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DirectDriveSynchronousMachineModelsfor

StabilityAssessmentofWindFarms

SebastianAchillesandMarkusP¨oller

Abstract—Theincreasingsizeofwindfarmsrequirespower

systemstabilityanalysisincludingdynamicwindgeneratormod-els.Forturbinesabove1MW,doubly-fedinductionmachinesarethemostwidelyusedconcept.However,especiallyinGer-many,direct-drivewindgeneratorsbasedonconverter-drivensyn-chronousgeneratorconceptshavereachedconsiderablemarketpenetration.Thispaperpresentsconverterdrivensynchronousgeneratormodelsofvariousorderthatcanbeusedforsimulatingtransientsanddynamicsinaverywidetimerange.

IndexTerms—Transmissionnetworks,variablespeedwindturbines,directdrivesynchronousmachines,off-shorewindpowergeneration,powersystemstability

W

I.INTRODUCTION

ORLDWIDE,thetotalinstalledwindpowercapacityaswellastheaverageratedpowerperwindmillisconstantlyincreasing.InmanyEuropeancountries,particularlyinDenmark,Germany,andSpain,wind-powerexperiencedarapidgrowthduringthepastyearsandcontributesnowadaysconsiderablytotheoverallelectricitypro-duction.

Asaresult,thereisastrongneedfortransmissionsystemopera-torstocarryoutpowersystemstabilityanalysisofthewholesystemincludingdynamicmodelsofon-andoff-shorewindfarms,sothatallinteractionsbetweenconventionalpowerplantsandwindpowergen-eratorsForthiscanbepurpose,assessed.

standardmodelsareneededthatcanbeusedforsimulatingcompletepowersystemswithhundredsofgenerators.Fortheseapplicationsthemodelcomplexitymustbechoseninawaythatmodelaccuracyissuf cientwithregardtotheresponsetowind uc-tuationsandfaultsintheelectricalnetworkbutthecalculationtimeforsimulatinglargesystemsremainsinanacceptablerange.

Nowadays,forunitsabove1MW,variablespeedconceptsareusu-allyappliedthatareeitherbasedondoubly-fedinductionmachines(e.g.GEWind-Power,Vestas,REPower,Nordex,NEG-Micon)orconverter-drivensynchronousmachines(e.g.ENERCON,P eiderer).Someofthelargestunitscurrentlyavailableandthereforeespe-ciallysuitedforoff-shoreapplicationsarebuiltonthedirectdrivesynchronousmachineconcept.Thispaperpresentsanapproachforstandardvariablespeedwindgeneratormodelsbasedonthisconcept.Itincludesmodelsofallcomponents,generatorandconverter(diodeorPWM),typicalapproachesforthecontrolcircuitsandaerodynamicsoftheturbine.

Thefocusofthepresentedmodelsisonpowersystemstabilityanal-ysis.However,someofthepresentedmodelscanalsobeusedfor ickeranalysis,harmonicsstudiesandelectromagnetictransientssim-ulations.

AllmodelshavebeenimplementedandtestedinthepowersystemanalysisprogramDIgSILENTPowerFactory[11].

II.CONVERTERDRIVENSYNCHRONOUSGENERATORS

Figure1showstheprincipalarrangementofadirectdrivesyn-chronousgenerator.Rotorandgeneratorshaftsaremountedtothe

SebastianAchillesandMarkusP¨ollerarewithDIgSILENTGmbH,Heinrich-Hertz-Str.9,72810Gomaringen,Germany(email:m.poller@digsilent.de,s.achilles@digsilent.de)

Fig.1.Direct-drivesynchronousgenerator

sameshaftwithoutgear-box.Thegeneratorisahigh-polesynchronousgeneratordesignedforlowspeed.Itcaneitherbeanelectricallyex-citedsynchronousgeneratororapermanentmagnetmachine.

Fig.2.Converter-drivensynchronousgeneratorwithgear-box

Synchronousmachinesofdirect-drivewindgeneratorstendtobeverylargeduetothehighnumberofpoles.Figure2showsacom-promisebetweenadirectdriveconceptandadesignwithgearbox.Here,arobust,single-stagegearboxwithlowratioisusedallowingforalowernumberofpolesandhenceasmallergeneratorthaninapuredirectdrivewindgenerator.Electrically,thisconceptbehavesanalogouslytothedirectdriveconceptof gure1.

Permanentmagnetmachineshaveusuallyahigheref ciencyandaremorecompactthanelectricallyexcitedmachines.However,theyarestillconsiderablymoreexpensiveandrequiremoreadvancedrec-ti ersbecausetheydon’tallowforreactivepowerorvoltagecontrol.Themostcommonconceptfordirectdrivewind-generatorsisthe”Enercon”-conceptusingasalientpole,electricallyexcitedsyn-chronousgenerator[3].

Forallowingvariablespeedoperation,thesynchronousgeneratormustbeconnectedtothegridthroughafrequencyconverter.Thereisanalmostin nitenumberofpossibilitiesrealizingfrequencycon-vertersandmostofthemhavealreadybeenusedforwindpowerap-plicationsinthepast.Theconcepthavingnowadaysthewidestspreadisshownin gure3.ThegeneratorisconnectedtoanintermediateDC-circuitbyadioderecti er.Thegrid-sideconnectionisrealizedbyaselfcommutatedpulse-widthmodulated(PWM)-converterthatim-posesapulse-widthmodulatedvoltagetotheAC-terminal.ThePWM-converterisconnectedtothenetworkthrougha lter,symbolizedbytheL-Ccircuitin gure3.Thelevelofharmonicsinthevoltageattheconnectionpointisextremelylow.

Thecontrolconceptfortheinverterof gure3consistsoffastcur-rentcontrollersthatregulateACcurrentsinanAC-voltageoriented