电气毕业设计外文翻译

TransformerOneofthemostvaluableapparatusinelectricpowersystemisthetransformer,foritenablesustoutilizedifferentvoltagelevelsacrossthesystemforthemosteconomicalvalue.Generationofpoweratthesynchronousmachinelevelisnormallyatarelativelylowvoltage，whichismostdesirableeconomically．Steppingupofthisgeneratedvoltagetohighvoltage，extra-highvoltageoreventoultra-highvoltageisdonethroughpowertransformerstosuitthepowertransmissionrequirementtominimizelossesandincreasethetransmissioncapacityofthelines．Thistransmissionvoltagelevelisthensteppeddowninmanystagesfordistributionandutilizationpurposes．Atransformerisastaticdevicefortransferringelectricenergyfromonecircuittoanotherelectromagnetically，thatis，byinductioninsteadofbyconduction．Itsusualfunctionistotransferenergybetweencircuitsofdifferentvoltage．Atransformerhasamagneticcoreonwhichtherearetwoormorewindings．Thesewindingsareinsulatedfromeachotherandfromground．Inautotransformers，however，thewindingsareconnectedtogether．Theassemblyofcoreandcoilsisnormallyinsulatedandcooledbyimmersioninmineraloilorothersuitableliquidwithinanenclosingtank．Connectiontothewindingsisbymeansofinsulatingbushings，usuallythroughthecover.The“ratiooftransformation”isdeterminedbytherelativenumberofturnsineachofhewindings．Thisisknownasthe“turnratio”anditistheratiooftheno-loadvoltages．Whentheunitiscarryingload,theratiooftheactualvoltagesisslightlydifferentbecauseofthedropcausedbytheflowof1oadcurrentthroughtheimpedanceofthetransformerwindings．Atratedload,thisdropisknownasthevoltageregulation.Theamountofvoltagedropvarieswiththepowerfactoroftheoutputevenwhenthekilovolt-amperesremainconstant．Practicallyallpowertransformersandmanydistributiontransformershavetapsinoneormorewindingsforchangingtheturnratio．Changingtheratioisdesirablefortworeasonstocompensateforvaryingvoltagedropinthesystemandtoassurethatthetransformeroperatesasnearlyaspossibleatthecorrectcoredensity．Forthelatterpurpose,thetapsshouldbeinthewindingsubjecttothevoltagevariation.De-energizedtapchangersareusedwhenitisexpectedthattheratiowillneedtobechangedonlyinfrequently,becauseofloadgrowthorsomeseasonalchange．Thedesiredtapisselectedbymeansofaratioadjuster(noloadtaps)．Loadtapchanger(LTC)isusedwhenchangesinratiomaybefrequentorwhenitisundesirabletode-energizethetransformertochangeatap．Itseldommakesmuchdifferencetotheuserwhichwindingorwindingsaretapped；therefore，thechoiceisusuallymadebythedesigneronthebasisofcostandgooddesign.BothwindingcurrentandvoltagemustbeconsideredwhenapplyingLTCequipment．HighvoltageandhighcurrentapplicationsrequirespecialconsiderationstoarriveatanoptimumlocationfortheLTCequipment．StepdownunitsusuallyhaveLTCinthelow-voltagewindingandde-energizedtapsinthehighvoltagewinding．Whenenergyistobetransferredbetweentwocircuitsofnearlythesamevoltage．Theuseofautotransformersaffordscostsavingsovertwo-windingunits．Thenearerthevoltagesaretoeachother,thesmallerwillbetheautotransformerperkilovolt-ampereofoutput，andthegreaterthesavings．Thesimplicityofphasingoutsystemshasincreaseditsuse．MostautotransformersareY-connected，andithasbeenastandardAmericanpracticetoaddalowcapacity,deltawinding．Thisisfrequentlyreferredtoasa“deltatertiary“．Itsprimarypurposehasbeentoprovideaninternalpathforthethirdharmoniccurrents(requiredforexcitation)，thusreducingthosecurrentsonthepowersystem．Italsohelpstostabilizetheneutralandtogroundthesystembetter．Inrecentyears，theuseofshieldedtelephonecablehasreducedtherequirementsforthedeltatertiary.Becauseanautotransformerdoesnotaffordelectricalseparationbetweenthetwocircuits,disturbancesoriginatingononecircuitcanbecommunicatedtotheother．Thisdifficultyisminimizedbysolidlygroundingtheneutraloftheautotransformer.Solidlygroundingtheneutral，however,causes(amongotherthings)currentofshort-circuitmagnitudetoflowthroughthedelta-connectedtertiarywindingduringgroundfaultsoneithersystem.Autotransformersarenotinherentlyself-protectingand,therefore，allwindingsmustbeexaminedformechanicalstrengthasappliedtothesystemwheretheywillbeused.Tertiariesarenormally35％ofthephysicalsizeofthelargestwindingoftheautotransformer,unlessotherwisespecifiedbytheuser.Thebulkofa11transformers，exceptforlargeextra-highvoltage(EHV)anddistributionunitsarethree-phaseunits．Intheearlydaysoftheindustry，ItwasalmostuniversalpracticeintheUnitedStatestousethreesingle-phaseunitsconnectedinathree-phasebank．InsulationclearancesandshippinglimitationsforcertainlargeEHVunitsnowrequirethisdesign．Thedistributionsystemsservemainlysingle-phaseloadsinresidentialareasandareservedfromsingle-phasetransformers．中文译文：变压器在电力系统中最有价值的一个装置是变压器,因为它使我们能够在系统中利用不同的电压水平获得最经济的价值。同步电机的发电电压水平通常是在一个相对较低的电压,这是最令人满意也是最经济的。把发电电压升高到高电压,超高压甚至超高电压是通过电源变压器实现的,这样可以适应电力传输要求,减少电压损失,提高线路的传输容量。在不同的阶段，我们可以根据配电和使用目的的要求把高电压通过变压器降到低电压。一个变压器是一个静态装置，因为电能量从一个电路转移到另一个电路是通过电磁的形式实现的,也就是说,能量的转移是通过感应而不是通过传导。变压器通常的功能是传递不同电压等级电路之间的能量。变压器有一个磁芯，磁芯上有两个或两个以上的绕组，这些绕组相互之间是绝缘的并且与大地也是绝缘的。然而自藕变压器中的绕组是直接连在一起的的。磁芯和绕组的组装通常是在密闭容器内进行的并且浸没在矿物油或其他适当的液体中。线圈的连接是通过绝缘套管,通常穿过表面。“变压比”是由每个绕组上线圈的相对匝数决定的。我们称为“匝数比”,这也是空载电压比。当变压器承载负荷时,实际电压的比例略有不同,因为当负载电流通过变压器时会因为绕组