英语翻译Fig.4.4showsaminimumpowersystem.Thesystemconsistsofanenergysource,aprimemover,agenerator,andaload.Theenergymaybecoal,gas,oroilburnedinafurnacetoheatwaterandgeneratorsteaminaboiler.Theprimemovermaybea

　　英语翻译

　　Fig.4.4showsaminimumpowersystem.Thesystemconsistsofanenergysource,aprimemover,agenerator,andaload.Theenergymaybecoal,gas,oroilburnedinafurnacetoheatwaterandgeneratorsteaminaboiler.Theprimemovermaybeasteam-driventurbine,ahydraulicorwaterwheel,oraninternalcombustionengine.Eachoneoftheseprimemovershastheabilitytoconvertenergyintheformofheat,fallingwater,orfuelintorotationofashaft,whichinturnwilldrivethegenerator.Theelectricalloadonthegeneratormaybelights,motors,heaters,orotherdevices,aloneorincombination.Thecontrolsystemfunctionstokeepspeedofthemachinessubstantiallyconstantandthevoltagewithinprescribedlimits,eventheroadmaychange.Thecontrolsystemmayincludeamanstationedinthepowerplantwhowatchesasetofmetersonthegenerator-outputterminalsandmakesthenecessaryadjustmentsmanually.Inamodernstation,thecontrolsystemisaservomechanismthatsensesagenerator-outputconditionsandautomaticallymakesthenecessarychangesinenergyinputandfieldcurrenttoholdtheelectricaloutputwithincertainspecifications.

　　Modernpowersystemsareusuallylarge-scale,geographicallydistributed,andwithhundredstothousandsofgeneratorsoperatinginparallelandsynchronously.Theymayvaryinsizeandstructurefromonetoanother,buttheyallhavebasiccharacteristics:

　　(1)Arecomprisedofthree-phaseACsystemsoperatingessentiallyatconstantvoltage.Generationandtransmissionfacilitiesusethree-phaseequipment.Industrialloadsareinvariablythree-phase;single-phaseresidentialandcommercialloadsaredistributedequallyamongthephasessoastoeffectivelyfromabalancedthree-phasesystem.

　　(2)Usesynchronousmachinesforgenerationofelectricity.Primemovesconverttheprimaryenergy(fossil,nuclear,andhydraulic)tomechanicalenergythatis,inturn,convertedtoelectricalenergybysynchronousgenerators.

　　(3)Transmitpoweroversignificantdistancestoconsumersspreadoverawidearea.Thisrequiresatransmissionsystemcomprisingsubsystemsoperatingatdifferentvoltagelevels.