电气专业毕业设计外文翻译电力变压器工作原理.docx

电气专业毕业设计外文翻译电力变压器工作原理.docx

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电气专业毕业设计外文翻译电力变压器工作原理

外文资料翻译

PowerTransformerPrinciples

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

Itshouldbenotedthattheno-loadcurrentisdistortesandnonsinusoidal.Thisistheresultofthenonlinearbehaviorofthecorematerial.Ifitisassumedthattherearenootherlossesinthetransformer,theinducedvoltageIntheprimary,Epandthatinthesecondary,Escanbeshown.Sincethemagneticfluxsetupbytheprimarywinding，therewillbeaninducedEMFEinthesecondarywindinginaccordancewithFaraday’slaw,namely,

.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

where