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建筑资料下蓑就在筑龙网变法器摘要变压器是变电所的主要设备，功能是实现电网电压的等级变换，基本工作原理是电磁感应变配电所是实现电压等级变换和电能分配的场所对供电电源进行电压等级变换，应对电能进行重新分配的场所称为变电所建筑变电所是供配电系统的枢纽，供电电源由电网引到变电所，在变电所完成降压，电能分配等功能关键词变电所；变压器；继电保护；介绍
1.要从远端发电厂送出电能，必须应用高压输电因为最终的负荷，在一些点高电压必须降低变压器能使电力系统各个部分运行在电压不同的等级本文我们讨论的原则和电力变压器的应用双绕组变压器
2.变压器的最简单形式包括两个磁通相互耦合的固定线圈两个线圈之所以相互耦合，是因为它们连接着共同的磁通在电力应用中，使用层式铁芯变压器（本文中提到的）变压器是高效率的，因为它没有旋转损失，因此在电压等级转换的过程中，能量损失比较少典型的效率范围在92到99%,上限值适用于大功率变压器从交流电源流入电流的一侧被称为变压器的一次侧绕组或者是原边它在铁圈中建立了磁通（P，它的幅值和方向都会发生周期性的变化磁通连接的第二个绕组被称为变压器的二次侧绕组或者是副边磁通是变化的；因此依据楞次定律，电磁感应在二次侧产生了电压变压器在原边接收电能的同时也在向副边所带的负荷输送电能这就是变压器的作用变压器的工作原理
3.当二次侧电路开路是，即使原边被施以正弦电压Vp,也是没有能量转移的外加电压在一次侧绕组中产生一个小电流I这个空载电流有两项功能
（1）在铁芯中产生电磁通，该磁通在零和土（pm之间做正弦变化，（pm是铁芯磁通的最大值；
（2）它的一个分量说明了铁芯中的涡流和磁滞损耗这两种相关的损耗被称为铁芯损耗变压器空载电流10一般大约只有满载电流的2%-5%因为在空载时，原边绕组中的铁o芯相当于一个很大的电抗,空载电流的相位大约将滞后于原边电压相位90°o显然可见电流分量Im=I0sin90,被称做励磁电流,它在相位上滞后于原边电压VP90%就是这个分量在铁芯中建立了磁通；因此磁通（p与Im同相第二个分量Ie=10sin00,与原边电压同相这个电流分量向铁芯提供用于损耗的电流两个相量的分量和代表空载电流，即10=Im+Ie应注意的是空载电流是畸变和非正弦形的这种情况是非线性铁芯材料造成的如果假定变压器中没有其他的电能损耗一次侧的感应电动势Ep和二次侧的感应电压Es可以表示出来因为一次侧绕组中的磁通会通过二次绕组，依据法拉第电磁感应定律，二次侧绕组中将产生一个电动势E,即E=NA（p/At相同的磁通会通过原边自身，产生一个0电动势Ep正如前文中讨论到的，所产生的电压必定滞后于磁通90°,因此，它于施加的电压有180°的相位差因为没有电流流过二次侧绕组，Es=Vso一次侧空载电流很小，仅为满载电流的百分之几因此原边电压很小，并且Vp的值近乎等于Ep原边的电压和它产生的磁通波形是正弦形的；因此产生电动势Ep和Es的值是做正弦变化的产生电压的平均值如下给定时间内磁通变化量Eavg=turnsx给时间即是法拉第定律在瞬时时间里的应用它遵循2（pm）（Eavg=N I®/=4fN pm其中N是指线圈的匝数从交流电原理可知，有效值是一个正弦波，其值为平均电压的
1.11倍；因此（E=
4.44fN pm因为一次侧绕组和二次侧绕组的磁通相等，所以绕组中每匝的电压也相同因此（Ep=
4.44fNp pm并且建筑资料下蓑就在筑龙网Es=
4.44fNs pm其中Np和Es是一次侧绕组和二次侧绕组的匝数一次侧和二次侧电压增长的比率称做变比用字母a来表示这个比率，如下式Ep Npa=二Es Ns假设变压器输出电能等于其输入电能一一这个假设适用于高效率的变压器实际上我们是考虑一台理想状态下的变压器；这意味着它没有任何损耗因此Pm=Pout或者Vplp xprimary PF=Vsls xsecondary PF这里PF代表功率因素在上面公式中一次侧和二次侧的功率因素是相等的；因此Vplp=Vsls从上式我们可以得知Vp IpEpaVs—Is Es它表明端电压比等于匝数比，换句话说，一次侧和二次侧电流比与匝数比成反比匝数比可以衡量二次侧电压相对于一次恻电压是升高或者是降低为了计算电压，我们需要更多数据终端电压的比率变化有些根据负载和它的功率因素实际上，变比从标识牌数据获得，列出在满载情况下原边和副边电压当副边电压Vs相对于原边电压减小时，这个变压器就叫做降压变压器如果这个电压是升高的，它就是一个升压变压器在一个降压变压器中传输变比a远大于la
1.0,同样的，一个升压变压器的变比小于laLO当a=l时，变压器的二次侧电压就等于起一次侧电压这是一种特殊类型的变压器，可被应用于当一次侧和二次侧需要相互绝缘以维持相同的电压等级的状况下因此，我们把这种类型的变压器称为绝缘型变压器显然，铁芯中的电磁通形成了连接原边和副边的回路在第四部分我们会了解到当变压器带负荷运行时一次侧绕组电流是如何随着二次侧负荷电流变化而变化的业也目从电源侧来看变压器，其阻抗可认为等于Vp/Ip从等式忆=%9a中我们可知Vp=aEsVs并且Ip=Is/a根据Vs和Is,可得Vp和Ip的比例是oVp_aVs a2Vs—Is ICl—Is但是Vs/Is负荷阻抗ZL,因此我们可以这样表示（）Zm primary=a2ZL这个等式表明二次侧连接的阻抗折算到电源侧，其值为原来的a2倍我们把这种折算方式称为负载阻抗向一次侧的折算这个公式应用于变压器的阻抗匹配有载情况下的变压器
4.一次侧电压和二次侧电压有着相同的极性，一般习惯上用点记号表示如果点号同在线圈的上端，就意味着它们的极性相同因此当二次侧连接着一个负载时，在瞬间就有一个负荷电流沿着这个方向产生换句话说，极性的标注可以表明当电流流过两侧的线圈时，线圈中的磁动势会增加总的来说，变压器为了保持磁通是常数，对磁通变化的响应是瞬时的更重要的是，在空载和满载时，主磁通的降落是很少的（一般在）1至3%其需要的条件是E降落很多来使电流Ip增加在一次侧，电流Ip在一次侧流过以平衡Is产生的影响它的磁动势Nplp，只停留在一次侧因为铁芯的磁通（p0保持不变，变压器空载时空载电流10必定会为其提供能量故一次侧电流Ip是电流Ip与10的和因为空载电流相对较小，那么一次侧的安匝数与二次侧的安匝数相等的假设是成立的因为在这种状况下铁芯的磁通是恒定的因此我们仍旧可以认定空载电流10相对于满载电流是极其小的当一个电流流过二次侧绕组，它的磁动势（Nsls）将产生一个磁通，于空载电流10产生的磁通（p0不同，它只停留在二次侧绕组中因为这个磁通不流过一次侧绕组,所以它不是一个公共磁通另外，流过一次侧绕组的负载电流只在一次侧绕组中产生磁通，这个磁通被称为一次侧的漏磁二次侧漏磁将使电压增大以保持两侧电压的平衡一次侧漏磁也一样因此，这两个增大的电压具有电压降的性质，总称为漏电抗电压降另外，两侧绕组同样具有阻抗，这也将产生一个电阻压降把这些附加的电压降也考虑在内，这样一个实际的变压器的等值电路图就完成了由于分支励磁体现在电流里，为了分析我们可以将它忽略这就符我们前面计算中可以忽略空载电流的假设这证明了它对我们分析变压器时所产生的影响微乎其微因为电压降与负载电流成比例关系，这就意味着空载情况下一次侧和二次侧绕组的电压降都为零建筑资料下蓑就在筑龙网TRANSFORMER AbstractThe transformeris the transformer substationmajor installation,the functionrealizes thenetworkvoltage ranktransformation,the keyjob principleis theelectromagnetic induction.Changes thesubstation isrealizesthe voltageclass transformationand the electrical energyassignment place.Carries on the voltageclasstransformation to the electricpower supply,deals with the placewhich theelectrical energycarries onredistributesto becalled the transformer substation.Constructs the transformer substationis forelectricalpower distributionsystems keyposition,theelectricpower supplydirects thetransformer substationfrom theelectricalnetwork,completes the voltage droppingin thetransformer substation,functions andso onelectricalenergy assignment.；；；Key-words:substation transformerRelay protection
1.INTRODUCTIONThe high-voltage transmissionwas needfor thecase electricalpower isto beprovided atconsiderabledistance froma generatingstation.At somepoint thishigh voltage must bereduced,because ultimatelyis mustsupplya load.The transformermakes itpossible forvarious partsof apower systemto operateat differentvoltage levels.In this paper wediscuss powertransformer principlesand applications.
2.TOW-WINDING TRANSFORMERSAtransformer inits simplestform consistsof twostationary coilscoupled bya mutualmagnetic flux.Thecoils aresaid tobe mutuallycoupled becausethey linka commonflux.In powerapplications,laminated steelcore transformersto whichthispaperis restrictedare used.Transformers areefficient becausethe rotationallosses normallyassociated withrotating machineare absent,so relativelylittle poweris lostwhen transformingpower fromone voltagelevel to another.Typicalefficiencies arein therange92to99%,the highervalues applyingto thelarger powertransformers.The currentflowing in the coilconnected to the acsource iscalled the primary windingor simplytheprimary.It setsup theflux pin the core,which variesperiodically bothin magnitudeand direction.The fluxlinks the secondcoil,called the secondary windingor simplysecondary.The fluxis changing;therefore,itinduces avoltage in the secondaryby electromagneticinduction inaccordance withLenzs law.Thus theprimaryreceives itspower fromthe sourcewhile the secondary suppliesthis powerto the load.This actionisknown astransformer action.
3.TRANSFORMER PRINCIPLESWhen a sinusoidalvoltage Vp is appliedto the primary withthe secondaryopen-circuited,there will beno energytransfer.The impressedvoltage causesa smallcurrent10to flowin the primary winding.Thisno-load currenthas twofunctions:1it producesthe magnetic flux in the core,which variessinusoidallybetween zeroand±pm,where pmis themaximum value of the core flux;and2it providesa componenttoaccount forthe hysteresisand eddycurrent lossesin the core.There combinedlosses are normally referred toas the core losses.The no-load current10is usuallyfew percentof therated full-load currentof thetransformer about2to5%.Since at no-load the primary windingacts as a largereactance dueto theiron core,the no-load currentwilllag the primary voltageby nearly90°.It isreadily seen that the current componentIm=lOsinGO,calledthe magnetizingcurrent,is90°in phasebehind the primary voltageVP.It isthis componentthat setsup theflux in thecore;pis therefore inphase withIm.The secondcomponent,Ie=IOsin0O,is inphase withthe primary voltage.It isthe currentcomponent thatsuppliesthecorelosses.The phasorsum of these twocomponents representsthe no-load cuiTent,or10=Im+IeIt shouldbe notedthat theno-load current is distortesand nonsinusoidal.This isthe resultof thenonlinearbehavior of thecorematerial.If it is assumedthat therearenoother lossesin thetransformer,the inducedvoltageIn the primary,Ep andthat in the secondary,Es canbe shown.Since the magneticfluxset upby theprimary winding,there willbe aninduced EMFE in the secondary winding inaccordance withFaradays law,namely,E=NA p/At.This sameflux alsolinksthe primary itself,inducing init anEMF,Ep.As discussedearlier,the inducedvoltagemustlag theflux by90°,therefore,they are180°out ofphase withthe appliedvoltage.Since nocurrent flowsin thesecondary winding,Es=Vs.The no-load primarycurrent10is small,afew percentof full-load current.Thus thevoltage inthe primary is smalland Vpis nearlyequal toEp.Theprimary voltageand the resulting fluxare sinusoidal;thus the induced quantitiesEp andEs varyasasinefunction.The averagevalueof theinducedvoltage givenbychange influx ina giventimeEavg=turnsx timewhichis Faradayslaw appliedtoafinite timeinterval.It followsthat2PmEavg=N1/2/=4fN pmwhichN isthe numberof turnson thewinding.Form accircuit theory,the effectiveor root-mean-squarerms voltagefor asine waveis
1.11times theaverage voltage;thusE=
4.44fN pmSince the sameflux linkswiththeprimary and secondary windings,thevoltageper turnin each windingis the same.HenceEp=
4.44fNp pmandEs=
4.44fNs pmwhereEp andEs arethe numberof turn on theprimary and secondary windings,respectively.The ratioofprimary tosecondary inducedvoltage iscalled the transformation ratio.Denoting thisratio bya,it is seenthat建筑资料下载就在筑龙网Ep Np=Es-NsAssume that the outputpower ofa transformerequals itsinput power,not abad sumptionin practiceconsideringthe highefficiencies.What wereally aresaying isthat weare dealingwith anideal transformer;that is,it hasno losses.ThusPm=PoutVplp xprimary PF=Vsls xsecondary PFwherePF isthe power factor.For theabove-stated assumptionit meansthat thepower factoron primaryandsecondary sidesare equal;thereforeVplp=Vslsfrom whichis obtainedEp名也aIt showsthat as an approximationthe terminal voltage ratioequals the turns ratio.The primary andsecondary current,ontheother hand,are inverselyrelated to theturnsratio.The turnsratio givesa measureofhow muchthesecondary voltage israised orlowered inrelation to theprimaryvoltage.To calculatethevoltage regulation,we needmore information.The ratiooftheterminalvoltagevaries somewhatdepending onthe load and itspowerfactor.In practice,the transformationratio isobtained fromthe nameplatedata,which listtheprimaryandsecondary voltageunder full-load condition.When thesecondaryvoltageVs isreduced comparedtotheprimaryvoltage,the transformationis saidtobe a step-down transformer:conversely,if thisvoltage israised,it iscalled astep-up transformer.In astep-down transformerthetransformationratio ais greaterthan unitya
1.0,while forastep-up transformerit is smallerthan unitya
1.
0.In theevent thata=l,thetransformersecondaryvoltageequals theprimaryvoltage.This isa specialtype oftransformer usedin instanceswhere electricalisolation isrequired betweentheprimaryandsecondary circuitwhile maintainingthe samevoltagelevel.Therefore,this transformerisgenerally knowsas anisolation transformer.As isapparent,it isthemagneticfluxinthecorethat formsthe connectinglink betweenprimary andsecondarycircuit.In section4it is shown howtheprimarywinding currentadjusts itselftothesecondary loadcurrentwhen thetransformer suppliesa load.Looking intothetransformerterminals fromthe source,an impedanceisseenwhich bydefinitiona%上=卜=弘二二equals Vp/Ip.From a,we haveVp aVs andIp Is/a.In termsof Vsand Isthe ratioofEsVp toTp isVPaVs a2VsIp=[s/a—IsBut Vs/Is isthe loadimpedance ZLthus wecan saythatZm primary=a2ZLThis equationtells usthat whenan impedanceis connectedtothesecondary side,it appearsfrom建筑资料下载就在筑龙网the sourceasanimpedance havinga magnitudethat isa2times itsactual value.We saythat the loadimpedance isreflected orreferredtotheprimary.It isthis propertyof transformersthat isused inimpedance-matching applications.
4.TRANSFORMERS UNDERLOADThe primaryandsecondaryvoltages shownhave similarpolarities,as indicatedby the“dot-making“convention.The dotsnear theupper endsofthewindings havethe samemeaning asin circuittheory;the marked terminals havethesamepolarity.Thus whena loadis connectedtothesecondary,theinstantaneous load currentisinthedirection shown.In otherwords,the polaritymarkings signifythat whenpositivecurrent entersboth windingsatthemarkedterminals,the MMFsofthetwo windingsadd.In general,it willbe foundthat thetransformer reactsalmost instantaneouslyto keepthe resultantcoreflux essentiallyconstant.Moreover,thecore flux p0drops veryslightly betweennoloadandfull loadabout1to3%,a necessarycondition ifEp isto fallsufficiently toallow anincrease inIp.On theprimary side,Ip isthecurrent that flowsintheprimary tobalance thedemagnetizing effectof Is.Its MMFNplp setsup a flux linkingtheprimaryonly.Since thecore flux p0remains constant.10must bethesame cuiTentthat energizesthetransformeratnoload.The primarycurrent Ipisthereforethe sumof thecurrentIp and
10.Because theno-load currentis relativelysmall,itiscorrect toassume that theprimaryampere-turns equalthesecondary ampere-turns,since itis underthis conditionthat thecorefluxis essentiallyconstant.Thus wewillassume that10is negligible,as itis onlya smallcomponent ofthe full-load current.Whenacurrent flowsinthesecondary winding,theresultingMMF Nslscreates aseparate flux,apartfrom thefluxp0produced by10,which linksthesecondarywinding only.This fluxdoes nolink with theprimarywindingand istherefore nota mutualflux.In addition,theloadcurrentthatflows throughtheprimarywinding createsafluxthat linkswiththeprimarywinding only;itiscalled theprimary leakage flux.The secondary-leakagefluxgives riseto aninducedvoltage thatis notcounter balancedby anequivalent inducedvoltage intheprimary.Similarly,thevoltage inducedintheprimaryisnot counterbalancedinthesecondarywinding.Consequently,these twoinducedvoltages behavelike voltage drops,generally calledleakage reactancevoltage drops.Furthermore,eachwindinghas someresistance,which producesa resistivevoltagedrop.When takeninto account,theseadditional voltage drops wouldcomplete theequivalent circuitdiagram ofa practicaltransformer.Note thatthemagnetizing branchisshownin thiscircuit,which forour purposeswillbedisregarded.This followsourearlier assumptionthattheno-loadcurrentis assumednegligible inour calculations.This isfurther justifiedinthat itis rarelynecessary topredict transformerperformance tosuch accuracies.Sincethevoltage dropsare alldirectlyproportional totheloadcurrent,it meansthat atno-load conditionsthere willbe novoltagedropsineither winding.zhulongxom建筑资料下蓑就在筑龙网。
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