Electric Power System

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1、UnitOneElectricPowerSystemTextAComponentsofPowerSystemsModempowersystemsareusuallylarge—scale，geographicallydistributed，andwithhundredstothousandsofgeneratorsoperatinginparallelandsynchronously．Theymayvaryinsizeandstructurefromonetoanother,buttheyallhavethesamebasiccharacteristics：(1

2、)Arecomprisedofthree—phaseACsystemsoperatingessentiallyatconstantvoltage．Generationandtransmissionfacilitiesusethree—phaseequipment．Industrialloadsareinvariablythree--phase；single·-phaseresidentialandcommercialloadsaredistributedequallyamongthephasesSOastoeffectivelyformabalancedthre

3、e—phasesystem．(2)Usesynchronousmachinesforgenerationofelectricity．Primemoversconverttheprimaryenergy(fossil，nuclear,andhydraulic)tomechanicalenergythatis，inturn，convertedtoelectricalenergybysynchronousgenerators．(3)Transmitpoweroversignificantdistancestoconsumersspreadoverawidearea．T

4、hisrequiresatransmissionsystemcomprisingsubsystemsoperatingatdifferentvoltagelevels．ThebasicelementsofamodempowersysteminUSA．areshowninFig．6—1．Electricpowerisproducedatgeneratingstations(GS)andtransmittedtoconsumersthroughacomplexnetworkofindividualcomponents，includingtransmissionlin

5、es，transformers，andswitchingdevices．Itiscommonpracticetoclassifythetransmissionnetworkintothefollowingsubsystems：Transmissionsystem；Subtransmissionsystem；Distributionsystem．Fig．1BasicelementsofapowersystemThetransmissionsysteminterconnectsallmajorgeneratingstationsandmainloadcentersi

6、nthesystem．Itformsthebackboneoftheintegratedpowersystemandoperatesatthehighestvoltagelevels(typically,230kVandaboveinU．S．A．)。Thegeneratorvoltagesareusuallyintherangeof11to35kV．Thesearesteppeduptothetransmissionvoltagelevel，andpoweristransmittedtotransmissionsubstationswherethevoltage

7、saresteppeddowntothesubtransmissionlevel(typically,69to138kV)．Thegenerationandtransmissionsubsystemsareoftenreferredtoasthebulkpowersystem．Thesubtransmissonsystemtransmitspowerinsmallquantifiesfromthetransmissionsubstationstothedistributionsubstations．Largeindustrialcustomersarecommo

8、nlysuppliedd