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外文翻译
外文原文
DesignStudyofDoubly-FedInductionGenerators
fora2MWWindTurbine
ABSTRACT
Adesignstudyfora2MWcommercialwindturbineispresentedtoillustratetwoconnectionmethodsforastandarddoubly-fedinductionmachinewhichcanextendthelowspeedrangedownto80%slipwithoutanincreaseintheratingofthepowerelectronicconverter.Thisfarexceedsthenormal30%lowerlimit.Thelowspeedconnectionisknownasinductiongeneratormodeandthemachineisoperatedwithashortcircuitedstatorwindingwithallpowerflowbeingthroughtherotorcircuit.AtwoloopcascadedPIcontrolschemehasbeendesignedandtunedforeachmode.Thepurposeofthispaperistopresentsimulationresultswhichillustratethedynamicperformanceofthecontrollerforbothdoubly-fedinductiongeneratorconnectionmethodsfora2MWwindturbine.Asimpleanalysisoftherotorvoltageforthedoubly-fedconnectionmethodisincludedasthisdemonstratesthedominantcomponentsthatneedtobeconsideredwhendesigningsuchadvancedcontrolstrategies.
Keywords:
Doubly-fed,Inductiongenerator,Windturbine
LISTOFIMPORTANTSYMBOLS
vrdqDirectandquadraturerotorvoltage
irdqDirectandquadraturerotorcurrent
λsdqDirectandquadraturestatorfluxlinkage
PsStatorrealpower
QsStatorreactivepower
pfsStatorpowerfactor
TeTorque
pDifferentialoperator
LmMagnetisingreactance
RrRotorresistance
LrRotorreactance
σTotalleakageinductance
ωsfSlipfrequency
‘s’Statorreferred
‘s’Rotorreferred
‘*’Referencevalue
1.INTRODUCTION
Thereiscontinuinginterestinwindturbines,especiallythosewitharatedpowerofmanymegawatts.Thispopularityislargelydrivenbybothenvironmentalconcernsandalsotheavailabilityoffossilfuels.Legislationtoencouragethereductionofthesocalledcarbonfootprintiscurrentlyinplaceandsointerestinrenewablesiscurrentlyhigh.Windturbinesarestillviewedasawellestablishedtechnologythathasdevelopedfromfixedspeedwindturbinestothenowpopularvariablespeedtechnologybasedondoubly-fedinductiongenerators(DFIGs).ADFIGwindturbineisvariablespeedwiththerotorconverterbeingcontrolledsothattherotorvoltagephaseandmagnitudeisadjustedtomaintaintheoptimumtorqueandthenecessarystatorpowerfactor[1,2,3].DFIGtechnologyiscurrentlywelldevelopedandiscommonlyusedinwindturbines.ThestatorofaDFIGisdirectlyconnectedtothegridwithapowerelectronicrotorconverterutilisedbetweentherotorwindingandthegrid.Thevariablespeedrangeisproportionaltotheratingoftherotorconverterandsobylimitingthespeedrangeto±30%[4,5,6,7]therotorconverterneedonlyberatedfor30%ofthetotalDFIGpowerwhilstenablingfullcontroloverthefullgeneratoroutputpower.Thiscanresultinsignificantcostsavingsfortherotorconverter[4].Theslipringconnectiontotherotorwindinghowevermustbemaintainedforreliableperformance.
Thepower–generatorspeedcharacteristicshowninfigure1isforacommercial2MWwindturbine.Thegeneratorspeedvarieswithwindspeedhoweverthisrelationissetforaspecificlocation.Aswindspeed,andthereforemachinespeed,fallsthepoweroutputofthegeneratorreducesuntilthewindturbineisswitchedoffwhenthepowerextractedfromthewindislessthanthelossesofthegeneratorandconverter.Anoperatingmodehasbeenproposedbyawindturbinemanufacturerthatisclaimedtoextendthespeedrangesothatatlowerspeedthepowerextractedfromthewindisgreaterthanthelossesinthesystemandsothesystemcanremainconnected.Thisproposedthatthestandarddoubly-fed(DF)connectionisusedoverthenormalDFspeedrangeandtheso-calledinductiongenerator(IG)modeisusedtoextendthelowspeedoperation.PreviousworkhasillustratedthatIGmodeenablestheDFIGtooperatedownto80%slip[8].ThischangeinoperationisachievedbydisconnectingthestatorfromthegridinDFmodeandthenshortcircuitingthestatortoenableIGoperation.AllofthegeneratorpowerflowsthroughtherotorconverterinIGmode.TheIGcurveisidenticaltotheDFcurvefor±30%slip.TheestimatedIGpowerextractedfromthewindatlowspeedsisobtainedbyextrapolatingthecurvefortheDFmode.
Thereferencetorquerequiredbybothcontrollers(DFandIGmode)caneasilybederivedfromthiscurve.Thetorque–speeddatacanthenbestoredinalook-uptablesothereferencetorqueisautomaticallyvariedwithspeed.
ThecapabilityofmodernDFwindturbinestovarythereactivepowerabsorbedorgenerated[6,9,10]allowsawindturbinetoparticipateinthereactivepowerbalanceofthegrid.Thereactivepoweratthegridconnectionconsideredinthisworkisdescribed,fortheUK,bytheConnectionConditionsSectionCC.6.3.2[11]availablefromtheNationalGrid.Thereactivepowerrequirementforawindfarmisdefinedbyfigure2.
PointA-MVArequivalentfor0.95leadingpowerfactoratratedMW
PointB-MVArequivalentfor0.95laggingpowerfactoratratedMW
PointC-MVAr-5%ofratedMW
PointD-MVAr5%ofratedMW
PointE-MVAr-12%ofratedMW
TheobjectiveofthispaperistoinvestigatethecontrollerperformanceofDFandIGmodefora2MW,690V,4-poleDFIGusingmachineparametersprovidedbythemanufacturer.Thisisfurtherresearchbuildingonapreviouspaperwhichdemonstratedthesteady-stateperformanceofthetwomodesofoperation,DFandIGmode[8].In[8]theauthorsdiscussedthesteady-stateefficiencyforbothconnections.Thesteady-stateperformanceworkillustratedthattherewerebenefitstooperatingthemachineinoneconnectionmethodasopposedtotheother.
Thispaperexaminesthecontrollability(i.e.transientperformance)ofthe2MWwindturbine.Resultsofthefulldynamiccontroller(currentregulation,decouplingequationsandvectorcontrol)inbothDFmodeandIGmodeareshown.AdetailedanalysisofthecomponentsthatformtherotorvoltageoverthefulloperatingrangeinDFIGmodeispresentedasthisenablesthedominantcontrolcomponentstobeidentified.Thisisparticularlyimportantwhendesigningadvancedcontrolschemesasanoverviewoverthefulloperatingrangecanbeidentified.Simulationmodels,whichhavebeenvalidatedagainsta7.5kWlaboratoryrig[12],areappliedtoarealistic2MWwindturbinetoenableconclusionstobemaderegardingtheproposeduseofIGmodeinarealwindturbine
2.CONNECTIONMETHODS
Doubly-fedinductionmachinesarecommonlyconnectedasshowninfigure3.Thegridsideinverter(GSI)iscontrolledtomaintainafixeddclinkvoltagewithagivenpowerfactoratthegrid(inourcaseunity).Therotorsideinverter(RSI)iscontrolledsothemaximumenergyisextractedfromthekineticenergyofthewindwhilstenablingthestatorpowerfactortobecontrolledwithinthelimitsofthegridrequirementsthoughunitypowerfactorisoftendesirable.
Analternativeconnectionmethodforadoubly-fedmachineisshowninfigure4,herecalledtheinductiongenerator(IG)connection.Thestatorisdisconnectedfromthegridandisshort-circuited.Therotorcircuitisunchangedfromfigure3.TheGSIiscontrolledasinDFmode.TheobjectiveoftheRSIistocontrolthestatorfluxlinkagewhileextractingthemaximumpowerfromthekineticwindenergy.
3.CONTROLLERPERFORMANCE
AclosedloopcontrollerforbothDFmodeandIGmodehasbeendiscussedinpriorwork[12]butonlyfora7.5kWlaboratorytestrig.Thedynamicsofa2MWsystemaresomewhatdifferentandareinvestigatedinthispaper.TheperformanceofthedynamiccontrollerforbothDFandIGmodeareshowninthissectionfora2MWwindturbine.
3.1.DFIGMode(TandQControl)
ThereferencevaluesforthecontrollerinDFmodearetorque(seefigure1)andstatorreactivepowertoenablethegridcoderequirement[11]tobeachieved,figure2.Twospeedsareinvestigatedinthissectiontoenabletheperformanceofthecontrollertobeshownbothaboveandbelowthe20%ofratedpowerlimitfromthegridcoderequirements.Anominalgeneratedpowerof320kWisachievedat1150rpm(lessthan20%ofratedpower)and
anominalpowerof1.25MWisachievedat1550rpm(greaterthan20%oftheratedpower).Thereferenceandactualtorque,Te,andstatorreactivepower,Qs,areshownforbothspeeds
infigure5.
Thevalueofreferencetorque,Te*,forbothspeedsisthespecificnominaltorqueforagivenspeedcalculatedfromfigure1;−2672Nmfor1150rpmand−7701Nmfor1550rpm.Astepof200Nmisappliedatbothspeedstoillustratethedynamicresponsetoastepchangeintorque.Thevalueofreferencestatorreactivepower,Qs*,at1150rpmisvariedbetweenthelimitsspecifiedbythegridcoderequirements;initially−5%ofthegeneratedpowerwithastepatt=3.5sto+5%ofthegeneratedpower.At1550rpmthestatorpowerfactor,pfs*,isinitially0.95leadingwithastepchangeatt=3stounitypfsandafinalstepatt=4stoa0.95laggingpfs.Thevectorcontrolloopsaretunedforatimeconstantof0.1sand0.9sfortheTeandtheQsloopsrespectively.Thevectorcontrolisdesignedtohaveaslowerbandwidththanthecurrentregulation.
Theactualrotorcurrentdirect,irds,andquadrature,irqs,componentscorrespondingtofigure5areshowninfigure6.TheeffectofthestepchangeinTe*isapparentontheirqs(thesuperscript‘s’indicatesthatthevariableisreferredtothestator)asexpected.Theirqs*componentat1550rpmcontainssmalltransientresponsesatt=3sandt=4sthatareduetothestepchangesintheQsvalue.ThestepchangeinQs*,showninfigure5,causesafastchangeinirds*,figure6,asthereisinitiallyanerrorbetweenthereferen