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1、GeneratorandElectricalEquipmentsGeneratorIntroductionElectricgeneratorsconvertmechanicalenergytoelectricalenergy,whichismoreeasilytransmittedtoremotelylocatedpointsofapplication.Thefirstlargeelectricgeneratingsystemsuseddirect-current(dc)generators,mainlybecausedirectcurrentwasbetterunderstoodthan
2、alternatingcurrent(ac).However,dcgeneratorsarelimitedtogeneratingpoweratrelativelylowvoltages,largelyduetoproblemsattheircommutates.Aspowernetworksdeveloped,higherandhighervoltageswererequiredtotransmitlargeblocksofpoweroverlongerandlongerdistances.Electrictransformerscaneasilychangethenormallylow
3、voltagegeneratedtothehighvoltagesneededforefficientpowertransmission,andofcourse,transformersonlyworkonalternatingcurrent.Acgenerators,oralternatorsastheyarecommonlycalled,aresomuchsimplermechanically,somuchmoreefficient,andrequiresomuchlessmaintenancethandcmachinesthatalllargegeneratingplantsoutp
4、utalternatingcurrenttoday.Althoughdetransmissionlinescantransportextremelylargeblocksofpowerveryefficientlyoverlongdistances,thepowerisalwaysgeneratedasalternatingcurrent,transformedtothevoltagerequired,rectifiedandtransmittedasdirectcurrent,andtheninvertedbacktoalternatingcurrentatthepointofappli
5、cation.MechanicalEnergyThemechanicalenergyfordrivingthegeneratormustbederivedfromasourcewithenoughreliabilityandcapacitytomakeiteconomicallyfeasibletodevelopandtransmittheenergyelectricallytothepointofuse.Asmallwatersupplyrunningonlyduringexceptionallywetyearsorlocatedatagreatdistancefromelectrica
6、lconsumerswouldprobablynotbesuitable.Mechanicalenergysourceswhichcannotbemoved,suchashydraulicturbinesorevenwindmachines,musthavethecostoftransportingtheenergyproduced(amongotherfactors)takenintoaccountwhenoverallcostsarecalculated.Steam-turbinepowerplants,however,canbelocatednearacoalseam,lumberm
7、ill,orareliablesourceofcoolingwatertosaveontransportationcosts.Somemechanicalpowermaybeobtainedfromsourcesmoreeasilylocatednearthepointofutilization.Gasturbinesandreciprocatinggasordieselenginesfallintothiscatego