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外文翻译风力发电中的自我激励与谐波.pdf

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外文翻译风力发电中的自我激励与谐波.pdf

rugged,egenerationcurrguidelines2PowerSystemNetworkDescriptionbinecanenterselfexcitationoperation.Thevoltageandfrequencyduringoffgridoperationaredeterminedbythebalancebetweenthesystemsreactiveandrealpower.Downloaded28Mar2008to211.82.100.20.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmWeinvestigateaverysimplepowersystemnetworkconsistingofone1.5MW,fixedspeedwindturbinewithaninductiongeneratorconnectedtoalinefeederviaatransformerH208492MVA,3phase,60Hz,690V/12kVH20850.Thelowspeedshaftoperatesat22.5rpm,andthegeneratorrotorspeedis1200rpmatitssynchronousspeed.AdiagramrepresentingthissystemisshowninFig.1.ThepowersystemcomponentsanalyzedincludethefollowingAninfinitebusandalonglineconnectingthewindturbinetothesubstationAtransformeratthepadmountOnepotentialproblemarisingfromselfexcitationisthesafetyaspect.Becausethegeneratorisstillgeneratingvoltage,itmaycompromisethesafetyofthepersonnelinspectingorrepairingthelineorgenerator.Anotherpotentialproblemisthatthegeneratorsoperatingvoltageandfrequencymayvary.Thus,ifsensitiveequipmentisconnectedtothegeneratorduringselfexcitation,thatequipmentmaybedamagedbyover/undervoltageandover/underfrequencyoperation.Inspiteofthedisadvantagesofoperatingtheinductiongeneratorinselfexcitation,somepeopleusethismodefordynamicbrakingtohelpcontroltherotorspeedduringanemergencysuchasagridlosscondition.WiththeproperchoiceofcapacitanceandresistorloadH20849todumptheenergyfromthewindturbineH20850,selfexcitationcanbeusedtomaintainthewindturbineatasafeoperatingspeedduringgridlossandmechanicalbrakemalfunctions.Theequationsgoverningthesystemcanbesimplifiedbylookingattheimpedanceoradmittanceoftheinductionmachine.ToContributedbytheSolarEnergyDivisionofTHEAMERICANSOCIETYOFMECHANICALENGINEERSforpublicationintheASMEJOURNALOFSOLARENERGYENGINEERING.ManuscriptreceivedFebruary28,2005revisedreceivedJuly22,2005.AssociateEditorDaleBerg.JournalofSolarEnergyEngineeringNOVEMBER2005,Vol.127/581Copyright©2005byASMEE.MuljadiC.P.ButterfieldNationalRenewableEnergyLaboratory,Golden,Colorado80401H.RomanowitzOakCreekEnergySystemsInc.,Mojave,California93501R.YingerSouthernCaliforniaEdison,Rosemead,California91770SelfExcitationWindPowerTraditionalwindturbinestheyareinexpensive,tiongeneratorsrequirisoftenused.Becausethecapacitorcompensationamongthewindturbine,tantaspectsofwindcontentintheoutputenaandgivessomeH20851DOI10.1115/1.20475901IntroductionManyoftodaysoperatingwindturbineshavefixedspeedinductiongeneratorsthatareveryreliable,rugged,andlowcost.Duringnormaloperation,aninductionmachinerequiresreactivepowerfromthegridatalltimes.Thus,thegeneralpracticeistocompensatereactivepowerlocallyatthewindturbineandatthepointofcommoncouplingwherethewindfarminterfaceswiththeoutsideworld.Themostcommonlyusedreactivepowercompensationiscapacitorcompensation.Itisstatic,lowcost,andreadilyavailableindifferentsizes.Differentsizesofcapacitorsaregenerallyneededfordifferentlevelsofgeneration.Abankofparallelcapacitorsisswitchedinandouttoadjustthelevelofcompensation.Withpropercompensation,thepowerfactorofthewindturbinecanbeimprovedsignificantly,thusimprovingoverallefficiencyandvoltageregulation.Ontheotherhand,insufficientreactivepowercompensationcanleadtovoltagecollapseandinstabilityofthepowersystem,especiallyinaweakgridenvironment.Althoughreactivepowercompensationcanbebeneficialtotheoveralloperationofwindturbines,weshouldbesurethecompensationisthepropersizeandprovidespropercontrol.Twoimportantaspectsofcapacitorcompensation,selfexcitationH208511,2H20852andharmonicsH208513,4H20852,arethesubjectsofthispaper.InSec.2,wedescribethepowersystemnetworkinSec.3,wediscusstheselfexcitationinafixedspeedwindturbineandinSec.4,wediscussharmonics.Finally,ourconclusionsarepresentedinSec.5.andHarmonicsinGenerationarecommonlyequippedwithinductiongeneratorsbecauseandrequireverylittlemaintenance.Unfortunately,inducreactivepowerfromthegridtooperatecapacitorcompensationthelevelofrequiredreactivepowervarieswiththeoutputpower,mustbeadjustedastheoutputpowervaries.Theinteractionsthepowernetwork,andthecapacitorcompensationareimporthatmayresultinselfexcitationandhigherharmonicent.Thispaperexaminesthefactorsthatcontrolthesephenomonhowtheycanbecontrolledoreliminated.H20852CapacitorsconnectedinthelowvoltagesideofthetransformerAninductiongeneratorFortheselfexcitation,wefocusontheturbineandthecapacitorcompensationonlyH20849therighthalfofFig.1H20850.Forharmonicanalysis,weconsidertheentirenetworkshowninFig.1.3SelfExcitation3.1TheNatureofSelfExcitationinanInductionGenerator.Selfexcitationisaresultoftheinteractionsamongtheinductiongenerator,capacitorcompensation,electricalload,andmagneticsaturation.Thissectioninvestigatestheselfexcitationprocessinanoffgridinductiongeneratorknowingthelimitsandtheboundariesofselfexcitationoperationwillhelpustoeitherutilizeortoavoidselfexcitation.Fixedcapacitorsarethemostcommonlyusedmethodofreactivepowercompensationinafixedspeedwindturbine.Aninductiongeneratoralonecannotgenerateitsownreactivepoweritrequiresreactivepowerfromthegridtooperatenormally,andthegriddictatesthevoltageandfrequencyoftheinductiongenerator.Althoughselfexcitationdoesnotoccurduringnormalgridconnectedoperation,itcanoccurduringoffgridoperation.Forexample,ifawindturbineoperatinginnormalmodebecomesdisconnectedfromthepowerlineduetoasuddenfaultordisturbanceinthelinefeeder,thecapacitorsconnectedtotheinductiongeneratorwillprovidereactivepowercompensation,andtheturDownloaded28Mar2008to211.82.100.20.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmoperateinanisolatedfashion,thetotaladmittanceoftheinductionmachineandtherestoftheconnectedloadmustbezero.Thevoltageofthesystemisdeterminedbythefluxandfrequencyofthesystem.Thus,itiseasiertostarttheanalysisfromanodeatoneendofthemagnetizingbranch.Notethatthetermimpedanceinthispaperistheconventionalimpedancedividedbythefrequency.Thetermadmittanceinthispapercorrespondstotheactualadmittancemultipliedbythefrequency.3.2SteadyStateRepresentation.Thesteadystateanalysisisimportanttounderstandtheconditionsrequiredtosustainortodiminishselfexcitation.Asexplainedabove,selfexcitationcanbeagoodthingorabadthing,dependingonhowweencounterthesituation.Figure2showsanequivalentcircuitofacapacitorcompensatedinductiongenerator.Asmentionedabove,selfexcitationoperationrequiresthatthebalanceofbothrealandreactivepowermustbemaintained.EquationH208491H20850givesthetotaladmittanceofthesystemshowninFig.2YSYMH11032YRH110320,H208491H20850whereYSH11005effectiveadmittancerepresentingthestatorwinding,thecapacitor,andtheloadseenbynodeMYMH11032H11005effectiveadmittancerepresentingthemagnetizingbranchasseenbynodeM,referredtothestatorsideYRH11032H11005effectiveadmittancerepresentingtherotorwindingasseenbynodeM,referredtothestatorsideH20849NotethesuperscriptH11032indicatesthatthevaluesarereferredtothestatorside.H20850EquationH208491H20850canbeexpandedintotheequationsforimaginaryandrealpartsasshowninEqs.H208492H20850andH208493H20850R1L/H9275H20849R1L/H9275H208502L1L2RRH11032/SH9275H20849RRH11032/SH9275H208502LLRH1103220H208492H20850whereFig.1ThephysicaldiagramofthesystemunderinvestigationFig.2Perphaseequivalentcircuitofaninductiongeneratorunderselfexcitationmode582/Vol.127,NOVEMBER20051LMH11032L1LH20849R1L/H9275H208502L1L2LLRH11032H20849RRH11032/SH9275H208502LLRH1103220H208493H20850R1LRSRLH20849H9275CRLH2085021L1LLLS−CRLH20849H9275CRLH2085021RSH11005statorwindingresistanceLLSH11005statorwindingleakageinductanceRRH11032H11005rotorwindingresistanceLLRH11032H11005rotorwindingleakageinductanceLMH11032H11005statorwindingresistanceSH11005operatingslipH9275H11005operatingfrequencyRLH11005loadresistanceconnectedtotheterminalsCH11005capacitorcompensationR1LandL1Laretheeffectiveresistanceandinductance,respectively,representingthestatorwindingandtheloadasseenbynodeM.Oneimportantaspectofselfexcitationisthemagnetizingcharacteristicoftheinductiongenerator.Figure3showstherelationshipbetweenthefluxlinkageandthemagnetizinginductanceforatypicalgeneratoranincreaseinthefluxlinkagebeyondacertainlevelreducestheeffectivemagnetizinginductanceLMH11032.Thisgraphcanbederivedfromtheexperimentallydeterminednoloadcharacteristicoftheinductiongenerator.Tosolvetheaboveequations,wecanfixthecapacitorH20849CH20850andtheresistiveloadH20849RLH20850valuesandthenfindtheoperatingpointsfordifferentfrequencies.FromEq.H208492H20850,wecanfindtheoperatingslipataparticularfrequency.Then,fromEq.H208493H20850,wecanfindthecorrespondingmagnetizinginductanceLMH11032,and,fromFig.3,theoperatingfluxlinkageatthisfrequency.Theprocessisrepeatedfordifferentfrequencies.Asabaseline,weconsideracapacitorwithacapacitanceof3.8mFH20849millifaradH20850connectedtothegeneratortoproduceapproximatelyratedVARH20849voltamperereactiveH20850compensationforfullloadgenerationH20849highwindH20850.AloadresistanceofRL1.0H9024isusedasthebaselineload.TheslipversusrotorspeedpresentedinFig.4showsthattheslipisroughlyconstantthroughoutthespeedrangeforaconstantloadresistance.Thecapacitancedoesnotaffecttheoperatingslipforaconstantloadresistance,butahigherresistanceH20849RLhighlowergeneratedpowerH20850correspondstoalowerslip.ThevoltageattheterminalsoftheinductiongeneratorH20849presentedinFig.5H20850showstheimpactofchangesinthecapacitanceFig.3AtypicalmagnetizationcharacteristicTransactionsoftheASMEDownloaded28Mar2008to211.82.100.20.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmandloadresistance.AsshowninFig.5,theloadresistancedoesnotaffecttheterminalvoltage,especiallyatthehigherrpmH20849higherfrequencyH20850,butthecapacitancehasasignificantimpactonthevoltageprofileatthegeneratorterminals.Alargercapacitanceyieldslessvoltagevariationwithrotorspeed,whileasmallercapacitanceyieldsmorevoltagevariationwithrotorspeed.AsshowninFig.6,foragivencapacitance,changingtheeffectivevalueoftheloadresistancecanmodulatethetorquespeedcharacteristic.TheseconceptsofselfexcitationcanbeexploitedtoprovidedynamicbrakingforawindturbineH20849asmentionedaboveH20850topreventtheturbinefromrunningawaywhenitlosesitsconnectiontothegridonesimplyneedstochoosethecorrectvaluesforcapacitanceH20849ahighvalueH20850andloadresistancetomatchtheturbinepoweroutput.Appropriateoperationoverarangeofwindspeedscanbeachievedbyincorporatingavariableresistanceandadjustingitdependingonwindspeed.3.3DynamicBehavior.Thissectionexaminesthetransientbehaviorinselfexcitationoperation.Wechooseavalueof3.8mFcapacitanceandaloadresistanceof1.0H9024forthissimulation.Theconstantdrivingtorqueissettobe4500Nm.Notethatthewindturbineaerodynamiccharacteristicandtheturbinecontrolsystemarenotincludedinthissimulationbecausewearemoreinterestedintheselfexcitationprocessitself.Thus,wefoFig.4VariationofslipforatypicalselfexcitedinductiongeneratorFig.5TerminalvoltageversusrotorspeedfordifferentRLandCJournalofSolarEnergyEngineeringcusontheelectricalsideoftheequations.Figure7showstimeseriesoftherotorspeedandtheelectricaloutputpower.Inthiscase,theinductiongeneratorstartsfromrest.Thespeedincreasesuntilitreachesitsratedspeed.Itisinitiallyconnectedtothegridandatt3.1secondsH20849sH20850,thegridisdisconnectedandtheinductiongeneratorentersselfexcitationmode.Att6.375s,thegeneratorisreconnectedtothegrid,terminatingtheselfexcitation.Therotorspeedincreasesslightlyduringselfexcitation,but,eventually,thegeneratortorquematchesthedrivingtorqueH208494500NmH20850,andtherotorspeedisstabilized.Whenthegeneratorisreconnectedtothegridwithoutsynchronization,thereisasuddenbrieftransientinthetorqueasthegeneratorresynchronizeswiththegrid.Oncethisoccurs,therotorspeedsettlesatthesamespeedasbeforethegriddisconnection.Figure8H20849aH20850plotsperphasestatorvoltage.Itshowsthatthestatorvoltageisoriginallythesameasthevoltageofthegridtowhichitisconnected.DuringtheselfexcitationmodeH208493.1sH11021tH110216.375sH20850,whentherotorspeedincreasesasshowninFig.7,thevoltageincreasesandthefrequencyisabithigherthan60Hz.Thevoltageandthefrequencythenreturntotheratedvalueswhentheinductiongeneratorisreconnectedtothegrid.Figure8H20849bH20850isanexpansionofFig.8H20849aH20850betweent3.0sandt3.5stobetterillustratethechangeinthevoltagethatoccursduringthattransient.4HarmonicAnalysis4.1SimplifiedPerPhaseHigherHarmonicsRepresentation.Inordertomodeltheharmonicbehaviorofthenetwork,wereplacethepowernetworkshowninFig.1withtheperphaseequivalentcircuitshowninFig.9H20849aH20850.Inthiscircuitrepresentation,ahigherharmonicormultipleof60HzisdenotedFig.6Thegeneratortorquevs.rotorspeedfordifferentRLandCFig.7Thegeneratoroutputpowerandrotorspeedvs.timeNOVEMBER2005,Vol.127/5834.1.2Transformer.WeconsiderathreephasetransformerwithleakagereactanceH20849XxfH20850of6percent.BecausethemagnetizDownloaded28Mar2008to211.82.100.20.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmbyh,wherehistheintegermultipleof60Hz.Thush5indicatesthefifthharmonicH20849300HzH20850.Forwindturbineapplications,theinductiongenerator,transformer,andcapacitorsarethreephaseandconnectedineitherWyeorDeltaconfiguration,sotheevenharmonicsandthethirdharmonicdonotexistH208515,6H20852.Thatis,onlyh5,7,11,13,17,...,etc.exist.4.1.1InfiniteBusandLineFeeder.TheinfinitebusandthelinefeederconnectingthewindturbinetothesubstationarerepresentedbyasimpleTheveninrepresentationofthelargerpowersystemnetwork.Thus,weconsiderasimpleRLlinerepresentation.Fig.8Theterminalvoltageversusthetime.„aVoltageduringselfexcitation.„bVoltagebeforeandduringselfexcitation,andafterreconnection.Fig.9Theperphaseequivalentcircuitofthesimplifiedmodelforharmonicanalysis584/Vol.127,NOVEMBER2005ingreactanceofalargetransformerisusuallyverylargecomparedtotheleakagereactanceH20849XMH11032H11015H11009→opencircuitH20850,onlytheleakagereactanceisconsidered.Assumingtheefficiencyofthetransformerisabout98percentatfullload,andthecopperlossisequaltothecorelossH20849ageneralassumptionforanefficient,largetransformerH20850,thecopperlossandcorelossareeachapproximately1percentor0.01perunit.Withthisassumption,wecancomputethecopperlossinperunitatfullloadcurrentH20849I1FullH6018Load1.0perunitH20850,andwecandeterminethetotalwindingresistanceoftheprimaryandsecondarywindingH20849aboutonepercentinperunitH20850.4.1.3CapacitorCompensation.Switchedcapacitorsrepresentthecompensationofthewindturbine.Thewindturbineweconsiderisequippedwithanadditional1.9MVARreactivepowercompensationH208491.5MVARabovethe400kVARsuppliedbythemanufacturerH20850.Thewindturbineiscompensatedatdifferentlevelsofcompensationdependingonthelevelofgeneration.ThecapacitorisrepresentedbythecapacitanceCinserieswiththeparasiticresistanceH20849RcH20850,representingthelossesinthecapacitor.Thisresistanceisusuallyverysmallforagoodqualitycapacitor.4.1.4InductionGenerator.TheinductiongeneratorH208491.5MW,480V,60HzH20850usedforthiswindturbinecanberepresentedastheperphaseequivalentcircuitshownFig.9H20849aH20850.TheslipofaninductiongeneratoratanyharmonicfrequencyhcanbemodeledasShhH9275s−H9275rhH9275sH208494H20850whereShH11005slipforhthharmonichH11005harmonicorderH9275sH11005synchronousspeedofthegeneratorH9275rH11005rotorspeedofthegeneratorThusforhigherharmonicsH20849fifthandhigherH20850theslipiscloseto1H20849Sh1H20850andforpracticalpurposesisassumedtobe1.4.2SteadyStateAnalysis.Figure9H20849bH20850showsthesimplifiedequivalentcircuitoftheinterconnectedsystemrepresentinghigherharmonics.Notethatthemagnetizinginductanceofthetransformersandtheinductiongeneratorareassumedtobemuchlargerthantheleakagesandarenotincludedforhighharmoniccalculations.Inthissection,wedescribethecharacteristicsoftheequivalentcircuitshowninFig.9,examinetheimpactofvaryingthecapacitorsizeontheharmonicadmittance,andusetheresultofcalculationstoexplainwhyharmoniccontentsofthelinecurrentchangeasthecapacitanceisvaried.Fromthesuperpositiontheorem,wecananalyzeacircuitwithonlyonesourceatatimewhiletheothersourcesareturnedoff.Forharmonicsanalysis,thefundamentalfrequencyvoltagesourcecanbeturnedoff.Inthiscase,thefundamentalfrequencyvoltagesourceH20849infinitebusH20850,Vs,isshortcircuited.Windfarmoperatorexperienceshowsusthatharmonicsoccurwhenthetransformeroperatesinthesaturationregion,thatis,athigherfluxlevelsasshowninFig.3.Duringtheoperationinthissaturationregion,theresultingcurrentcanbedistortedintoasharplypeakedsinusoidalcurrentduetothelargermagnetizingcurrentimbeddedintheprimarycurrent.Thisnonsinusoidalcurrentcanexcitethenetworkatresonantfrequenciesofthenetwork.FromthecircuitdiagramwecancomputetheimpedanceH20849atanycapacitanceandharmonicfrequencyH20850seenbytheharmonicsource,Vh,withEq.H208495H20850,wherethesignH20648representsthewordsinparallelwithTransactionsoftheASME

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