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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