub-microgrid A can maintain a balance. PIR regulatorsutilized in the grid-connected inverter control system sup-pressed power output fluctuations. And the super capacitorbidirectional converter maintained the sub-microgrid DCbus voltage. Hence, during the microgrid operated in LVRTstatus, the sub-microgrid DC bus voltage fluctuations can begreatly inhibited. The sub-microgrid A’s DC bus voltage hadno significant fluctuations, providing a strong guarantee forthe load. We can also learn that the power dissipated by DCload and AC load maintained stability. The loads had almostnot been impacted frommicrogrid main feeder voltage drop.

7. Conclusion

We designed a microgrid with a LVRT capability for thefirst time. Simulations for single-phase ground fault and two-phase ground fault in distribution network are implementedin PSCAD/EMTDC. From the simulation results, the follow-ing conclusions can be drawn.

(1) The designed microgrid consists of several DC sub-microgrids and hybrid energy storage system. Oneof the main advantages of the DC microgrid is thatit can save a lot of electronic devices. Therefore,the microgrid described in this paper also has thisadvantage.

(2) Themicrogridwedesigned canmaintainDGand loadoperation normally when an asymmetry ground faultoccurs in the distribution network. In this case, DGand load need not to be removed. The microgrid hasa LVRT capability.

(3) During the microgrid operation in LVRT status, itexports active power according to the distributionnetwork’s requirement, providing support for thedistribution network’s fault recovery.

(4) Because the distribution network’s fault phase voltagedrop deeper and frequency of power harmonic istoo high in the experiment and other reasons, powerfluctuations near PCC are not well suppressed. Wemust take measures to improve for it in the future.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgment

This work is supported by National Natural Science Founda-tion of China (51177015).

References

[1] S. A. Aoyang, D. Xing,W.Minghao et al., “To response the highpenetration ofmicro power in grid network access,”Automationof Electric Power Systems, vol. 34, no. 1, pp. 78–83, 2010.

[2] S. Huang, L. Xiao, K. Huang, Z. Chen, and S. Xiong, “Operationand control on the grid-side converter of the directly-drivenwind turbine with PM synchronous generator during asymmet-rical faults,” Transactions of China Electrotechnical Society, vol.26, no. 2, pp. 173–180, 2011.

[3] S.-Y. Hou, Y. Fang, J.-X. Zeng, and Z.-N. Yin, “Application ofsupercapacitors to improve wind power system’s low voltageride through capability,” Electric Machines and Control, vol. 14,no. 5, pp. 26–31, 2010.

[4] C. Zhang, J. Zhang, W. Wu, and D. Xu, “High-frequency linkinverter waveform control with resonant controller based ondelta operator,” Transactions of China Electrotechnical Society,vol. 23, no. 7, pp. 81–85, 2008.

[5] P. C. Loh, M. J. Newman, D. N. Zmood, and D. G. Holmes, “Acomparative analysis of multiloop voltage regulation strategiesfor single and three-phase UPS systems,” IEEE Transactions onPower Electronics, vol. 18, no. 5, pp. 1176–1185, 2003.

[6] D. N. Zmood and D. G. Holmes, “Stationary frame currentregulation of PWM inverters with zero steady-state error,” IEEETransactions on Power Electronics, vol. 18, no. 3, pp. 814–822,2003.

[7] L.Wenbin,Research on Energy Storage Systems to StabilizeWindFarm Power Fluctuations, Chongqing University, Chongqing,China, 2012.

[8] Photovoltaic power plants connected to the grid technologytechnical requirements of State Grid Corporation (Trial), 2009.

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Research Article Design of a Microgrid with Low-Voltage