GRID-CONNECTION-OF-DOUBLY-FED-INDUCTION-GENERATOR-

1INTRODUCTIONTheincreasingpenetrationofwindenergyinthepowersystemisreshapingthewaywindfarmsareoperated.Denmarkmaintainsa15%yearlyaverageofthetotalpowergeneratedbywindfarms,andduringcertainperiodsofhighwindandlowconsumption,mostofthepowerisgeneratedbythewind.Othercountriesmayfaceasimilarsituationinashorttime.Theshareofwindpowerinrelationtothestrengthofelectricitygridsandotherpowerplantsisreachinglevelswheretheymaycauseproblemstosystemoperatorssuchasvoltagevariations,gridoutputunbalanceandgridinstability.Windfarmscannolongerbeconsideredasasimpleenergysource,VARneutralorconsumer,shuttingdownwhensystemfaultsoccurandwithlocalcontrolofregulation.Nowtheymustoperateaspowerplants,theymustbeabletoprovidereactivepower,remainconnectedduringsystemfaultsandadapttheircontroltotheneedsofthesystem.SystemOperatorsinDenmarkandGermany[1][2]havebeenthefirstcountriesadaptingtheirgridcodestothisnewscenario.Manyothercountrieshavefollowedtheexample,andothersarestilladaptingtheirgridcodes.Agoodreviewofnewgridcodesadaptedforwindfarmintegrationisshownin[3][4].Windfarmsmayeasilycontributetogridfrequencycontrolbymodifyingactivepowercontrolstrategies.Voltagecontrolcanalsobeimplementedbecausemostwindpowerfarmscancontrolreactivepower,sobyadaptingthereactivepowercontrolstrategyVoltagecontrolisachieved.Themostdemandingrequisiteforwindfarms,speciallywithDoublyFedInductionGenerators(DFIG)istheFaultRideThroughcapability.Windfarmsconnectedtohighvoltage(usuallyabove100kV)transmissionsystemmuststayconnectedwhenavoltagedipoccursinthegrid,otherwise,thesuddendisconnectionofagreatamountofwindpowermaycontributetothevoltagedip,withterribleconsequences.Windfarmsmustremainconnectedwhenthevoltagedipprofileisabovethelineshowninfigure1undertheE.ONregulation[2].Theperunitvoltageatthepointofconnectiontothegridisshownintheverticalaxesandtheduration(seconds)ofthefaultinthehorizontalaxes.ThiscoderequiresFaultRide-Through(FRT)capabilityduringvoltagedropsinTransmissionSystem(110kVandabove)of15%ofnominalvoltageduring300mswithrecoveryupto80%ofnominalvoltageafter3s,withtheslopeshowninfigure1.Othergridcodesproposesimilarprofiles,adaptedtotheregionsfaultstatistics.3000Timeafterfault[ms]0Voltage(p.u.)300ms3sec.0.80.1510.20.40.60.8Figure1.E.ONFaultRideThroughVoltagelimitingcurveActualgridcodesinvigouronlymentiondepthanddurationforbalancedvoltagedips.Theeffectofsuchavoltagedipinthewindturbineisdifferentfordifferentwindturbinesystemtechnologies.Directlycoupledsynchronousgeneratorsandsquirrelcageasynchronousgeneratorswithfullpowerback-to-backconvertersmaycopewiththevoltagedipreadaptingtheircontrolsandpowerratings.OntheotherhandDoublyFedInductionGenerators(DFIG)withthepowerconvertersconnectedtotherotormustbedisconnectedfromthegridunlessadditionalprotectionsystemsareprovided.ThismayGRIDCONNECTIONOFDOUBLYFEDINDUCTIONGENERATORWINDTURBINES:ASURVEYI.MARTINEZDEALEGRIA1,J.L.VILLATE2,J.ANDREU1,I.GABIOLA2,P.IBAÑEZ2,(1):UPV-EHU,UrkixoZumarkalea,48013Bilbao,Spain,[e-mail]él:[phone],Fax:[phone](2):ROBOTIKER,ParqueTecnológicoEdif.20248170Zamudio(Bizkaia),[e-mail]él:[phone],Fax:[phone]ABSTRACT:ThesuitabilityofWindTurbineswithDoublyFedInductionGeneratorfornewgridoperatornormsthatrequireRideThroughoperationisanalysed.Newgridcodesrequirethatthewindturbinesremainconnectedduringvoltagedipsinthegrid.AsurveyoftheproblemsassociatedwithvoltagedipsandmethodsforRideThroughoperationofthistypeofsystemarepresented.DifferentalternativesforLowVoltageRideThrougharedescribed.SimulationresultsofPowerErrorVectorControlmethodfortheVoltageSourceInverterconnectedtotherotorduringavoltagedipareshown.Thesimulatedresultsshowthatthistypeofcontrolmayeliminatetheneedforacrowbar,oratleastagreatreductionintherotorcurrentsisobtainedandactivationofcrowbarmaybelimitedtoveryextremecaseswhenverystrongwindgustsandverydeepvoltagedipstakeplaceatthesametime.Keywords:Doubly-FedinductionGenerator,LowVoltageRideTrough,VoltageDipbecomeaseriousdrawbackforthewindturbineindustrybecausemostofthewindturbinesmanufacturedactuallyuseDoubly-FedInductionGenerators.2DFIGLOWVOLTAGERIDETHROUGHPROBLEMDESCRIPTIONExperienceofwindfarmdevelopersandresearchstudieshavedemonstratedthatgridfaultsnegativelyaffectwindfarms.Thedifferenttypesoffaultsthatmayaffectawindfarmaresummarizedin[5]andaremainlyclassifiedassymmetricalandnon-symmetricalfaults.Actualgridcodesonlyspecifysymmetricalfaults,becausetheyaffectmoreseverelythegridstability.Ontheotherhand,non-symmetricalfaultsaremoredifficulttodealwithDFIGgenerators.ThestatorfluxoftheDFIGmachineisdeterminedbythefollowingexpression:()dtIRVtsssss∫−+=00ψψ(1)TheproblemwithaDFIGwhenavoltagedipoccursisthatthestatorfluxcannotfollowthesuddenchangeinstatorvoltageandaDCcomponentinthestatorfluxappears,becausetheintegraltermisreducedandthestatorfluxvectorbecomesalmoststationary.Therotorkeepsturningandhighslipisgenerated,whichtendstointroduceover-voltageandover-currentintherotorcircuitsduetotheeffectofspeed-voltage.Non-symmetricalfaultscreatehigherover-currentsandover-vo