Voltage-Sag Tolerance of DFIG Wind Turbine With a series grid side passive-impedance network

Voltage-Sag Tolerance of DFIG Wind Turbine With a series grid side passive-impedance network

1048IEEETRANSACTIONSONENERGYCONVERSION,VOL.25,NO.4,DECEMBER2010

Voltage-SagToleranceofDFIGWindTurbineWitha

SeriesGridSidePassive-ImpedanceNetwork

XiangwuYan,Member,IEEE,GiriVenkataramanan,SeniorMember,IEEE,PatrickS.Flannery,Member,IEEE,

YangWang,StudentMember,IEEE,QingDong,andBoZhang

Abstract—Duetotheincreaseofthenumberofwindturbinesconnecteddirectlytotheelectricutilitygrid,newregulatorcodeshavebeenissuedthatrequirelow-voltageride-throughcapabilityforwindturbinessothattheycanremainonlineandsupporttheelectricgridduringvoltagesags.Conventionalride-throughtech-niquesforthedoublyfedinductiongenerator(DFIG)architectureresultincompromisedcontroloftheturbineshaftandgridcur-rentduringfaultevents.Inthispaper,aseriespassive-impedancenetworkatthestatorsideofaDFIGwindturbineispresented.Itiseasytocontrol,capableofoff-lineoperationforhighef ciency,andlowcostformanufacturingandmaintenance.ThebalancedandunbalancedfaultresponsesofaDFIGwindturbinewithaseriesgridsidepassive-impedancenetworkareexaminedusingcomputersimulationsandhardwareexperiments.

IndexTerms—Doublyfedinductiongenerator(DFIG),lowvolt-ageride-through(LVRT),voltagesagridethrough,windturbine.

I.INTRODUCTION

A

NOMENCLATURE

ICurrentspacevector.

Voltagespacevector.V

ZImpedance.

Fluxspacevector.λ

ωSpeed.

Allparametersandquantitiesareconsideredtobetransformedtothestatorside.Subscriptsa,b,cThree-phasestationaryreferenceframe.dcDirectcurrent.PCCPointofcommoncouplingbetweenwindfarmand

grid.

q,dRealaxisandnegativeimaginaryaxisofsyn-chronousreferenceframe.s,r,gStator,rotor,gridside.wWind.

ManuscriptreceivedJanuary5,2010;revisedApril8,2010,andMay19,2010;acceptedMay22,2010.DateofpublicationAugust12,2010;dateofcurrentversionNovember19,2010.ThisworkwassupportedinpartbytheChinaScholarshipCouncilandNaturalScienceFoundationofHebeiunderGrantE2009001400andinpartbytheNationalNaturalScienceFoundationofChinaunderGrant50977027.Paperno.TEC-00004-2010.

X.Yan,Q.Dong,andB.ZhangarewiththeDepartmentofElectricalEngineering,NorthChinaElectricPowerUniversity,Baoding071003,China(e-mail:xiangwuy@;dq.d@;adamn-cepu@).

G.VenkataramananandY.WangarewiththeDepartmentofElectricalMa-chineandComputerEngineering,UniversityofWisconsin–Madison,Madison,WI53706USA(e-mail:giri@engr.wisc.edu;ywang38@.wisc.edu).

P.S.FlanneryiswiththeAmericanSuperconductorinMiddleton,Middleton,WI53562USA(e-mail:patrick. annery@).

Colorversionofoneormoreofthe guresinthispaperareavailableonlineat.

DigitalObjectIdenti er10.1109/TEC.2010.2054097

Saresultofthedoublyfedinductiongenerator(DFIG)windturbine’slargebutlightweightmechanicalstruc-tureandpowerelectronicsinterface,duringextremepointofcommoncoupling(PCC)voltagesags,veryhighcurrentsareinducedintherotorcircuitwhichcandamagetherotor-sideconverterandcauseunduefatigueonthegearbox[1].Olderutility-connectioncodesallowedwind-turbinedisconnectionintheeventofgridvoltagesagbelow0.8p.u.(perunit)[2],[3].Intherecentyears,duetotheincreaseofthenumberofwindtur-binesconnecteddirectlytotheelectric-utilitygrid,newregula-torcodeshavebeenissuedthatrequirelow-voltageride-through(LVRT)capabilityforthewindturbine.Insteadofdisconnec-tion,thewindturbineshavetosupporttheelectricgridduringvoltagesags[3],[4].Fromtheaspectofpreventingvoltagecollapse,therequirementtypicallyemphasizesprovisionofre-activecurrentasafunctionofthesagdepthwithintheturbinescapability.Inordertomanagetheseproblems,severalride-throughoptionsfortheconventionalDFIGarchitecturehavebeenproposed.Twomodi cationstotherotorcircuitinclud-ingtheadditionofeitherasilicon-controlledrecti er(SCR)rotor-crowbarcircuit[1]orathree-phaserecti erandmodu-latedresistiveloadhavebeendemonstratedtoimproveintheDFIGride-throughcapability[5],[6].Asanalternative,briefdisconnectionofthestatorwindingsduringvoltagesagviaanSCRstaticswitch[7]hasalsobeenshowntoreducetorqueandcurrentspikesforsagsdownto15%ofthenominalvolt-age.Amodi edrotor-currentcontrolmethodhasbeenshowntoprotectthemachine-sideconverter(MSC)forwind-turbineter-minalvoltagedowntoabout30%ofthenominalvoltage,withresidualtorquespikesandoscillations[8].Anexplorationoftheseriesgrid-sideconverter(GSC)DFIGarchitecture[9]–[11]revealednotonlyanexcellentpotentialforvoltagesagridethroughbutalsotheshortcomingsinpowerprocessingcapa-bility.Auni edDFIGarchitectureinwhichtheseriesGSCispartneredwithaparallelgrid-siderecti erispresentedasanalternativeforbothDFIGwind-turbinepowerprocessingandrobustvoltagesagridethroughin[12]–[14].FurtheranapproachforLVRTforaDFIGwindturbineusingthepassive-impedancenetworkswaspresentedin[15].Thepresenceofthepassive-impedancenetworkinserieswiththestatorsideduringthegridfaultallowsadirectmechanismtopreventuncontrollablestator uxdynamics.SimulationresultsshowitsexcellentcapacityforaDFIGwind-turbineLVRTinbalancedgridfault.

Basedonthepracticalityandcost,anoptimizedpassive-impedancenetworkinserieswiththestatorsideforthepur-posesofdampingsynchronousframestator- uxoscillationsis

0885-8969/$26.00©2010IEEE

Voltage-Sag Tolerance of DFIG Wind Turbine With a series grid side passive-impedance network

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TABLEI

POSITIVEANDNEGATIVESEQUENCEPHASORS(PERUNIT)FOREACHSAGTYPE

ASAFUNCTIONOFPERUNITCHARACTERISTICVOLTAGEN

V

presentedinthispaper.TheeffectsofunbalancedgridfaultsonaDFIGwindturbinewithseriespassive-impedancenetworkandwithoutanycountermeasurearediscussedindetail.Furthersim-ulationandexperimentalresultsshowthispassive-impedancenetworkinserieswiththeDFIGstatorsidehasexcellentca-pacityforaDFIGridethroughPCCvoltagesagto15%ofthenominalvoltage.

II.VOLTAGESAGANDITSEFFECTANDNEWREQUIREMENTS

OFLVRTCAPABILITYFORTHEWINDTURBINEA.VoltageSaganditsEffect

Inathree-phasegrid,anumberofdifferentfaulttypescanoc-cursuchassingle-phase-to-neutral,phase-to-phase,two-phase-to-neutral,andthree-phasefaults.Differentfaulttypesleadtodifferentvoltagesags.Aclassi cationcommonlyusedisthatthevoltagesagsareclassi edintoseventypesasdiscussedin[16]and[17],namely,A,B,C,D,E,F,andG.Thevoltagesagattheterminalsofthewindturbinedependsonseveralfactorsin-cludingtheequivalent-networkmodel,faultlocation,faulttype,andpropertiesoftheinterfacetransformer[16].Forunbalancedfaults,themethodofsymmetricalcomponentsmustbeusedtodeterminetheremainingphasevoltages[18],[19].Thefourdif-ferenttypesoffaultsresultindifferentvoltagephasorresponsesatthewind-turbineterminals,dependingonthenatureofthetransformerconnectionsbetweenthefaultpointandthewindturbine[16],[17].AsthepresenceofΔ/Ytransformersbe-tweenthePCCandstatorterminalspreventstheoccurrenceofazero-sequencecomponentvoltageattheDFIGwindturbineterminalsduringalltypesofsagevents,voltagesagtypesBandEareomittedfromdiscussionsincetheyarenotseenatthewind-turbineterminals,leavingonlyfourtypesofpossiblevoltagesags,namely,A,C,D,andF.SagtypeGisalsocon-sideredinthecaseofawindturbinewithonlyoneinterfacetransformer.

Thep.u.characteristicvoltageN

˙v,thep.u.ratiosN˙pandN˙nofpositiveandnegativesequencevoltagesagisde nedas

N

˙V

˙v=V˙,N˙V˙p=pV,andN˙V˙n=nV˙.

(1)

nom˙nomnomN

˙Positiveandnegativesequencessagp.u.phasorsN

˙pandnforeachsagtypeseenatthewind-turbineterminalsarepresentedinTableI[14].

ThevoltageseenatstatorterminalsofaDFIGwindturbineinatypicalwindfarmduringvoltagesagscanberepresented

withpositiveandnegativesequencecomponentsexpressedinthestationaryframeas

V

˙PCC=N˙pV˙nomejωet+N˙nV˙nome jωet.(2)

B.NewRequirementsofLVRTCapability

forDFIGWindTurbine

Consideringtherelevantvoltage-sagride-throughstandardsandsafeoperation,aDFIGwindturbinehastoful llthefol-lowingrequirements[2],[4].

1)DFIGwindgeneratorsarerequiredtowithstandathree-phasefaultwithninecycles(150ms)atthevoltagesagupto15%ofthenominalvoltageandsinglelinetogroundfaultswithdelayedclearingtime.

2)Uninterruptedfeedingofade nedcurrentintothegridforgridprotectionandsystemsafetyduringgridfaults.3)Maintaininginstantaneousdc-linkvoltagewithindeviceratingandinstantaneousdevice(orphase)currentswithintwicenominalratings.

4)Avoidingoftorquetransientsbeyondthepermittedstressleveltogearanddriveshafts,i.e.,2.0–2.5p.u.torque.5)Feedingthemaximumpossibleactivepowertothegridassoonaspossibleafterthefaultiscleaned.III.PRINCIPLEOFVOLTAGE-SAGRIDETHROUGHOFADFIGWINDTURBINEWITHPASSIVE-IMPEDANCENETWORKA.ConceptofVoltage-SagRide-ThroughofaDFIGWindTur-bineWithPassive-ImpedanceNetwork[15]

Asweknow,aDFIGwindturbineconvertsaero-kineticen-ergytoelectricalenergy.ADFIGwindturbineprovideselec-tricalpowertothegrid,whichcanbeequivalentlyconsideredasapowersourceconnectedtoanin nite-buspowersystem.Asingle-phasesimpli edequivalentcircuitisshowninFig.1(a).Ifathree-phasenetworkshortcircuithappensatPCC,thesystemisdividedintotwoparts;theDFIGisshortedthroughtrans-formerZTandpartoflineZl.Asimulatedresponseof2MWDFIGwindturbinetoPCCvoltagesagto15%ofthenominalvoltageispresentedin[15].Thestator uxλpletely,wecanalsoseethetransientvalueofstatorschangescom-currentIandrotorcurrentIfourtimesoftheratedvalue,sdrivershafttorquerreachesalmostTechangestoalmostseventimesoftheratedvalue.Therefore,grid-voltagesagscanbedetrimentaltotheDFIGwindturbines.

Oneideatostabilizethestator uxλs,currentIs,Ir,andtorqueTeundergrid-voltagesagistoinsertanequivalentimpedancebetweenDFIGandgridasshowninFig.1(b).TheequivalentsystemseenfromtheDFIGisillustratedinFig.1(b),iftheconnectedimpedancemeetstheexpressionasfollowing:

ZeqIs=Vg+ZgIsZ(3)

eq=

VgIs

+Zg

whereVZgisprefaultequivalentelectromotiveforceofgrid,gisprefaultequivalentimpedanceofgrid,andIsisprefaultequivalentstatorcurrent.

Voltage-Sag Tolerance of DFIG Wind Turbine With a series grid side passive-impedance network

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Fig.1.Singlephasesimpli edequivalentcircuitofDFIGwindturbinewithpassive-impedancenetworks.(a)DFIGwindturbineonnormaloperation.(b)DFIGwithseriesequivalentimpedanceduringgridvoltagefault.(c)DFIGwithpassive-impedancenetworksongridvoltagerecovery.(d)DFIGwithseriespassive-impedancenetworksonnormaloperation.

Wecan ndequivalentimpedanceZeqaccordingtoexpres-sion(3).Intheidealcase,theequivalentimpedanceZeqcanbe

insertedintosystemwithoutanydelay;thestator uxλandthetorqueTdurings,thecurrentIthegrid-voltagesandIr,ewillnotchangeatallsag.Further,inordertomeetnewcoderequire-ments,suchasuninterruptedfeedingofade nedcurrentintothegridforgridprotectionandsystemsafety,andtobalancewindpower,whichisusefulforcontrollerregulationofDFIGwindturbineduringthegridfault,amoredetailedpassive-impedancenetworktopologyispresentedasFig.1(c).However,

voltage

Fig.2.Single-phaseequivalentcircuitmodelofthepassive-impedancenetwork.

recoveryisalsoastepchange,weneedtoavoidlargechangesinthestator uxλs,thecurrentIitisnecessarytoholdthesandItopologyr,andthetorqueTe;therefore,onlineuntilfullgridvoltagerecoveryasshowninFig.1(c).Afterthefullrecov-ery,switchSsisclosedandswitchSpisopenedthroughvoltagecross-pointorothermethodsasshowninFig.1(d).Thisdescrip-tionpresentstheconceptofthepassive-impedancenetworkforDFIGwind-turbinevoltagesagridethrough.B.ModelofPassive-ImpedanceNetworkanditsControlStrategy

Asingle-phasesimpli edequivalentcircuitoftheproposedsystemispresentedinFig.2.Ascanbeobservedfromthe gure,thepassive-impedancenetworkconsistsofserieselementwithasolid-statebypassswitchandashuntelementwithasolid-stateisolatingswitch.Theseriesimpedanceisusedformodifyingthestator ux,limitingshort-circuitcurrent,maintaininggridconnection,anduninterruptedlyfeedingcurrentintogridduringthegridfault.Theshuntimpedanceisusedtobalancetheenergyofthewindturbineduringthegridfault.Theshuntelementandserieselementsareininactivemodeduringsteady-stateoperation,i.e.,theserieselementisbypassedandtheshuntelementisisolatedonnormaloperation.

Here,Zpisathree-phaseshuntimpedance,Zscisathree-phaseseriesimpedance,Spisathree-phasesolid-stateisolatingswitch,andSsisathree-phasesolid-statebypassswitch.Thealgebraicmodelofthepassive-impedancenetworkcanbeex-pressedasfollows:

IVs=Ss

(Vp

g Vs)Z+S(4)

psZsc

whereSp=1meansisolatedswitchSpisclosed,Sp=0meansisolatedswitchSpisopened.SwitchSsissimilartoswitchSp,Ss=1meansthebypassswitchSsisclosed.andSs=0meansthebypassswitchSsisopened.Controllogicofthepassive-impedancenetworkisexpressedinFig.3

Tomeetthestator-currentlimit(2p.u.),theseriesimpedanceZscvalueissettoZb,andZb=UAN/IA,whereUANistheratedvalueofthestator-phasevoltageofDFIG,IAistheratedvalueofthestator-phasecurrentofDFIG.BasedontherequirementsoftheDFIGpowerbalanceduringthegridfault,theshuntimpedanceZpvalueissettoequaltoZb.ThebypassswitchSsandtheisolationswitchSpoftheimpedancenetworkaredesignedasthethree-phaseswitchoperatedatthesametime,

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Fig.3.Controllogicofthepassive-impedance

network.

Fig.4.Modelblockdiagramofagridconnectingwindturbinewithadoublyfedinductiongenerator.

respectively,forallofthebalancedandunbalancedvoltagesags.Basedontheanalyticalanalysis,thetimingrequirementsoftheinsertion/switchingofthenetworkarenotveryhigh,especiallyahalf-cycledelayintheinsertionofthenetworkisacceptable;moreover,thenetworkshouldbewithdrawnwithinoneto vecyclesafterthegridvoltagerecovery.

IV.INFLUENCEOFBALANCEDANDUNBALANCEDGRID

FAULTSONADFIGWINDTURBINEA.SimulationModelofaGridConnectingDFIG-BasedWindTurbineWithControlSystem

Acompletewind-turbinemodelincludesthewind-speedmodel,theaerodynamicmodelofthewind-turbinerotor,themechanicalmodelofthetransmissionsystem,andmodelsoftheelectricalcomponents,namely,theDFIG,thepulsewidthmodulationback-to-backvoltagesourceconverters,thetrans-former,andthewind-farmcollectionfeedernetwork.Fig.4showsamodelblockdiagramofagridconnectingmegawattscalewindturbinewithadoublyfedinductiongenerator,anditsmainparametersareshowninTableII.

Ad–qsynchronousreferenceframeischosenformodelingtheDFIG.Themodelofthedoublyfedinductionmachineisbasedonthe fth-ordertwoaxesrepresentationsthatarecom-monlyknownasthe“Parkmodel”[20].Thesynchronousro-tatingreferenceframeisusedwiththedirect-axisalignedwiththestator-voltagevector.Inthisway,thedecoupledcontroloftheelectricaltorqueandtherotor-excitationcurrentisobtained.WhenmodelingtheDFIG,themotorconventionwillbeused,whichmeansthatthecurrentsareinputsandthatrealpowerandreactivepowerhaveanegativesignwhentheyarefedintothegrid.Thefollowingsimplifyingassumptionsaremadeinthedevelopmentofthemodel.

TABLEII

WIND-TURBINESYSTEMSIMULATIONP

ARAMETERS

1)Theironlosses,mechanicallosses,andpower-converterlossesarenegligible.

2)Themagneticcircuitofthemachinecanberepresentedbyalinearmodel.

3)Theentiremechanicalsystemcanbemodeledusingalumpedinertiaparameterreferredtotheelectricalangleandspeedoftheinductiongenerator.

4)Thepowerconverterscanbemodeledusingstate-spaceaveragedrepresentationtorepresenttheirlowfrequencydynamics.

5)Thewind-farmcollectionnetworktoPCCiselectricallystiff.

Afullfeedback-controlmodelincludescontrolofthewind-turbinetorque,powerextraction,andgridreactivepowerbycontrollingthecurrentsGSCandMSC,respectively.AhighbandwidthPIregulatorisimplementedtocontroltherotorcur-rentviatheMSC.Withd–qsynchronousreferenceframealign-ment,q-axiscomponentoftherotorcurrentisproportionaltotorqueandd-axiscomponentofrotorcurrentsuppliesreactivepower.

B.DynamicResponseofaGridConnectingDFIG-BasedWindTurbineWithandWithoutSeriesPassive-ImpedanceNetworkDuringGridBalancedandUnbalancedFaults

Theproposedride-throughapproachforDFIGusingpassive-impedancenetworkunderbalancedconditionswassimulated

Voltage-Sag Tolerance of DFIG Wind Turbine With a series grid side passive-impedance network

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Fig.5.Responseofa2MWDFIGwindturbinewithandwithoutseriespassive-impedancenetworkatType“A”(3 fault)voltagesagdownto15%ofthenominalvoltage.(a)“A”typeofvoltagesagwithoutanycountermeasures.(b)“A”typeofvoltagesagwithpassive-impedancenetwork.

TABLEIII

PARAMETERSUSEDINHARDWARES

YSTEM

indetailforacandidate2MWsystemaspresentedin[15].UnbalancedvoltagesagcaseswerediscussedtoshowtheirdetrimentaleffectstoaDFIGwindturbineontherotor-sideconverterandrotorshaftwhennoLVRTcountermeasureistaken[21].ThispaperstudiestheperformancesofaDFIGwindturbinewithseriespassive-impedancenetworkforLVRTunderunbalancedvoltagesagcases.

AscanbeseeninFigs.5(a)–8(a),thegridfaultcanleadtocon-siderableover-currents,over-voltages,andover-torque,puttingthewholefacilityunderstresswhennoLVRTcountermeasureistaken.Indetail,itisclearthattheA-typevoltagesagleadstothehigheststressonthewholefacility,D-andF-typeofvoltagesagsaresimilar,theynotonlyleadtoveryhighover-currents,over-voltages,andover-torque,butalsoleadtotorquereverse.TheC-andG-typeofvoltagesagsaresimilartoo,buttheyhaveshallowerstresstotheterminalconnectedtowindturbinesatthesamevoltagesagdepth.Atthesametime,thedc-linkvoltageduringtypeA,D,andFvoltagesagsexceedsthelimits,likelydestroyingtheback-to-backconverter.Itshouldbenotedthatthemostserioussituationhappensatthevoltage-recoverystageinallkindofvoltagesags.DuetothespacelimitationsandthatthevoltagesagtypesDandFissimilar,thecharacteristicsofvoltagesagtypeFarenotincludedinthisdiscussion.

Further,Figs.5(b)–8(b)demonstratethedynamicresponsesofaDFIGwindturbineusingseriespassive-impedancenetworkundertypicalvoltagesags.ItisshownthataDFIGwindturbineusingseriespassive-impedancenetworkcanfullyrelievethestressofthewholefacilitythroughmitigatingsageffectsonthestator ux.Therotorcurrent,dc-linkvoltage,andtorqueful lltherequirementsatthesametime.Inaddition,theDFIGwindturbinescanuninterruptedlyfeedcurrent,activepower,andreactivepowerintothegridduringvoltagefaults.Afterthefault,theDFIGwindturbinecanfeedthemaximumpossibleactivepowertothegridalmostimmediatelyafterthefaultiscleaned.

Voltage-Sag Tolerance of DFIG Wind Turbine With a series grid side passive-impedance network

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Fig.6.Responseofa2MWDFIGwindturbinewithandwithoutseriespassive-impedancenetworkattype“C”(2 fault)voltagesagdownto15%ofthenominalvoltage.(a)“C”typeofvoltagesagwithoutanycountermeasures.(b)“C”typeofvoltagesagwithpassive-impedance

network.

Fig.7.Responseofa2MWDFIGwindturbinewithandwithoutseriespassive-impedancenetworkattype“D”(1 faulttoground)voltagesagdownto15%ofthenominalvoltage.(a)“D”typeofvoltagesagwithoutanycoun-termeasures.(b)“D”typeofvoltagesagwithpassive-impedancenetwork.

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Fig.8.Responseofa2MWDFIGwindturbinewithandwithoutseriespassive-impedancenetworkattype“G”(2 faulttoground)voltagesagdownto15%ofthenominalvoltage.(a)“G”typeofvoltagesagwithoutanycoun-termeasures.(b)“G”typeofvoltagesagwithpassive-impedance

network.

Fig.9.Illustrationoftheexperimentalhardware

setup.

Fig.10.Experimentalresults,voltagesagtype“A”(3 fault),Nv=0.15,ωrλ=1.20p.u.Fromtoptobottom:(a)vfabc(25V/div);(b)irabc(10A/div);(c)qds(0.2Wb/div);(d)vdc(25V/div),isab(10A/div);50

ms/div.

Fig.11.Experimentalresults,voltagesagtype“C”(2 fault),Nv=0.15,ωrλ=1.20p.u.Fromtoptobottom:(a)vf(d)vabc(25V/div);(b)irabc(10A/div);(c)qds(0.2Wb/div);dc(25V/div),isab(10A/div);50ms/div.

BORATORYSCALETESTDEMONSTRATION

Thesimulationresultsinthissectionindicatedexcellentper-formanceofthe2MWDFIGwind-turbinesystemwithseriespassive-impedancenetworkunderavarietyofsagconditions.Hardwaredemonstrationswerecarriedoutona2-kWDFIGsetupasdescribedinthissection.

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Fig.12.Experimentalresults,voltagesagtype“D”(1 faulttoground),Nv=0.15,ωr=1.20p.u.Fromtoptobottom:(a)vf(10A/div);(c)λ(0.2Wb/div);(d)vabc(25V/div);(b)irabcqdsdc(25V/div),isab(10A/div);50

ms/div.

Fig.13.Experimentalresults,voltagesagtype“G”(2 faulttoground),Nv=0.15,ωr=1.20p.u.Fromtoptobottom:(a)vfabc(25V/div);(b)ir(10A/div);(c)λV/div),isabcqds(0.2Wb/div);(d)vdc(25ab(10A/div);50ms/div.

A.ExperimentalHardwareSetup

Anillustrationoftheexperimentalhardwaresetupispre-sentedinFig.9.TheelectricalparametersareshowninTableIII.TheshaftoftheDFIGisdrivenbyapermanentmag-netacmotor.Agridemulatorgeneratesdesiredacvoltagesagsatthepointofinterconnection.TherotorwindingsoftheDFIGareaccessedviaslipringsandconnectedtotheMSC.ThedcbusoftheMSCissharedbytheGSC.EachoftheinvertersiscontrolledbyoneofthetwoDSP/FPGAcontrolboards,andissine-trianglemodulatedwiththird-harmonicinjection.Switch-ingandsamplefrequenciesarebothsetto5kHz.Therotorpositionisdeterminedfromanencoder,andisalsousedforrotor-speedestimationinstatefeedbackdecouplingterms.TheangleofthevoltagevfisestimatedfromaphaselockedloopforuseinsynchronousframecontrollersfortheMSCandGSC.

B.HardwareResults

Scopecapturesfrombalancedandunbalancedvoltagesageventsonthe2kWlaboratoryscalehardwaretest-bedarepre-sentedinFigs.10–13.Ineachcase,theDFIGspeedis1.20p.u.,runningnearratedpoweratnominalvoltageandfrequency.ThescopeimagecapturesshowresponseoftheDFIGtotypeA,C,D,andGsagsrepresentativeofthree-phase,phase-to-phase,one-phase-to-ground,andtwo-phase-to-groundfaults,eachwithacharacteristicp.u.voltage,Nvof0.15.Therotorcurrents(seeframe(b)of gures)havesomesecond-harmoniccomponent,buttheyarewithintheboundsoftheMSCcurrentrating.

VI.CONCLUSION

ThispaperhasdemonstratedthecapabilityofLVRTofaDFIGwindturbineusingastator-sideseriespassive-impedancenetworkatbalancedandunbalancedshort-circuitgridfaults.Thepresenceoftheseriespassive-impedancenetworkallowsamechanismtomitigatetheeffectsofsagsonthestator ux.SimulationandexperimentalresultsofthedesignedsystemshowexcellentperformanceofLVRTinthebalancedandunbal-ancedgridshort-circuitfault.Italsoindicatesmanyadvantagesbyusingstator-sideseriespassive-impedancenetworkforridethroughatavarietyofsagevents,e.g.,thesimplecontrollogics,avoidingthephenomenonoftheimpactofPCCvoltagesagonorientationvector,andmorepracticalandreliableimplementa-tions.Furthermore,theassistantequipmentusedforDFIGwindturbinefortheLVRTisfullyindependent,thus,itisconvenienttoinstall,operate,andmaintain.Finally,thisassistantequip-mentalsoindicateslowercostforapplicationforMWscaleDFIGwindturbines.

APPENDIX

SeeTableIIandTableIII.

ACKNOWLEDGMENT

TheauthorswouldliketothankforthesupportandmotivationprovidedbytheWisconsinElectricMachinesandPowerElec-tronicsConsortium(WEMPEC)oftheUniversityofWisconsin–Madison.

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1–10.

XiangwuYan(M’09)receivedtheB.E.degreeinelectricalengineeringfromtheHunanUniversity,Changsha,China,in1986,theM.S.degreefromtheNorthChinaElectricPowerUniversity,Baoding,China,in1990,andthePh.D.degreefromtheHarbinInstituteofTechnology,Harbin,China,in1997.

HewasanHonoraryFellowoftheWis-consinElectricMachinesandPowerElectronicsConsortium(WEMPEC),UniversityofWisconsin–Madison,Madison.HejoinedtheNorthChinaElec-tricPowerUniversityasaFacultyMember,where

heiscurrentlyinvolvedintheresearchinelectronicpowerconversion,powerquality,andrenewableenergygenerationasa

Professor.

GiriVenkataramanan(M’92–SM’06)receivedtheB.E.degreeinelectricalengineeringfromtheGov-ernmentCollegeofTechnology,Coimbatore,India,in1986,theM.S.degreefromtheCaliforniaInsti-tuteofTechnology,Pasadena,in1987,andthePh.D.degreefromtheUniversityofWisconsin–Madison,Madison,in1992.

AfterteachingelectricalengineeringatMontanaStateUniversity,Bozeman,hereturnedtotheUniver-sityofWisconsinasaFacultyMemberin1999,whereheiscurrentlyinvolvedintheresearchinvariousar-easofelectronicpowerconversionasanAssociateDirectoroftheWisconsinElectricMachinesandPowerElectronicsConsortium(WEMPEC),Madison.HeholdsseveralU.S.patentsandisthecoauthorofmorethan100technical

publications.

PatrickS.Flannery(M’99)receivedtheB.S.de-greeinmechanicalengineeringfromthePennsyl-vaniaStateUniversity,UniversityPark,in1998,theM.S.andPh.D.degreesinelectricalengineeringfromtheUniversityofWisconsin–Madison,Madison,in2003and2008,respectively.

From1998to2001,hewasanElectromechani-calEngineeratCSAEngineeringinMountainView,CA.HeiscurrentlyaPrincipalEngineeratAmer-icanSuperconductorinMiddleton,Middleton,WI.Hisresearchinterestsincludetheapplicationpower

electronics,electricmachinesandcontroltorenewableenergygeneration.Dr.FlanneryisamemberoftheAmericanSocietyofMechanicalEngineers.

YangWang(S’09)receivedtheB.S.degreeinelectricalengineeringfromtheZhejiangUniversity,Hangzhou,China,in2007.HeiscurrentlyworkingtowardtheM.S.andPh.D.degreesinelectricalengineeringfromtheUniversityofWisconsin–Madison,Madison.

Hiscurrentresearchinterestsincludepowerelectronics,drives,and

control.

QingDongreceivedtheB.S.,M.S.,andPh.D.de-greesfromtheNorthChinaElectricPowerUni-versity,Baoding,China,in1990,1994,and2003,respectively.

HeiscurrentlyanAssociateProfessorattheNorthChinaElectricPowerUniversity.Hisresearchinter-estincludespowersystemrobust

control.

BoZhangwasborninHebei,China,in1981.HereceivedtheB.S.andM.S.degreesinelectricalengi-neeringfromtheNorthChinaElectricPowerUniver-sity,Baoding,China,in2005and2008,respectively.HeiscurrentlywiththeDepartmentofElectri-calEngineering,NorthChinaElectricPowerUni-versity.Hisresearchinterestsincludetheapplica-tionofpowerelectronicsinpowersystemandPWMconverter.

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