Overheadandundergrounddistributionsystemscomponents,part1. Introduction: A.C.Electricalpowersystemswillhaveanyorallofthefollowingcomponents: generators (completewiththeirprimemoversandcontroldevices),isolatedphasebus,instrument transformers,switchgearassemblies,transmissionlines,transmissiontowers,circuitbreakers, fuses,relays,meters,transformers,disconnectswitches,loadbreakswitches,lightningarresters, poles,overheadconductors,undergroundcables,reclosers,sectionalizes,padmounted switchgear,cableterminations,splices,connectors,supports,synchronous/inductionmotorsand otherloads.Broadlyanyoftheaboveelementscanfallinoneormoreofthefollowing categories:generationandtransmission,distribution(includingsubtransmission)and utilization.Thevoltageingeneraloftheelectricalpowerequipmentcanbelow(upto1KV), medium(upto72KV)orhigh(over72KV&upto745)voltage. Soil: Soilclassification: Thesimplestwaytoclassifysoilsistoclassifyitintocohesiveornoncohesive.Anexampleof cohesivesoilisclayasitisafinegrainedsoil,coarsegrainedsoilsarenoncohesivelikesand. Furthersubclassificationtocohesiveandnoncohesiveispossiblebasedontheoriginor methodofdepositionorstructureofthesoil.Soilstructuremaybeclassifiedasdeposited (transportedfromtheirplaceofformation)orresidual(formedbyphysicaland/orchemical forces,breakingdownparentrocksorsoiltoamorefinelydividedstructure).Soilsdonot necessarilyretainconsistencyatvariousdepths,theyareinlayersofdifferentthicknessof unlikesoils.Soilsingeneralcanbeclassifiedaccordingtotheirhardnessinto9generalclasses from0to8,0beingthemostsolid(soundhardrock)likegranite,basalt,massivelimestoneand 8thesoftestlikeswampandmiscellaneousfills.Soilcanbetestedbysoiltestprobeswhich givereadingsinLBinchperprobepitch(thereadingisattheendofthepitch).Anothermethod oftestingthesoilisASTM1586bywhichthehardnessclassificationofthesoilisfunctionof thenumber(count)ofblows/ft.Soilcanalsobeclassifiedinto:loose,dense,honeycombed, dispersedandcomposite. Soilpropertiespertinenttoelectricalinstallations: The3propertiesofsoilthataffecttheelectricalinstallationsarethehardnessofthesoil,its thermalresistivity&itselectricalresistivity.Thefirstpropertywillaffectthemethodof erectingthepoleline&theselectionoftheanchorsusedinpoleguying.Thesecondwillaffect thecurrentcarryingcapacityofundergroundcable.Thethirdwillaffectthegroundingsystem, includingthegroundingrods(electrodes)&grid(mat),thatinfluencesthecurrentflowthrough thegroundpathandgroundvoltageunderfaultyconditions(flowofshortcircuitcurrents). Poles: 1

Wood: Woodisanorganicmaterial.Thetrunksandbrancheshave,fromtheoutsideinwardsthe following:thebark,sapwood,heartwoodandsmallcoreofsofttissue(inthecentre).Treesare classifiedintothesoftwood(cedars,pines,firs,larches)andhardwood.Woodhasindependent propertiesinthedirectionofthetree'saxesi.e.longitudinal,radialandtangential.The mechanicalpropertiesofwoodinclude:themodulusofelasticity(stressdividedbystraininthe elasticzoneofthecurve),themodulusofrigidityandthePoisson'sratio=lateralstrain/axial strain(allthesepropertiescanfallundertheheadingelasticproperties),modulusofstrengthin bending,maximumstressincompressionandtheshearstrength(strengthofwood),sound velocityanddampingcapacitycausedbyinternalfriction.Wooddefectscanberelatedtoeither decompositionofwoodfibrebyfungi(decay)ortobreakdownofcellwallsasaresultof appliedstressbeyondtheyieldstressofthecellofthewallmaterial(mechanicaldestruction). Installedpolescanbetestedtodeterminetheconditionofpoles.Theultrasonicmethodcanbe usedatgroundlineorbelow.Themostcommonlyusedspeciesoftreesusedforwoodpolesare: Southernyellowpine,Westernredcedar,Douglasfir,WesternlarchandJackpine. Treatmentthatawoodpole: Treatmentcanbeconductedunderorwithoutpressure.Preservativesthatcanbeusedareanyof thefollowing:creosoteoil,ammoniacolcopperfattyacid,pentachlorophenolandchromated copperarsenates(CCA).Chemicalpreservativesareusedtoprotectthewoodfromtheattackof biologicaldegraderslikefungi,insectsandmarineorganisms.Thepropertreatmentofthewood canimprove,significantly,theservicelifeofthepolesasitpreventsdeterioration. Testsconductedonwoodpoles: Installedpolescanbetestedtodeterminetheconditionofpoles.Theultrasonicmethodcanbe usedatgroundlineorbelow.Ultrasonictechnologydealswiththebehaviorofhighfrequency sound(beyondtherangeofhumanhearing).Frequenciesusedfortestingmetallicmaterials rangefrom2.2510MHZ,thosefortestingwoodandothernonmetallicmaterialsrange25100 KHZ.Ultrasonicpulsesareproducedbytransducersandduringthetestthepulsesarereceived byothertransducersfoundonthereceivingend. Thetypesofultrasonicpulsesare:longitudinalorcompressionpulses,transverseorshear (radial)pulses,surfaceorRayleighpulses,plateorLambpulses(inthinplates)&tangential pulses(inwoodonly).Thesonicvelocityindicatestheintegrityofthepole.Ahighersonic velocityindicatesalongerlifepolethanalowersonicvelocity. Othermaterialsusedtomanufacturepolesforelectricalapplications: Aluminumstructurescanbefoundwhenornamentalstreetlightingisused.Thehollowtubular streetlightingpolesgiveapleasingappearance.Thepolesarelightweight,thustheyareeasyto handle.Thesupplytothelampsfromtheundergroundelectricdistributionsystemsareeasier withthehollowpoles. Thepolesthataremadefromconcreteareusedforstreetlighting,distributionandtransmission 2

line.Concretepolesaremoreexpensivethanwoodenones,lowerininsulationlevel,more difficulttoclimb,heaviertohandleandmoredifficulttodrillthanwoodenones.The advantagesofconcretepolesare:theirlongerlifeandtheiravailabilityondemand.Holesare providedtosuitrequiredpoleframing,unusedholesmaybepluggedwithplasticcaps.The polesareclassifiedfromAtoJwhereAistheleaststrong600lb.andJthestrongestwith4500 lb.ultimateload.Theminimumrequiredinformationtospecifyaroundconcretedistribution poleare:lengthinft,topdiameter,minimumracewaydiameter,holes(spacinganddiameter), apertures,grounding,bars(galvanizedorcoated),surfacetreatment,regularorprestressedclass. Thehollowspunprestressedconcretepoleshaveahighdensityconcreteshell,completely, encasingareinforcingcagecontainingprestressedhightensilesteelwires.Prestressing producespoleswithahighstrengthtoweightratiosthatisusedfordistributionlines.Square taperedprestressedconcretepolesareconstructedbyplacingthestressedreinforcingmaterial (inaform)andpouringtheconcreteintotheform.Forgroundingpurposes,acopperwireis usuallycastedintothepole.Aplastictubemaybeusedtoobtainahollowpole. Fiberglasspoleswhencomparedtowoodpolesareimmunetofreezing,rotting,damagefrom nailsandpeckingofbirds.Fiberglasspolesaretooexpensive,butfiberglasscomponents(like insulatorsupports)arereasonablypriced.Fiberglasspolesareusedasstreetlightpolessupplied fromanundergrounddistributionsystem.Thistypeofpoledoesnotrequirepainting. Steelstructures(towers)havebeenusedextensivelytosupportsubtransmissionand transmissionlineconductors. Maximumweightoftheequipmenttobeinstalled(mounted)onapole: Themaximumweightthatmaybeinstalledonpolesisfunctionoftheheightofthepole,the class,themountingdistancefromthepoletoptotheuppermostattachment,fiberstrengthsof poles,hoistingcablestressesandthelinemen/equipmenttobeonthepoleatanytime. Distributioncircuitstructures,generally,consistofsinglepoles(wood),verticalorhorizontal linepostinsulators(V/HLP)orpintypeinsulatorswithfiberglasssupportbracketsor crossarms.Subtransmisionlinesoperatingatvoltagesof138KVorlowercanbebuiltonsingle woodpolestructures. Framing&guying: Framingmeansthedressingofthepolewiththeinsulatorsthatwillcarrytheconductors.Justto listafewexamples:singlephaseprimarycircuitwiththeinsulatoronthetoporsideofthepole, 3phasetriangulararmlesswith1insulatoronthetopand2onthesides,3phasetriangular armlesswiththe3insulatorsonthesides,verticaldeadendanddoublecircuitframing. Aguyisabraceorcablefastenedtothepoletostrengthenitandkeepitinposition.Guysare usedwhereverthewirestendtopullthepoleoutofitsnormalpositionandtosustaintheline duringtheabnormalloadscausedbysleet,windandotheweatherconditions.Guyscounteract theunbalancedforcesimposedonthepolebydeadendingconductors,changesinconductor sizes/types/tension,anglesinthedistributionpoleline.Guyassembliescanbeclassifiedinto: 3

anchor(down),span,head,arm,stubandpush.Themaincomponentsoftheanchorguy assemblyare:galvanizedmachineboltwithnut,locknut,squarecurvedgalvanizedwasher, galvanizedsteelguywire,porcelainguystraininsulator,prefabricatedguydeadendgrips, plasticguyguard,anglethimbleye,eyenut,steelanchorrod,powerinstalledscrewanchor.Refer tofig.1.1forthecommonpoleguyingconfigurations.

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Pole'sfires: Firesonwoodpolesandcrossarmscanbeinitiatedbyleakagecurrents.Wheninsulatorson woodpolesbecomecontaminatedbydustorchemicals,thenlightrainfogorwetsnowmoistens theinsulators,thepolesandthecrossarms(ifavailable),theseconditionswillcauseleakage 5

currentstoflowtotheground.Theleakagecurrentsgotogroundthroughagroundwireonthe poleorthebaseofthepole.Theflowofcurrentisimpededbythedryareasontheassembly. Whenthemediuminthedryzone(exampleair)issubjectedtodielectricstress(voltage gradient)thatexceedsthedielectricstrengthofthisgap,anarcwillbeestablished.Thearc,if closetocombustiblemateriallikewood,itwillignitethewoodinthedryarea.Theleakage currentscanmaintainthearcandthefire(afterignition). Selectionofapole: Thefactorscanbeclassifiedbroadlyintoavailability&requiredstrength.Theavailabilityofthe materialofthepoleeitherwoodorotherwiseintheareawherethepolelinewillbebuiltorthe polewillbeerectedisveryimportant.Regardingwoodpoles,thefactorsthataffectthechoice ofonetypeovertheotherare:thephysicalrequirementsofthepoles,thatareneededto constructtheline,andthecostofshipmentfromthelocationwherethespeciesaregrown.The transportationissueofthepoleshavetobeconsideredcarefullyduetothequantityofthepoles requiredtobuildtheline,thelengthofsuchpolesaswellastheirweights.Thestrengthofthe poleshavetobeenoughtowithstandalltheexpectedloadsthatthepoleswillbesubjectedto duringitsexpectedlifespan. Majorloadsactingonapole: Themajorloadsthatmaybeactingonapoleareice&windloadingwhichcanbeclassified intoeitherlight,mediumorheavyloading,transversewindloadingonthepole&conductors, polelineangle,longitudinalloading(alongthelineconductor)duetodeadendingorabroken wireandverticalloading(weightofpole,crossarms,pins,insulators,attachments,guys&ice coveredwires).Loadingonpolesmustbecalculatedforthefollowingconditions:crossingsof railways,waterways&highways,crossingsofotherpowerorcommunicationlines,poleangles anddeadendings. Thewoodenpolesaredefinedasfollows:theclass(1to74500LBto1200minimumhorizontal breakingloadwhenapplied2ft.frompoletop),theminimumcircumferenceatpoletoplevel (27"to15"),lengthofpole(25to110ft,generally),minimumcircumferenceatthegroundlevel (distancefrombutt),thewoodspecies(WesternRedCedar,SouthernYellowPine,DouglasFir, WesternLarch),thetreatmentagainstattackfromfungiandinsects(eg.creosoteoil,ammonical copperfattyacid,pentachlorphenolorchromatedcopperarsenate)andtheweightperpole.The concretepolesaredefinedaccordingly:ultimateload(classAtoJ,600LBto4500LB, respectively),thelength,themanufacturingprocess(regularorprestressedclass),thesteel reinforcingrods(cage)tensilestrength,thediameter,theracewaydiameter,spacingand diameterofholesinthepole,groundingbars(galvanizedorcoated)surfacetreatment. Switches: Differenttypesofswitches&differentconfigurations/installationmethods: Switchescanbedividedintofourgeneralclasses.Airswitches,oilswitches,vacuumswitches andSF6switches.Airswitchescanbefurtherclassifiedinto:circuitbreakers,airbreak,load 6

breakanddisconnectswitches.Oilswitchescanbecircuitbreakersoroilcircuitreclosers. Vacuumcanbeavacuumcircuitbreakeroravacuumrecloser.SF6canbeacircuitbreakerora circuitswitcherorarecloser.Circuitbreakersaremostlyusedinindoorsubstations(unit, transformerordistribution)orinoutdoor(onstructure,asstandalone)installations.Overhead switchescanalsobeclassifiedaccordingtotheirmethodofoperationi.e.,manualvs. manual/motorandalsoaccordingtothepossibilityofremote/localoperationoronlylocal operationoftheswitch.Overheadswitchescanalsobeclassifiedaccordingtotheirtypeof installation:vertical(tiered),horizontalorriserpoleorinline(midspanopeners),triangularor poletop(fig.1.2).Switchescanalsobeclassifiedaccordingtotheirtypeofbreaki.e.vertical, sideordouble.

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Breakingtheload&extinguishingthearcinairbreakswitches: Wheninterrupted,thearcisformedbetweenthemetalhornsofthecircuitcarryingthecurrent. Thedistancebetweenthehornsincreasesastheswitchcontinuestoopen.Thearciscooledby windandextendsinlengthuntilextinction.Airbreakswitchesareusuallymountedon substationstructuresoronpolesandareoperatedfromthegroundlevel.Theswitchesare operatedbyahandleconnectedtotherod,extendingfromtheswitchtoalevelclosetothe groundwheretheopening/closingoperationsareperformed.Airbreakswitchescanbe automated(motorized)toallowforremoteoperation. 8

Interruptingtheloadcurrentinloadbreakswitches: Loadbreak(interrupter)switchescomeinsingleandthreepoleconfigurations.Whencurrentis interrupted,thearcisconfinedandextinguishedintheinterrupter(noexternalarcinterruption orseparatedeviceisrequired).Theseswitchescaninterruptlinecurrents(loadsplitting,load dropping),transformerloadandmagnetizingcurrents,cablechargingandloadcurrents. Generally,thebladesaremadeofharddrawncopperwithsilvertosilvercontacts,thesilver platedstationarycontactmaybeformedofcopperandincludeataperedprofiletoimprove bladeclosure.Thearcextinguishingprocessissimilartotheonewhichoccurswhenanindoor loadbreakswitchhastobreakthecurrentandextinguishesthearcinthearcchuteorarctube. Withtheswitchinthefullyclosedposition,currentflowsthroughthecopperbladeandthe silvercontacts(theinterrupteris,totally,outofthecircuit).Asthebladebeginstoopen,the currentistransferredtotheinterrupterbywipingactionbetweentheshuntcontactandthe interruptersexternalcontact.Afterwhich,themaincurrentcarryingcontactspart.Thenextstep istheopeningoftheinternalcontactsoftheinterrupterbythebladecam.Thearcinsidethe interrupterisextinguishedbythermalactionandthedeionizinggasesgeneratedfromtheliner andtheinternaldesignoftheinterrupter.Exhaustisquietandflamelessandisventedthrough theexhaustcap.Loadbreakswitchescanbemotoroperatedforremoteclose/openoperation. Theairbreakswitcheshaveagas(orvacuum)counterpartwherethearcisextinguishedinSF6 polesorvacuumbottles.Therearetwomajorconstructionsforgasfilledoverheadswitches whichare:onewithvisiblebreakandonewithout.ThevisiblebreakwillhaveanSF6bottle wherethecontactsbreak,afterwichthearcextinguishesandadisconnectswitch(interlocked withtheSF6contacts)isusedtoprovidethevisiblebreak(thedisconnectisinserieswiththe SF6pole).Theothertypewillhavetheloadinterrupting/switchingcontactsandoperating mechanismcontainedinahermeticallysealedweldedtank.Themotorizedoperating mechanismsareclassifiedintorotatingandreciprocatingtypes. Applicationsofdisconnectswitches: Disconnectswitchesareairbreakswitches,notequippedwitharcinghornsorotherloadbreak devices.Thedifferentconfigurationsofdisconnectswitches,asusedonoverheadsystems,are: thebranchfeederstyle,crossarmvertical,crossarminverted,stationvertical,stationinverted andmainfeederstyle. Toolsusedwithdisconnectswitches/fusecutoutstointerruptloadcurrents: Whenportableloadbreaktoolsareusedinconjunctionwiththeseswitches,switchingthe followingelementsispossible:transformers,lines,cableandcapacitors(withcertain limitations).Theseswitchesaredefinedbythefollowingparameters:insulationratings(the nominalvoltageinKVandtheBILinKV),ratedcontinuouscurrentinamperes,leakage distanceininches,dryarcingdistance,disconnectgapininchesandcantileverstrengthof insulatorinLBs.Ingeneral,loadbreaktoolscanbeclassifiedintoaloadinterruptertooland parallelingtool.Theparallelingloadbreakingtoolwillcreateatemporarybypassjumperacross 9

thedisconnect(inparallelwiththepermanenttapconnection).Afterthebladeofthetoolis closed,thepermanenttapcanbedisconnected.Theloadcanthenbedroppedbyopeningthe bladeofthetool.Thetooloperatesbysimpledisconnectstick.Theloadinterruptertoolis definedaccordingtoitsnominalvoltageanditsinterruptingcapability(nominaland maximum).Theloadbreaktool,usually,hasananchortohangontheattachmenthookofthe disconnect(cutoutorpowerfuseforthatmatter)andapullringhooktoengagetheswitchpull ring.Generally,theloadbreaktoolisattachedtoauniversalpole.Afterconnectingthetoolto thedisconnectaspreviouslymentioned,theuniversalpoleispulleddownward(firmlyand steadily)andasthetoolisextendingtoitsmaximumlength,thedisconnectisopenedandthe currentisdivertedthroughthetool.Atapredeterminedpointinthetoolopeningstroke,its internaltriggertrips,thechargedoperatingspringisreleased,theinternalcontactsareseparated andthecircuitisinterrupted.Thetoolhastoberesetforthenextoperation. Thedifferenttypesofoverheadswitchesare:eithersingleorthreephase,eithermanually operatedorelectric/manualoperated,eitherlocalcontrolorremote/localcontrol,oilinsulated orairorSF6.Thedifferenttypesofpadmountedswitchgearare:eithermanuallyor manually/motoroperated,controlledlocallyorlocally/remotely,airoroilorSF6/vacuum insulated,protectivedevicesareeitherfusesorelectronicdevices.Theconfigurationwill, generally,havefourcompartmentswithanycombinationoffuseorinterrupter,switch,solidor emptycompartment.Thedifferenttypesoflightningarrestersare:station,intermediate, distribution(heavyduty,normalorlightduty)andmayberiserpoletype. Applicationsofoverheadoilswitches: Thedifferentapplicationsofoverheadoilswitchesinutilitydistributionsystemsare:general purposeforinductiveandresistiveloads&capacitor(capacitivecurrentswitching).The definingparametersare:theratedmaximumvoltage,thebasicimpulselevel,thedielectric withstand,continuouscurrent,inductiveloadswitching,capacitiveswitchingcurrent,making current,momentarycurrent,shorttimecurrentrating;forthecontrolcircuit:nominalandrange ofoperatingvoltage,tripcoilcurrent.Theweight,dimensions,oilvolumeandspeedof operationfortheswitcharealsoimportantdefiningdata.Theothertwodevicesthatmayuseoil astheswitchingmediumarethesectionalizesandreclosers. Classificationofairinsulatedswitches: Thefollowingistheclassificationoftheairinsulatedswitchesaccordingtotheirbreakingtype: sidebreakswitches,verticalbreakanddoublebreak.Thedifferenttypesofmountingsforsuch switchesare:upright,vertical,triangular,tieredoutboardmountingandpedestal.Theinsulators oftheswitchmaybeepoxyorporcelain,thebaseisinsulatedorsteel.

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