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1、9576_C03.fmPage91Monday,October17,20058:12AM3UndergroundDistributionMuchnewdistributionisunderground.Undergrounddistributionismuchmorehiddenfromviewthanoverheadcircuits,andismorereliable.Cables,connectors,andinstallationequipmenthaveadvancedconsiderablyinthelastquarterofthe20thcentury,
2、makingundergrounddistributioninstalla-tionsfasterandlessexpensive.3.1ApplicationsOneofthemainapplicationsofundergroundcircuitsisforundergroundresidentialdistribution(URD),undergroundbranchesorloopssupplyingresidentialneighborhoods.Utilitiesalsouseundergroundconstructionforsubstationexi
3、tsanddropstopadmountedtransformersservingindustrialorcommercialcustomers.Otherusesarecrossings:rivercrossings,highwaycrossings,ortransmissionlinecrossings.All-undergroundconstruction—widelyusedfordecadesincities—nowappearsinmoreplaces.Undergroundconstructionisexpensive,andcostsvarywide
4、ly.Table3.1showsextractsfromonesurveyofcostsdonebytheCEA;thetwoutilitieshighlighteddifferbyafactoroften.Themainfactorsthatinfluenceunder-groundcostsare:•Degreeofdevelopment—Roads,driveways,sidewalks,andwaterpipes—theseandotherobstaclesslowconstructionandincreasecosts.•Soilcondition—Rock
5、sandfrozengroundincreaseovertimepayforcablecrews.•Urban,suburban,orrural—Urbanconstructionismoredifficultnotonlybecauseofconcrete,butalsobecauseoftraffic.Ruralconstruc-tionisgenerallytheleastexpensiveperlength,butlengthsarelong.•Conduit—Concrete-encasedductscostmorethandirect-buriedcon-d
6、uits,whichcostmorethanpreassembledflexibleconduit,whichcostmorethandirectlyburiedcablewithnoconduits.91Copyright©2006Taylor&FrancisGroup,LLC9576_C03.fmPage92Monday,October17,20058:12AM92ElectricPowerDistributionEquipmentandSystemsTABLE3.1ComparisonofCostsofDifferentUndergroundConstructi
7、onsatDifferentUtilitiesUtilityConstruction$/ftaTAURuralorurban,1phase,#2Al,25kV,trenched,directburied6.7Rural,3phase,#2Al,25kV,trenched,directburied13.4Urbancommercial,3phase,#2Al,25kV,trenched,directburied13.4Urbanexpress,3phase,500-kcmilAl,25kV,trenched,directburied23.5WHUrban,1pha
