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1、TransformerOneofthemostvaluableapparatusinelectricpowersystemisthetransformer,foritenablesustoutilizedifferentvoltagelevelsacrossthesystemforthemosteconomicalvalue.Generationofpoweratthesynchronousmachinelevelisnormallyatarelativelylowvoltage，whichismostdesirableeconomicall
2、y．Steppingupofthisgeneratedvoltagetohighvoltage，extra-highvoltageoreventoultra-highvoltageisdonethroughpowertransformerstosuitthepowertransmissionrequirementtominimizelossesandincreasethetransmissioncapacityofthelines．Thistransmissionvoltagelevelisthensteppeddowninmanystage
3、sfordistributionandutilizationpurposes．Atransformerisastaticdevicefortransferringelectricenergyfromonecircuittoanotherelectromagnetically，thatis，byinductioninsteadofbyconduction．Itsusualfunctionistotransferenergybetweencircuitsofdifferentvoltage．Atransformerhasamagneticcore
4、onwhichtherearetwoormorewindings．Thesewindingsareinsulatedfromeachotherandfromground．Inautotransformers，however，thewindingsareconnectedtogether．Theassemblyofcoreandcoilsisnormallyinsulatedandcooledbyimmersioninmineraloilorothersuitableliquidwithinanenclosingtank．Connectiont
5、othewindingsisbymeansofinsulatingbushings，usuallythroughthecover.The“ratiooftransformation”isdeterminedbytherelativenumberofturnsineachofhewindings．Thisisknownasthe“turnratio”anditistheratiooftheno-loadvoltages．Whentheunitiscarryingload,theratiooftheactualvoltagesisslightly
6、differentbecauseofthedropcausedbytheflowof1oadcurrentthroughtheimpedanceofthetransformerwindings．Atratedload,thisdropisknownasthe"voltageregulation".Theamountofvoltagedropvarieswiththepowerfactoroftheoutputevenwhenthekilovolt-amperesremainconstant．Practicallyallpowertransfo
7、rmersandmanydistributiontransformershavetapsinoneormorewindingsforchangingtheturnratio．Changingtheratioisdesirablefortworeasonstocompensateforvaryingvoltagedropinthesystemandtoassurethatthetransformeroperatesasnearlyaspossibleatthecorrectcoredensity．Forthelatterpurpose,thet
8、apsshouldbeinthewindingsubjecttothevoltagevariation.De-energizedtapchangersareused
