电力变压器的选择和冷却-电气毕业设计外文翻译Word格式.docx
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《Special English for ArchitecturalElectricEngineeringandAutomation》
文献、资料来源:
期刊(著作、网络等)文献、资料发表(出版)日期:
2006.4 院(部):
信息与电气工程学院学院
专业:
电气工程与自动化 班级:
电气093 姓名:
孙德艺
学 号:
2009081102
指导教师:
陈红艳
翻译日期:
2013.6.30
山东建筑大学毕业设计外文文献及翻译
外文文献一:
Transformer
Oneofthemostvaluableapparatusinelectricpowersystemisthetransformer,foritenablesustoutilizedifferentvoltagelevelsacrossthesystemforthemosteconomicalvalue.Generationofpoweratthesynchronousmachinelevelisnormallyatarelativelylowvoltage,whichismostdesirableeconomically.Steppingupof
thisgeneratedvoltagetohighvoltage,extra-highvoltageoreventoultra-high
voltageisdonethroughpowertransformerstosuitthepowertransmissionrequirementtominimizelossesandincreasethetransmissioncapacityofthelines.Thistransmissionvoltagelevelisthensteppeddowninmanystagesfordistributionandutilizationpurposes.
Atransformerisastaticdevicefortransferringelectricenergyfromonecircuit
toanotherelectromagnetically,thatis,byinductioninsteadofbyconduction.Itsusualfunctionistotransferenergybetweencircuitsofdifferentvoltage.Atransformerhasamagneticcoreonwhichtherearetwoormorewindings.Thesewindingsareinsulatedfromeachotherandfromground.Inautotransformers,however,thewindingsareconnectedtogether.Theassemblyofcoreandcoilsisnormallyinsulatedandcooledbyimmersioninmineraloilorothersuitableliquidwithinanenclosingtank.Connectiontothewindingsisbymeansofinsulatingbushings,usuallythroughthecover.
The“ratiooftransformation”isdeterminedbytherelativenumberofturnsineachofhewindings.Thisisknownasthe“turnratio”anditistheratiooftheno-loadvoltages.Whentheunitiscarryingload,theratiooftheactualvoltagesisslightlydifferentbecauseofthedropcausedbytheflowof1oadcurrentthroughtheimpedanceofthetransformerwindings.Atratedload,thisdropisknownasthe"
voltageregulation"
.Theamountofvoltagedropvarieswiththepowerfactoroftheoutputevenwhenthekilovolt-amperesremainconstant.
Practicallyallpowertransformersandmanydistributiontransformershavetaps
inoneormorewindingsforchangingtheturnratio.Changingtheratioisdesirable
fortworeasonstocompensateforvaryingvoltagedropinthesystemandtoassurethatthetransformeroperatesasnearlyaspossibleatthecorrectcoredensity.Forthelatterpurpose,thetapsshouldbeinthewindingsubjecttothevoltagevariation.De-energizedtapchangersareusedwhenitisexpectedthattheratiowillneedtobechangedonlyinfrequently,becauseofloadgrowthorsomeseasonalchange.Thedesiredtapisselectedbymeansofaratioadjuster(noloadtaps).Loadtapchanger(LTC)isusedwhenchangesinratiomaybefrequentorwhenitisundesirabletode-energizethetransformertochangeatap.Itseldommakesmuchdifferencetotheuserwhichwindingorwindingsaretapped;
therefore,thechoiceisusuallymadebythedesigneronthebasisofcostandgooddesign.BothwindingcurrentandvoltagemustbeconsideredwhenapplyingLTCequipment.HighvoltageandhighcurrentapplicationsrequirespecialconsiderationstoarriveatanoptimumlocationfortheLTCequipment.StepdownunitsusuallyhaveLTCinthelow-voltagewindingandde-energizedtapsinthehighvoltagewinding.
Whenenergyistobetransferredbetweentwocircuitsofnearlythesame
voltage.Theuseofautotransformersaffordscostsavingsovertwo-windingunits.Thenearerthevoltagesaretoeachother,thesmallerwillbetheautotransformerperkilovolt-ampereofoutput,andthegreaterthesavings.Thesimplicityofphasingoutsystemshasincreaseditsuse.MostautotransformersareY-connected,andithasbeenastandardAmericanpracticetoaddalowcapacity,deltawinding.Thisisfrequentlyreferredtoasa“deltatertiary“.Itsprimarypurposehasbeentoprovideaninternalpathforthethirdharmoniccurrents(requiredforexcitation),thusreducingthosecurrentsonthepowersystem.Italsohelpsto
stabilizetheneutralandtogroundthesystembetter.Inrecentyears,theuseof
shieldedtelephonecablehasreducedtherequirementsforthedeltatertiary.Becauseanautotransformerdoesnotaffordelectricalseparationbetweenthetwocircuits,disturbancesoriginatingononecircuitcanbecommunicatedtotheother.Thisdifficultyisminimizedbysolidlygroundingtheneutraloftheautotransformer.Solidlygroundingtheneutral,however,causes(amongotherthings)currentofshort-circuitmagnitudetoflowthroughthedelta-connectedtertiarywindingduringground
faultsoneithersystem.Autotransformersarenotinherentlyself-protectingand,therefore,allwindingsmustbeexaminedformechanicalstrengthasappli