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1、9576_C01.fmPage1Friday,October14,20059:04AM1FundamentalsofDistributionSystemsElectrificationintheearly20thcenturydramaticallyimprovedproductivityandincreasedthewell-beingoftheindustrializedworld.Nolongeraluxury—nowanecessity—electricitypowersthemachinery,theco
2、mputers,thehealth-caresystems,andtheentertainmentofmodernsociety.Givenitsbenefits,electricityisinexpensive,anditspricecontinuestoslowlydecline(afteradjustingforinflation—seeFigure1.1).Electricpowerdistributionistheportionofthepowerdeliveryinfrastruc-turethattak
3、estheelectricityfromthehighlymeshed,high-voltagetrans-missioncircuitsanddeliversittocustomers.Primarydistributionlinesare“medium-voltage”circuits,normallythoughtofas600Vto35kV.Atadistributionsubstation,asubstationtransformertakestheincomingtrans-mission-level
4、voltage(35to230kV)andstepsitdowntoseveraldistributionprimarycircuits,whichfanoutfromthesubstation.Closetoeachenduser,adistributiontransformertakestheprimary-distributionvoltageandstepsitdowntoalow-voltagesecondarycircuit(commonly120/240V;otherutilizationvolta
5、gesareusedaswell).Fromthedistributiontransformer,thesecondarydistributioncircuitsconnecttotheenduserwheretheconnectionismadeattheserviceentrance.Figure1.2showsanoverviewofthepowergenerationanddeliveryinfrastructureandwheredistributionfitsin.Func-tionally,distr
6、ibutioncircuitsarethosethatfeedcustomers(thisishowthetermisusedinthisbook,regardlessofvoltageorconfiguration).Somealsothinkofdistributionasanythingthatisradialoranythingthatisbelow35kV.Thedistributioninfrastructureisextensive;afterall,electricityhastobedeliver
7、edtocustomersconcentratedincities,customersinthesuburbs,andcustomersinveryremoteregions;fewplacesintheindustrializedworlddonothaveelectricityfromadistributionsystemreadilyavailable.Distributioncircuitsarefoundalongmostsecondaryroadsandstreets.Urbanconstruc-ti
8、onismainlyunderground;ruralconstructionismainlyoverhead.Subur-banstructuresareamix,withagooddealofnewconstructiongoingunderground.Amainlyurbanutilitymayhavelessthan50ftofdistributioncircu
