某变电所毕业设计中英文对照中英文翻译.docx

某变电所毕业设计中英文对照中英文翻译.docx

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某变电所毕业设计中英文对照中英文翻译

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外文资料翻译

A1.1原文

TRANSFORMER

1.INTRODUCTION

Thehigh-voltagetransmissionwasneedforthecaseelectricalpoweristobeprovidedatconsiderabledistancefromageneratingstation.Atsomepointthishighvoltagemustbereduced,becauseultimatelyismustsupplyaload.Thetransformermakesitpossibleforvariouspartsofapowersystemtooperateatdifferentvoltagelevels.Inthispaperwediscusspowertransformerprinciplesandapplications.

2.TOW-WINDINGTRANSFORMERS

Atransformerinitssimplestformconsistsoftwostationarycoilscoupledbyamutualmagneticflux.Thecoilsaresaidtobemutuallycoupledbecausetheylinkacommonflux.

Inpowerapplications,laminatedsteelcoretransformers（towhichthispaperisrestricted）areused.Transformersareefficientbecausetherotationallossesnormallyassociatedwithrotatingmachineareabsent,sorelativelylittlepowerislostwhentransformingpowerfromonevoltageleveltoanother.Typicalefficienciesareintherange92to99%,thehighervaluesapplyingtothelargerpowertransformers.

Thecurrentflowinginthecoilconnectedtotheacsourceiscalledtheprimarywindingorsimplytheprimary.Itsetsupthefluxφinthecore,whichvariesperiodicallybothinmagnitudeanddirection.Thefluxlinksthesecondcoil,calledthesecondarywindingorsimplysecondary.Thefluxischanging;therefore,itinducesavoltageinthesecondarybyelectromagneticinductioninaccordancewithLenz’slaw.Thustheprimaryreceivesitspowerfromthesourcewhilethesecondarysuppliesthispowertotheload.Thisactionisknownastransformeraction.

3.TRANSFORMERPRINCIPLES

WhenasinusoidalvoltageVpisappliedtotheprimarywiththesecondaryopen-circuited,therewillbenoenergytransfer.TheimpressedvoltagecausesasmallcurrentIθtoflowintheprimarywinding.Thisno-loadcurrenthastwofunctions:

（1）itproducesthemagneticfluxinthecore,whichvariessinusoidallybetweenzeroand

φm,whereφmisthemaximumvalueofthecoreflux;and

（2）itprovidesacomponenttoaccountforthehysteresisandeddycurrentlossesinthecore.Therecombinedlossesarenormallyreferredtoasthecorelosses.

Theno-loadcurrentIθisusuallyfewpercentoftheratedfull-loadcurrentofthetransformer（about2to5%）.Sinceatno-loadtheprimarywindingactsasalargereactanceduetotheironcore,theno-loadcurrentwilllagtheprimaryvoltagebynearly90º.ItisreadilyseenthatthecurrentcomponentIm=I0sinθ0,calledthemagnetizingcurrent,is90ºinphasebehindtheprimaryvoltageVP.Itisthiscomponentthatsetsupthefluxinthecore;φisthereforeinphasewithIm.

Thesecondcomponent,Ie=I0sinθ0,isinphasewiththeprimaryvoltage.Itisthecurrentcomponentthatsupplies

thecorelosses.Thephasorsumofthesetwocomponentsrepresentstheno-loadcurrent,or

I0=Im+Ie

Itshouldbenotedthattheno-loadcurrentisdistortesandnonsinusoidal.Thisistheresultofthenonlinearbehaviorofthecorematerial.

Ifitisassumedthattherearenootherlossesinthetransformer,theinducedvoltageIntheprimary,Epandthatinthesecondary,Escanbeshown.Sincethemagneticfluxsetupbytheprimarywinding,therewillbeaninducedEMFEinthesecondarywindinginaccordancewithFaraday’slaw,namely,E=NΔφ/Δt.Thissamefluxalsolinkstheprimaryitself,inducinginitanEMF,Ep.Asdiscussedearlier,theinducedvoltagemustlagthefluxby90º,therefore,theyare180ºoutofphasewiththeappliedvoltage.Sincenocurrentflowsinthesecondarywinding,Es=Vs.Theno-loadprimarycurrentI0issmall,afewpercentoffull-loadcurrent.ThusthevoltageintheprimaryissmallandVpisnearlyequaltoEp.Theprimaryvoltageandtheresultingfluxaresinusoidal;thustheinducedquantitiesEpandEsvaryasasinefunction.Theaveragevalueoftheinducedvoltagegivenby

Eavg=turns×

whichisFaraday’slawappliedtoafinitetimeinterval.Itfollowsthat

Eavg=N

=4fNφm

whichNisthenumberofturnsonthewinding.Formaccircuittheory,theeffectiveorroot-mean-square（rms）voltageforasinewaveis1.11timestheaveragevoltage;thus

E=4.44fNφm

Sincethesamefluxlinkswiththeprimaryandsecondarywindings,thevoltageperturnineachwindingisthesame.Hence

Ep=4.44fNpφm

and

Es=4.44fNsφm

whereEpandEsarethenumberofturnontheprimaryandsecondarywindings,respectively.Theratioofprimarytosecondaryinducedvoltageiscalledthetransformationratio.Denotingthisratiobya,itisseenthat

a=

=

Assumethattheoutputpowerofatransformerequalsitsinputpower,notabadsumptioninpracticeconsideringthehighefficiencies.Whatwereallyaresayingisthatwearedealingwithanidealtransformer;thatis,ithasnolosses.Thus

Pm=Pout

or

VpIp×primaryPF=VsIs×secondaryPF

wherePFisthepowerfactor.Fortheabove-statedassumptionitmeansthatthepowerfactoronprimaryandsecondarysidesareequal;therefore

VpIp=VsIs

fromwhichisobtained

=

≌

≌a

Itshowsthatasanapproximationtheterminalvoltageratioequalstheturnsratio.Theprimaryandsecondarycurrent,ontheotherhand,areinverselyrelatedtotheturnsratio.Theturnsratiogivesameasureofhowmuchthesecondaryvoltageisraisedorloweredinrelationtotheprimaryvoltage.Tocalculatethevoltageregulation,weneedmoreinformation.

Theratiooftheterminalvoltagevariessomewhatdependingontheloadanditspowerfactor.Inpractice,thetransformationratioisobtainedfromthenameplatedata,whichlisttheprimaryandsecondaryvoltageunderfull-loadcondition.

WhenthesecondaryvoltageVsisreducedcomparedtotheprimaryvoltage,thetransformationissaidtobeastep-downtransformer:

conversely,ifthisvoltageisraised,itiscalledastep-uptransformer.Inastep-downtransformerthetransformationratioaisgreaterthanunity（a>1.0）,whileforastep-uptransformeritissmallerthanunity（a<1.0）.Intheeventthata=1,thetransformersecondaryvoltageequalstheprimaryvoltage.Thisisaspecialtypeoftransformerusedininstanceswhereelectricalisolationisrequiredbetweentheprimaryandsecondarycircuitwhilemaintainingthesamevoltagelevel.Therefore,thistransformerisgenerallyknowsasanisolationtransformer.

Asisapparent,itisthemagneticfluxinthecorethatformstheconnectinglinkbetweenprimaryandsecondarycircuit.Insection4itisshownhowtheprimarywindingcurrentadjustsitselftothesecondaryloadcurrentwhenthetransformersuppliesaload.

Lookingintothetransformerterminalsfromthesource,animpedanceisseenwhichbydefinitionequalsVp/Ip.From

=

≌

≌a,wehaveVp=aVsandIp=Is/a.IntermsofVsandIstheratioofVptoIpis

=

=

ButVs/IsistheloadimpedanceZLthuswecansaythat

Zm（primary）=a2ZL

Thisequationtellsusthatwhenanimpedanceisconnectedtothesecondaryside,itappearsfromthesourceasanimpedancehavingamagnitudethatisa2timesitsactualvalue.Wesaythattheloadimpedanceisreflectedorreferredtotheprimary.Itisthispropertyoftransformersthatisusedinimpedance-matchingapplications.

4.TRANSFORMERSUNDERLOAD

Theprimaryandsecondaryvoltagesshownhavesimilarpolarities,asindicatedbythe“dot-making”convention.

Thedotsneartheupperendsofthewindingshavethesamemeaningasincircuittheory;themarkedterminalshavethesamepolarity.Thuswhenaloadisconnectedtothesecondary,theinstantaneousloadcurrentisinthedirectionshown.Inotherwords,thepolaritymarkingssignifythatwhenpositivecurrententersbothwindingsatthemarkedterminals,theMMFsofthetwowindingsadd.

Sincethesecondaryvoltagedependsonthecorefluxφ0,itmustbeclearthatthefluxshouldnotchangeappreciablyifEsistoremainessentiallyconstantundernormalloadingconditions.Withtheloadconnected,acurrentIswillflowinthesecondarycircuit,becausetheinducedEMFEswillactasavoltagesource.ThesecondarycurrentproducesanMMFNsIsthatcreatesaflux.Thisfluxhassuchadirectionthatatanyinstantintimeitopposesthemainfluxthatcreateditinthefirstplace.Ofcourse,thisisLenz’slawinaction.ThustheMMFrepresentedbyNsIstendstoreducethecorefluxφ0.ThismeansthatthefluxlinkingtheprimarywindingreducesandconsequentlytheprimaryinducedvoltageEp,ThisreductionininducedvoltagecausesagreaterdifferencebetweentheimpressedvoltageandthecounterinducedEMF,therebyallowingmorecurrenttoflowintheprimary.ThefactthatprimarycurrentIpincreasesmeansthatthetwoconditionsstatedearlierarefulfilled:

（1）thepowerinputincreasestomatchthepoweroutput,and

（2）theprimaryMMFincreasestooffsetthetendencyofthesecondaryMMFtoreducetheflux.

Ingeneral,itwillbefoundthatthetransformerreactsalmostinstantaneouslytokeeptheresultantcorefluxessentiallyconstant.Moreover,thecorefluxφ0dropsveryslightlybetweennoloadandfullload（about1to3%）,anecessaryconditionifEpistofallsufficientlytoallowanincreaseinIp.

Ontheprimaryside,Ip’isthecurrentthatflowsintheprimarytobalancethedemagnetizingeffectofIs.ItsMMFNpIp’setsupafluxlinkingtheprimaryonly.Sincethecorefluxφ0remainsconstant.I0mustbethesamecurrentthatenergizesthetransformeratnoload.TheprimarycurrentIpisthereforethesumofthecurrentIp’andI0.

Becausetheno-loadcurrentisrelativelysmall,itiscorrecttoassumethattheprimaryampere-turnsequalthesecondaryampere-turns,sinceitisunderthisconditionthatthecorefluxisessentiallyconstant.