Embedding wind farm generation in power system tra

1Abstract—NowadaysWindEnergyConversionSystem(WECS)isoneofthemostemployedformofrenewablegenerationthatmayintroduceoperationproblemsinthepowersystem.Infact,becauseofthelargesizeofthelatestwindfarms,thepowerproducedbythisnewtechnologyisoftendeliveredtotransmissionnetwork.Asconsequence,windturbinescaninfluencethestabilityofpowersystem.Inthispaper,themodelsofthewindturbines,inductiongenerators(fixedandvariablespeed)togetherwiththecontroldevicesareintegratedintoapowersystemfortransientstabilitystudies.Thesimulationresultsshowthatwindgeneratorcharacteristicsandtheircontrolmayremarkablyinfluencepowersystemdynamics.IndexTerms—doublyfedinductiongenerator,fixedspeedinductiongenerator,transientstabilityanalysis,windfarms.I.INTRODUCTIONITHtheentryintoforceoftheKyotoProtocol(onFebruary2005),anumberofcountrieshavetoimplementnewrenewableenergytargets,whichrequiresignificantamountsofwindgeneration.Inparticular,theEuropeanCommission,initslastpublishedreport[1],foreseesatotalamountof79.9GWofwindpowercapacitytobeinstalledintheEUby2010.Thefastgrowthofwindgenerationleadstoconcernaboutitseffectsonelectricgrid.Windfarmsproliferationcouldseriouslycompromisethepowersystemsecurity.Infact,WindEnergyConversionSystems(WECSs)donottakepartinvoltageandfrequencycontroland,ifadisturbanceoccurs,WECSsareinstantaneouslydisconnectedandreconnectedonlywhennormalconditionshavebeenresumed.Thiswaspossibleaslongaswindpowerpenetrationwasstilllow.WhenaremarkableamountofWECSpowerispresent,theprotectivedisconnectionoflargeamountofwindpowermaycauseanimportantlossofgenerationthatmaythreatenthepowersystemstability.However,sincemostwindfarmsareofhighsize,theyaredirectlyconnectedinthetransmissionorsub-transmissionlevelaffectingtheirdynamics.Forthesereasons,dynamicsecuritystudiesonapowersystemmusttakeintoaccountWECSmodelling.Windturbinesusuallyareequippedwithfixedspeedinductiongenerators(FSIGs)thataredirectlygrid-coupledorwithvariablespeed,typically,doublyfedinductiongenerators(DFIGs)grid-coupledviapowerelectronicconverters.DependingontheadoptedWECStechnology,differentresponsestodisturbancescanbeexperienced.Therecentliteratureinthisfieldpresentsvariousstudies[2]-[5].Themainaspectsthatcharacterizethetwotypesofinductiongeneratorscanbesummarizedasfollows[6].FSIGconsistsofasquirrelcageinductiongeneratorthatiscoupledtothewindturbinerotorthroughagearbox.Thepowerextractedfromwindislimitedusingstalleffect.Thismeansthattherotorisdesignedinsuchawaythatitsefficiencydecreasesinhighwindspeeds,thuspreventingthemechanicalpowerextractedfromthewindtobecometoolarge.ThistypeofWECS,withregardtoDFIGs,hastheadvantageofbeingsimplerandcheaper,indeedthedisadvantagesare:(i)lackofcontrolpossibilitiesofbothactiveandreactivepower;(ii)largemechanicalloads,becausepowerfluctuationsaretranslatedintotorquepulsationsleadingtoagearboxfailure;(iii)largefluctuationsinoutputpower,becausenoenergybufferintheformofarotatingmasswithvaryingrotationalspeedispresent.Forthesereasons,windturbinemanufacturersareincreasinglymovingtovariablespeeddevices.IntheDFIG,windturbineutilizesawoundrotorinductiongenerator,whereboththerotorandstatorarefed.Likeinthefixedspeed,thewindturbinerotoriscoupledtothegeneratorthroughagearbox.Athighwindspeeds,thepowerextractedfromthewindcanbelimitedbypitchingtherotorblades.TheadvantagesofDFIGarethat(i)itoffersextensivecontrollabilityofbothactiveandreactivepower;(ii)itcaneasiercomplywiththerequirementsofgridcompanies,especiallywiththoseappliedtolargewindfarms;(iii)itshowslessfluctuationsinoutputpower,becauselargerotorinertiasmoothvariationsinwindspeed,canbeobserved;(iv)thepowerelectronicconverterhasaratingofonlyonethirdofthenominalwindturbinepower.ThecontributionofthispaperisinanalysingthetransientbehaviorofapowersystemwherearemarkablenumberofWECSsispresent.SuitablemathematicalmodelsforFSIGsandDFIGswiththeircontroldevicesareanalyzedandcompared.Simulationsarecarriedoutforeseeingaconsiderablepenetration(upto1500MW)ofwindgenerationforSouthernItalypowersystem.Theinfluenceonsynchronousgenerator(SG)dynamicsduetotheWECSpresenceaftershort-circuitfaultsareinvestigated.ResultsofsystemtimedomainsimulationspointoutthatanimprovementoftheglobaltransientbehaviorcanbeobservedwhenFSIGsarereplacedbytheDFIGs.Thepaperisorganizedasfollows:intheSectionII,adescriptionofthewindturbinemodelsisgiven.Then,inSectionIII,thepowersystemunderinvestigationismodelledinordertoembedWECSgeneration.TestresultsontheSouthernItalypowersystemarereportedinSectionIV.V.Bufano,M.Dicorato,Member,IEEE,A.Minoia,StudentMember,IEEE,M.TrovatoMember,IEEEDipartimentodiElettrotecnicaedElettronica-PolitecnicodiBari,ItalyEMBEDDINGWINDFARMGENERATIONINPOWERSYSTEMTRANSIENTSTABILITYANALYSISWAuthorizedlicenseduselimitedto:CHANGSHAUNIVOFSCIENCEANDTECHNOLOGY.DownloadedonDecember1,2008at08:54fromIEEEXplore.Restrictionsapply.2II.THEWECSMODELA.FixedspeedwindgeneratorThedetaileddynamicrepresentationoftheinductiongeneratorisusuallybasedonafifth-orderParkmodel[7].However,whentheinductionmachineispartofalargesystem,itisdesirabletousealower-ordermodel.Thi