Contents: 1. Lesson1:Overhead&undergrounddistributionsystemscomponents. 2. Lesson2:Switchgear,circuitbreakers,MCCs&contactors. 3. Lesson3:Powerlinedisturbances. 4. Lesson4:Maintenance. 5. Lesson5:Personnelvs.equipmentsafety.

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Lesson1:Overhead&undergrounddistributionsystemscomponents. Soil: 1)Howissoilclassified? 2)Inwhataspectsarethesoilpropertiesimportantwhenitcomestoelectricalinstallations? Poles: 3)Whatiswood? 4)Whatisthetreatmentthatawoodpolegets?Whatisthepurposeofsuchtreatment? 5)Whatarethetestsconductedonwoodpoles? 6)Whatothermaterialsareusedtomanufacturepolestorunand/orinstallelectricalconductorsand equipment? 7)Whataretheimportantfactorsthataffectthemaximumweightoftheequipmenttobeinstalled (mounted)onapole? 8)Whatdoesframingmean?Whatdoesguyingmean? 9)Whatcausepole'sfires? 10)Whatarethefactorsaffectingtheselectionofapole? 11)Poleclass&guyingarefunctionofpoleloading,whatarethemajorloadsactingonapole? Switches: 12)Listthedifferenttypesofswitchesfoundinoverheadsystems&different configurations/installationmethods? 13)Howdoesanairbreakswitchbreaktheload&extinguishthearc? 14)Howdoesaloadbreakswitchinterrupttheloadcurrent? 15)Whatarethedifferentapplicationsinwhichdisconnectswitchesareused? 16)Whattoolsareusedwithdisconnectswitches&fusecutoutstointerruptloadcurrents? Padmountedswitchgear: 17)Giveageneraldescriptionofpadmountedswitchgear? 18)Whatarethemajorcomponentsofanoilfilledpadmountedswitchgearunit? 19)WhatarethemajorcomponentsofSF6/vacuumunits? 20)Whataretheproductionteststhatareconductedonpadmountedswitchgearassemblies? Distributiontransformers: 21)Whatarethedifferentmethodsofmounting(installing)distributiontransformersonpoles? 22)Whatarethemainattachments(accessories)usedtocompletetheinstallationoftheoverhead(pole mounted)distributiontransformer? 23)Whatarethemaincomponents(accessories)thataremountedorincludedwiththepadmountedor vaulttypedistributiontransformerstocompleteitsinstallation? 24)Whatarethebasicpartsofatransformer?

25)Asusedwithtransformers,whataretheimportantcurvesthatdefinetheelectricsteelproperties? 26)Whichpropertiesoftheelectricsteelarestructuresensitive&whicharenot? 27)Whatareamorphousmetals? 28)Whatarethedifferenttypesofcoresusedindistributiontransformers? 29)Whatarethedifferentarrangementsofcoilsusedindistributiontransformersandhowdoesthe crosssectionareaofthewireslooklike? 30)Howcantheexcitingcurrentsoftransformersberesolved(i.e.whatarethecomponentsofthe excitingcurrents)? 31)Whatarethemethodsusedtoinsulatethecoilsofatransformer? 32)Whenatransformerisenergized,whatarethelossesencounteredifitisnotloaded&ifitis loaded? 33)Whatarethecausesofthegeneratedgasesinanoilfilledtransformer? 34)Whatfactorsaffectthegenerationofgasinoilimmersedtransformers? Cables: 35)Whatarethemethodsoflayingundergroundcables? 36)Whatarerubbers? 37)WhatisPVC? 38)Whatarepolyethylenes? 39)Whataretheelectriccharacteristicsofcables? 40)Whatarethefailuremodesincables? 41)Whataretherequirementsthatamanholemustfulfill? 42)Whatarethedissipationfactor&powerfactorofacableinsulation? 43)Howwouldtheinductance,capacitance&dissipationfactorofacablebemeasured? 44)Whatarethedifferentmethodstolocatefaultsinundergroundcables? 45)Whatistheneedforacabletermination? 46)Whywouldcablesplicesbeused? 47)Tomakeasplicethatwouldlastlong,whataretheconditionsthathavetobefulfilledwhiledoing thesplice? 48)Whatarefaultedcircuitindicators?Wheretheyaremounted? 49)Whatisroutetracing? 50)Drawasketchforanelbow&anelbowinsertshowingthemaincomponentsofsuchadevice? Conductors: 51)Whatarethedifferenttypesofconductorsusedinoverheaddistributionsystems? 52)Comparetheconductivity&strengthofaluminumconductorstothoseofcopper? 53)Whatdoescoldflowmeans? 54)Whatarethedifferenttypesofconnectorsusedinpowerdistributionsystems? 55)Whatarethefactorsaffectingtheselectionoflineconductors? 56)Howareconductorsgradedwhenitcomestoflexibility? Insulators: 57)Whatisthefunctionofaninsulator? 58)Whatarethetypesofinsulators?

59)Whatarethepropertiesofaninsulator? 60)Whatareporcelaininsulatorsmadeof? 61)Whatareglassinsulatorsmadeof? 62)Whatarefiberglassinsulatorsmadeof? 63)Whatarethepropertiesofsiliconethatisusedininsulators? 64)Whatarethetestsperformedonthebulkmaterial&thoseconductedonthecompleteinsulator? Fuses&cutouts: 65)Whatarethedifferenttypesoffusesusedinpowersystems? 66)Howarefusesusedtoprotectpolemountedtransformers? 67)Whatarethecomponentsofafusecutout?Howdoesafusecutoutoperate? 68)Howdoesapowerfuseextinguishthearc? 69)Howdoesacurrentlimitingfuse(c.l.f.)interruptcurrents? 70)Whatarethedefiningparametersoffuses? 71)Howarefusesusedtoprotectpadmountedtransformers? 72)Howarefusesusedtoprotectvaultinstalledtransformers? 73)Whatarethemaincomponentsofac.l.f.? EnergyManagement: 74)Whatisenergymanagement? 75)Whatisdemandsideenergymanagement? 76)Whatarehighefficiencymotors? 77)Whataretheofficeequipmentcommonlyused&whataretheirapproximatepowerconsumption? 78)Whataretherequireddatatocomparethetotalcostofthedifferentlightingoptionsortocalculate thepaybackperiodofthedifferentinstallations? 79)Whatarethedatarequiredtobeabletocomparetherequiredwattagefortheavailablealternative bulbs/fixtureoptions? 80)WhatarethemaintasksofaSCADAsystem? Geographicinformationsystems(GIS): 81)WhatareGIS? 82)WhatarethecomponentsofaGIS? 83)HowdoesaGISwork? 84)WhatarethetasksofaGIS? 1)Howissoilclassified? Thesimplestwaytoclassifysoilsistoclassifyitintocohesiveornoncohesive.Anexampleof cohesivesoilisclayasitisafinegrainedsoil,coarsegrainedsoilsarenoncohesivelikesand.Further subclassificationtocohesiveandnoncohesiveispossiblebasedontheoriginormethodofdeposition orstructureofthesoil.Soilstructuremaybeclassifiedasdeposited(transportedfromtheirplaceof formation)orresidual(formedbyphysicaland/orchemicalforces,breakingdownparentrocksorsoil toamorefinelydividedstructure).Soilsdonotnecessarilyretainconsistencyatvariousdepths,they areinlayersofdifferentthicknessofunlikesoils.Soilsingeneralcanbeclassifiedaccordingtotheir hardnessinto9generalclassesfrom0to8,0beingthemostsolid(soundhardrock)likegranite,basalt,

massivelimestoneand8thesoftestlikeswampandmiscellaneousfills.Soilcanbetestedbysoiltest probeswhichgivereadingsinLBinchperprobepitch(thereadingisattheendofthepitch).Another methodoftestingthesoilisASTM1586bywhichthehardnessclassificationofthesoilisfunctionof thenumber(count)ofblows/ft.Soilcanalsobeclassifiedinto:loose,dense,honeycombed,dispersed andcomposite. 2)Inwhataspectsarethesoilpropertiesimportantwhenitcomestoelectricalinstallations? The3propertiesofsoilthataffecttheelectricalinstallationsarethehardnessofthesoil,itsthermal resistivity&itselectricalresistivity.Thefirstpropertywillaffectthemethodoferectingthepoleline &theselectionoftheanchorsusedinpoleguying.Thesecondwillaffectthecurrentcarryingcapacity ofundergroundcable.Thethirdwillaffectthegroundingsystem,includingthegroundingrods (electrodes)&grid(mat),thatinfluencesthecurrentflowthroughthegroundpathandgroundvoltage underfaultyconditions(flowofshortcircuitcurrents). 3)Whatiswood? Woodisanorganicmaterial.Thetrunksandbrancheshave,fromtheoutsideinwardsthefollowing:the bark,sapwood,heartwoodandsmallcoreofsofttissue(inthecentre).Treesareclassifiedintothe softwood(cedars,pines,firs,larches)andhardwood.Woodhasindependentpropertiesinthedirection ofthetree'saxesi.e.longitudinal,radialandtangential.Themechanicalpropertiesofwoodinclude:the modulusofelasticity(stressdividedbystrainintheelasticzoneofthecurve),themodulusofrigidity andthePoisson'sratio=lateralstrain/axialstrain(allthesepropertiescanfallundertheheading elasticproperties),modulusofstrengthinbending,maximumstressincompressionandtheshear strength(strengthofwood),soundvelocityanddampingcapacitycausedbyinternalfriction.Wood defectscanberelatedtoeitherdecompositionofwoodfibrebyfungi(decay)ortobreakdownofcell wallsasaresultofappliedstressbeyondtheyieldstressofthecellofthewallmaterial(mechanical destruction).Installedpolescanbetestedtodeterminetheconditionofpoles.Theultrasonicmethod canbeusedatgroundlineorbelow.Themostcommonlyusedspeciesoftreesusedforwoodpolesare: Southernyellowpine,Westernredcedar,Douglasfir,WesternlarchandJackpine. 4)Whatisthetreatmentthatawoodpolegets?Whatisthepurposeofsuchtreatment? Treatmentcanbeconductedunderorwithoutpressure.Preservativesthatcanbeusedareanyofthe following:creosoteoil,ammoniacolcopperfattyacid,pentachlorophenolandchromatedcopper arsenates(CCA).Chemicalpreservativesareusedtoprotectthewoodfromtheattackofbiological degraderslikefungi,insectsandmarineorganisms.Thepropertreatmentofthewoodcanimprove, significantly,theservicelifeofthepolesasitpreventsdeterioration. 5)Whatarethetestsconductedonwoodpoles? Installedpolescanbetestedtodeterminetheconditionofpoles.Theultrasonicmethodcanbeusedat groundlineorbelow.Ultrasonictechnologydealswiththebehaviorofhighfrequencysound(beyond therangeofhumanhearing).Frequenciesusedfortestingmetallicmaterialsrangefrom2.2510MHZ, thosefortestingwoodandothernonmetallicmaterialsrange25100KHZ.Ultrasonicpulsesare producedbytransducersandduringthetestthepulsesarereceivedbyothertransducersfoundonthe receivingend. Thetypesofultrasonicpulsesare:longitudinalorcompressionpulses,transverseorshear(radial)

pulses,surfaceorRayleighpulses,plateorLambpulses(inthinplates)&tangentialpulses(inwood only).Thesonicvelocityindicatestheintegrityofthepole.Ahighersonicvelocityindicatesalonger lifepolethanalowersonicvelocity. 6)Whatothermaterialsareusedtomanufacturepolestorunand/orinstallelectricalconductors andequipment? Aluminumstructurescanbefoundwhenornamentalstreetlightingisused.Thehollowtubularstreet lightingpolesgiveapleasingappearance.Thepolesarelightweight,thustheyareeasytohandle.The supplytothelampsfromtheundergroundelectricdistributionsystemsareeasierwiththehollowpoles. Thepolesthataremadefromconcreteareusedforstreetlighting,distributionandtransmissionline. Concretepolesaremoreexpensivethanwoodenones,lowerininsulationlevel,moredifficulttoclimb, heaviertohandleandmoredifficulttodrillthanwoodenones.Theadvantagesofconcretepolesare: theirlongerlifeandtheiravailabilityondemand.Holesareprovidedtosuitrequiredpoleframing, unusedholesmaybepluggedwithplasticcaps.ThepolesareclassifiedfromAtoJwhereAistheleast strong600lb.andJthestrongestwith4500lb.ultimateload.Theminimumrequiredinformationto specifyaroundconcretedistributionpoleare:lengthinft,topdiameter,minimumracewaydiameter, holes(spacinganddiameter),apertures,grounding,bars(galvanizedorcoated),surfacetreatment, regularorprestressedclass. Thehollowspunprestressedconcretepoleshaveahighdensityconcreteshell,completely,encasinga reinforcingcagecontainingprestressedhightensilesteelwires.Prestressingproducespoleswithahigh strengthtoweightratiosthatisusedfordistributionlines.Squaretaperedprestressedconcretepolesare constructedbyplacingthestressedreinforcingmaterial(inaform)andpouringtheconcreteintothe form.Forgroundingpurposes,acopperwireisusuallycastedintothepole.Aplastictubemaybeused toobtainahollowpole. Fibreglasspoleswhencomparedtowoodpolesareimmunetofreezing,rotting,damagefromnailsand peckingofbirds.Fibreglasspolesaretooexpensive,butfibreglasscomponents(likeinsulatorsupports) arereasonablypriced.Fibreglasspolesareusedasstreetlightpolessuppliedfromanunderground distributionsystem.Thistypeofpoledoesnotrequirepainting. Steelstructures(towers)havebeenusedextensivelytosupportsubtransmissionandtransmissionline conductors. 7)Whataretheimportantfactorsthataffectthemaximumweightoftheequipmenttobe installed(mounted)onapole? Themaximumweightthatmaybeinstalledonpolesisfunctionoftheheightofthepole,theclass,the mountingdistancefromthepoletoptotheuppermostattachment,fiberstrengthsofpoles,hoisting cablestressesandthelinemen/equipmenttobeonthepoleatanytime.Distributioncircuitstructures, generally,consistofsinglepoles(wood),verticalorhorizontallinepostinsulators(V/HLP)orpintype insulatorswithfiberglasssupportbracketsorcrossarms.Subtransmisionlinesoperatingatvoltagesof 138KVorlowercanbebuiltonsinglewoodpolestructures. 8)Whatdoesframingmean?Whatdoesguyingmean? Framingmeansthedressingofthepolewiththeinsulatorsthatwillcarrytheconductors.Justtolista

fewexamples:singlephaseprimarycircuitwiththeinsulatoronthetoporsideofthepole,3phase triangulararmlesswith1insulatoronthetopand2onthesides,3phasetriangulararmlesswiththe3 insulatorsonthesides,verticaldeadendanddoublecircuitframing. Aguyisabraceorcablefastenedtothepoletostrengthenitandkeepitinposition.Guysareused whereverthewirestendtopullthepoleoutofitsnormalpositionandtosustainthelineduringthe abnormalloadscausedbysleet,windandotheweatherconditions.Guyscounteracttheunbalanced forcesimposedonthepolebydeadendingconductors,changesinconductorsizes/types/tension,angles inthedistributionpoleline.Guyassembliescanbeclassifiedinto:anchor(down),span,head,arm, stubandpush.Themaincomponentsoftheanchorguyassemblyare:galvanizedmachineboltwith nut,locknut,squarecurvedgalvanizedwasher,galvanizedsteelguywire,porcelainguystraininsulator, prefabricatedguydeadendgrips,plasticguyguard,anglethimbleye,eyenut,steelanchorrod,power installedscrewanchor.Refertofig.1.1forthecommonpoleguyingconfigurations.

9) Whatcausepole'sfires? Firesonwoodpolesandcrossarmscanbeinitiatedbyleakagecurrents.Wheninsulatorson woodpolesbecomecontaminatedbydustorchemicals,thenlightrainfogorwetsnowmoistens theinsulators,thepolesandthecrossarms(ifavailable),theseconditionswillcauseleakage

currentstoflowtotheground.Theleakagecurrentsgotogroundthroughagroundwireonthe poleorthebaseofthepole.Theflowofcurrentisimpededbythedryareasontheassembly. Whenthemediuminthedryzone(exampleair)issubjectedtodielectricstress(voltage gradient)thatexceedsthedielectricstrengthofthisgap,anarcwillbeestablished.Thearc,if closetocombustiblemateriallikewood,itwillignitethewoodinthedryarea.Theleakage currentscanmaintainthearcandthefire(afterignition). 10)Whatarethefactorsaffectingtheselectionofapole? Thefactorscanbeclassifiedbroadlyintoavailability&requiredstrength.Theavailablilityof thematerialofthepoleeitherwoodorotherwiseintheareawherethepolelinewillbebuiltor thepolewillbeerectedisveryimportant.Regardingwoodpoles,thefactorsthataffectthe choiceofonetypeovertheotherare:thephysicalrequirementsofthepoles,thatareneededto constructtheline,andthecostofshipmentfromthelocationwherethespeciesaregrown.The transportationissueofthepoleshavetobeconsideredcarefullyduetothequantityofthepoles requiredtobuildtheline,thelengthofsuchpolesaswellastheirweights.Thestrengthofthe poleshavetobeenoughtowithstandalltheexpectedloadsthatthepoleswillbesubjectedto duringitsexpectedlifespan. 11)Poleclass&guyingarefunctionofpoleloading,whatarethemajorloadsactingona pole? 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. 12)Listthedifferenttypesofswitchesfoundinoverheadsystems&different configurations/installationmethods? Switchescanbedividedintofourgeneralclasses.Airswitches,oilswitches,vacuumswitches andSF6switches.Airswitchescanbefurtherclassifiedinto:circuitbreakers,airbreak,load 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.

13)Howdoesanairbreakswitchbreaktheload&extinguishthearc? Wheninterrupted,thearcisformedbetweenthemetalhornsofthecircuitcarryingthecurrent. Thedistancebetweenthehornsincreasesastheswitchcontinuestoopen.Thearciscooledby windandextendsinlengthuntilextinction.Airbreakswitchesareusuallymountedon substationstructuresoronpolesandareoperatedfromthegroundlevel.Theswitchesare operatedbyahandleconnectedtotherod,extendingfromtheswitchtoalevelclosetothe groundwheretheopening/closingoperationsareperformed.Airbreakswitchescanbe automated(motorized)toallowforremoteoperation. 14)Howdoesaloadbreakswitchinterrupttheloadcurrent?

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. 15)Whatarethedifferentapplicationsinwhichdisconnectswitchesareused? Disconnectswitchesareairbreakswitches,notequippedwitharcinghornsorotherloadbreak devices.Thedifferentconfigurationsofdisconnectswitches,asusedonoverheadsystems,are: thebranchfeederstyle,crossarmvertical,crossarminverted,stationvertical,stationinverted andmainfeederstyle. 16)Whattoolsareusedwithdisconnectswitches/fusecutoutstointerruptloadcurrents? Whenportableloadbreaktoolsareusedinconjunctionwiththeseswitches,switchingthe followingelementsispossible:transformers,lines,cableandcapacitors(withcertain limitations).Theseswitchesaredefinedbythefollowingparameters:insulationratings(the nominalvoltageinKVandtheBILinKV),ratedcontinuouscurrentinamperes,leakage distanceininches,dryarcingdistance,disconnectgapininchesandcantileverstrengthof insulatorinLBs.Ingeneral,loadbreaktoolscanbeclassifiedintoaloadinterruptertooland parallelingtool.Theparallelingloadbreakingtoolwillcreateatemporarybypassjumperacross 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. 17)Giveageneraldescriptionofpadmountedswitchgear? Theseswitchingassembliescanbeclassifiedintoairinsulated,oilsealedinsulated,orSF6/load breakswitchesandvacuumfaultinterrupters.Typically,fortheairinsulatedtypewhenthe separableconnectorsareinplace,theconstructionwillhaveallenergizedpartsenclosedin grounded(shield)enclosure(deadfront).Verificationoftheopenswitch(visiblebreak)is possiblethroughplexiglassviewingwindows.Thethreephasegangoperatedswitchesinthe assemblyareoperatedwithouthavingtoopenthecablecompartment.The600Adeadbreak bushingsareexternallyreplaceable.Theunitwillhaveparkingstands,replacemantfusestorage pockets,groundpadsforgroundingprovisions,doorretainers,latching(3point)arrangement, fuseviewingwindows,liftingprovisions,faultindicatorsandfloorcover(ifrequired).Fig.1.3 showsthemajorcompartmentsandmountingmethodonprecastconcretepad.

18)Whatarethemajorcomponentsofanoilfilledpadmountedswitchgearunit? Theoilfilledunitswillhaveafillplug,adrainvalve,theoilgauge,thecableentrance,thesteel tank,thespringoperator,themechanicalinterlockoverthefusecompartment,grounding provision,parkingstandsandthehingedcover. 19)WhatarethemajorcomponentsofSF6/vacuumunits? Themajorelementsare:theheavygaugeenclosure,sealedswitchcompartment,operating handle(formanualoperation),fillvalve/gaspressuregauge,electroniccontrolpackage(pad lockable),electronicloadtaptripadjustments(knobs,pushbuttons),springassistedswitch operator(withmarkedpositionindicator),provisionsforpadlocking,cableentrancebushings, phaseindicationlabels,parkingstands,deadbreakelbowconnectors,doorlatches,hingeddoors withstoppers,viewingwindows,groundlugs,deepwelllowcurrent(eg.200A)bushings, mechanicaltripandresetlever,provisionsfordoorpadlocking,motoroperator/RTUs.The motor/RTUinstallationwouldprovideremoteoperationoftheswitches(fromacontrolstation) orlocaloperation(motoroperated).Withtheavailabilityofsensors/RTUinthepadmounted switchgearassembly,theremoteindicationoftheloadlevelsandfaults(currents)atthecontrol stationmayassisttheoperatorsinrunningthesystemmoreefficientlywithlessdowntimesand higherlevelsofsupplycontinuity. 20)Whataretheproductionteststhatareconductedonpadmountedswitchgear assemblies?Howcanapadmountedswitchgearbemodifiedtooperate&indicate

remotelythroughaSCADA? Theproductiontestsrunonsuchassembliesare:continuitytest(toassurecorrectinternal connections),hipot(dielectric),pressuretest(toassuretankissealed),theprotective (electronico/c)devicecharacteristicscurveandleakagetests(ifrequired). Toremotelyoperateandindicatewithpadmountedswitchgearandundergrounddistribution systems,thefollowingelementsaretobepartoftheswitchgearunit:onesetofthreecurrent sensorsontheloadsideofeachinterrupter(orswitch),sourcesidePTwith120V(forexample) secondaryvoltage,radios/modemsortherequiredcommunicationequipment,connectorfor remoteantenna(ifapplicable),localinterfaceinthecontroller(status,controlswitches, displays),localcommunicationporttoallowsetup/updatingsoftware/troubleshooting/report generationusingalaptop.Shouldductsbe,already,installedtheuseoffiberoptics communicationnetworkwouldbethemostsuitable(overothermeansofcommunications:RF, PLC,.....,etc.).Fiberopticsisimmunetoelectromagneticandradiointerference,thusitcanbe placedinductscontaininghighvoltageCuorAlcables(i.e.15,27.6kV).Theelementsthatare neededtobuildafiberopticnetworkbesidethecablearethetransceiverswiththeserialand opticalports(LEDtransmitterandphotodiodereceiver),theRTUs,opticalsplitters,multiplexes andmodems. 21)Whatarethedifferentmethodsofmounting(installing)distributiontransformerson poles? Thesetransformerscanbefasteneddirectlytothepoles,hungfromcrossarms,mountedon racksorplatformsormountedonbracketsattachedtothepoles.TheKVAratingsforsuch transformersarelowi.e.167or250.Thepolemountedtransformerscanbeinstalledinclusters of3transformersattachedtothesupportingbracketsofwhichthelatterareattachedtothe poles. 22)Whatarethemainattachments(accessories)usedtocompletetheinstallationofthe overhead(polemounted)distributiontransformer? Thehighvoltagebushingwiththeclamptypeconnectorisconnectedtotheprimary(medium voltagecircuit)andthelowvoltagecablesareconnectedtospadetypeconnectors.Thepole mountedtransformersuseoilastheinsulatingmaterial.Theyareinstalledinmany configurations.Ingeneral,thesetransformersareconnectedtotheprimarycircuitthrougha currentlimitingfuseandafusecutout.Toprotectthetransformersagainstlightningorvoltage surges,theprimaryofthetransformerwillhavealightningarresterconnectedacrossitandthe ground.Thereisanothertypeofpolemountedtransformerswhichisthecompletelyself protectedone(CSP).Primaryfusesandlightningarrestersareincludedwiththetransformer, thusthereisnoneedtoanyexternalprotectivedeviceexceptforacurrentlimitingfuse. 23)Whatarethemaincomponents(accessories)thataremountedorincludedwiththe padmountedorvaulttypedistributiontransformerstocompleteitsinstallation? Thedistributiontransformersratingforsinglephasevariesfrom10KVAto1MVA,forthree phase30KVAto2.5MVA.Thepowertransformerscomeinsizesfrom3MVAto150MVAfor 3phaseconstructions.Indistributionsystems,threephasetransformersandthreephasebanks

(i.e.3singlephaseconnectedtoprovideadeltaorwye3phaseconfiguration)arequite common.Ingeneral,theprotectionofthepowertransformersisthroughtheuseofprotective relay(o/cordifferential&overcurrentground)andgasrelays.Thedistributiontransformersare protectedbyfuses(currentlimitingandexpulsiontypes).Padmountscanbeclassifiedinto radialfeedandloopfeed.Thepadmountedtransformerswillhaveloadorfaultsensing (expulsion)typefuse.Inserieswiththisfuseacurrentlimitingbackupfuseundertheoil.For vaultmountedtransformers,aseriesofcurrentlimitingandexpulsiontypewithpowerfuseor fuselinkmountedonthepoleorthewallofthevaultaremostprobablyusedasprimary protection.Foratypicalgenerallayoutofavault,refertofig.1.4.Forvaultmountedandpad mounted,theprimaryconnectionismadethroughtheuseofelbows(wherethecablesare connected)andinsertsinthetransformersconnectedtothedeepwell(cavity)bushings;the secondarywindingsofthetransformersarebroughtoutthroughL.V.bushingsandspade terminals.Otheraccessoriesthatarefoundindistributiontransformersare:pressurerelief devices,fillerplugs,drainplugsand/orsamplingvalves,parkingstandsforelbows,tapchangers (offload),loadbreakswitchesforradialfeedpadmountsandsectionalizingswitchforloop feed.

24)Whatarethebasicpartsofatransformer? Thegeneralarrangementofanytransformerwillhavethefollowingbasicparts:anironcore consistingoflaminatedsheets,theprimaryandthesecondarywindings.Thereasonofhaving thecoreslaminatedwithinsulationbetweenthelaminationistoreducetheeddycurrents inducedbythealternatingmagneticflux.Theverticalpartsofthecoreareusuallytermedthe limbsandthehorizontalaretheyokes.Thetwodesignsforthecorearethecoretypewherethe ironcoreformingthelimbsaresurroundedbythewindingsandtheshelltypewherethe windingsmorecompletelysurroundedbytheiron(fig.1.5).Thematerialofthecoreiseither thegrainorientedsiliconsteelortheamorphousalloys.Thesiliconsteel(iron)containssilicon inthe31/2%level.Thethicknessofthelaminatesisintherangeof.014inches(29guage).For highefficiencytransformersormotorsthesteelusedwouldhavesiliconinthe45%range.The steelusedintheseapparatusisdesignatedforexampleasM2(.007"),M3(.009")orM6 (.016").

25)Asusedwithtransformers,whataretheimportantcurvesthatdefinetheelectricsteel properties? Thereareafewcurvesthatdefinetheimportantpropertiesoftheelectricsteelasusedin transformers,theyare:

BHloop:themagneticinduction(inweber/m2,forexample)vs.themagneticfieldstrength, alsotermedmagnetizingforce(inampereturns/cmorperm)itisknownasthehysteresisloop. thedcmagnetizationcurve:whichisthemagneticinduction(B)vs.themagnetizingforce(H). Coreloss:themagneticinductions(eg.inweber/sq.m)vs.thecorelossinwatts/LB. theVAlosscurve:theexcitingvoltampererms/LBvs.excitinglossesinVA/LB(Pa). theanglefromrollingdirection:itistheanglefromtherollingdirectionindeg.vs.Pa.

26) Whichpropertiesoftheelectricsteelarestructuresensitive&whicharenot? Thepropertiesofthemagneticmaterialsdependon:thechemicalcomposition,fabrication processandheattreatment.Saturation(magnetization)changesslowlybyvariationinchemical compositionbutisunaffectedbyfabrication(includingimpurities)orheattreatment. Permeability()whichequalsB/H,coerceiveforce(itisthedcmagnetizingforceatwhichthe magneticinductioniszerowhenthemayerialisinasymmetricallycyclicallymagnetized condition)andhysteresislossarestructuresensitivei.e.affectedbycomposition,impurities, strain,temperature,crystalstructureandorientation.Atconstantmagneticfield,thecoreloss increaseswithincreasedsheetthickness.Itistheeddycurrentcomponentinthecorelossesthat increaseswiththeincreaseinthethickness. 27)Whatareamorphousmetals?

Amorphousmetalsarealloyswithnoncrystallineatomicstructure.Theatomsarearranged randomlyinrelationtoeachother.Itiseasiertomagnetizethistypeofalloysthancrystalline ones.Ifthistypeofmaterialisusedtobuilddistributiontransformers,lowercorelosseswill produced.Thecoolingrateoftheliquidalloytoobtaintheamorphousmetalstructureisinthe orderof1milliondegreespersecond.Thereareafewmethodsofquenchingtoproducethis material.Theprocessthatisusedinpracticeistheplanarflowcasting.Thistechnique,ina simplifiedmannerproducesthesolidifiedmetalthroughthefollowingsteps: liquidalloyismeltedanddeliveredtoaholdingreservoir. thealloyisdeliveredthroughatapinthebottomofthereservoirtothecastingnozzleandthen thequenchingbelt. thequenchingbelthasthecoolingboxandleadsthequenchedribbontothemeasuringstand andwindingmachinery. 28)Whatarethedifferenttypesofcoresusedindistributiontransformers? Thecoreconstructioncanbeanyofthefollowing,functionoftheratingandthedesign:wound, butt,ormittered(fig.1.5a).Thewound(spiral)coremayhavethesteelsheetscuttopre determinedlengths(commonlyused)orsheetswithnocuts,thisisacommondesignwithoil filleddistributiontransformers.Thebutt(andlap)designwillhavetwodifferentsizesofsheets, thefirstmakeupthelegsandtheothertomaketheyokes.Inthisdesign,thegapsbetweenthe differentsteelparts(inthefluxpath)maybethereasonforthenoiseandtheincreaseinthe requiredampereturnstoachievethedesired(rated)fluxdensity.Themitteredcorewillhavethe sheetsforthelegsandyokescutat45inordertohavethefluxpathalwaysinthedirectionthe steelwasrolled(grainoriented).Hotrolledorcoldrolledsteelsareusedintransformerswith regulargrainorhighpermeabilitygrainorientedproperties.

29)Whatarethedifferentarrangementsofcoilsusedindistributiontransformersand howdoesthecrosssectionareaofthewireslooklike? Themajorwindingtypesaretheconcentric(thel.v.isclosertothecoreandtheh.v.iswound ontopofthel.v.)andthesandwiched(wherethesecondarywindingsandwichisontopand bottomoftheprimaryone),fig.1.6.Thewiresusedinformingthecoilsareinsulatedcopperor aluminum.Thecoilsareoftheprewound(formed)constructionandcanbeofthecylindricalor disctype.Cylindricalcoilsarewoundinhelicallayers,withlayersinsulatedfromeachother. Insulatingcylindersareplacedbetweenthecoreandthefirstcoil.Theyare,also,placedbetween thecylindricalwindings.Disccoilsmaybeoneormultiturnsperlayer.Multilayershavean insulatingmaterialbetweenthem.Acompletewindingconsistsofstackeddiscsofcoilswith intercoilinsulation.Thewindingconfigurationwillhaveaneffectonthetransientresponseof thetransformer.Indrytypetransformers,therearethreemajortypesofwindings/windings insulationcombination.Theyare:opencoil,castcoilandcoatedcoil.Thewirescanhaveanyof thefollowingforms:circular,rectangular(strip)oroval.

30)Howcantheexcitingcurrentsoftransformersberesolved(i.e.whatarethe componentsoftheexcitingcurrents)? Areminder,theexcitingcurrentcanbebrokendownintoafundamentalandafamilyofodd harmonics(usingFourieranalysis).Thefundamentalcomponentcanfurtherberesolvedinto twocomponents,oneinphasewiththecountere.m.f.andanotheroutofphaseby90.Thecore lossabsorbedbythehystersisandeddycurrentlossesinthecore,accountforthatcomponentin phasewiththee.m.f.Themagnetizingcurrentequalstheexcitingcurrentminusthecoreloss componentwhichmeansitisequaltoalltheharmonicsplustheoutofphasefundamental component.Atconstantmagneticinductionandmagneticstrengthfield,thetotallosses decreasewiththeincreaseinthepermeability.Forthesamepermeability,thelossesincrease withtheincreaseinthesheetthickness.Theincreaseofthegrainsizenumberreducesthe losses,aswellastheincreaseinpercentsilicon(increaseintheresistivity)reducesthelosses. Theincreaseinthetensilestrengthwillreducethelosses. 31)Whatarethemethodsusedtoinsulatethecoilsofatransformer? Theopencoilusesamethodwherebythetransformercoilsafterbeingwoundareimmersedin aninsulatingvarnishlikesilicone.Thevarnishfillstheairvoidsandcoatsthecoilsurfacewith about2mils(.002")protectivecoating.Thedifferenttypesofwoundcoilsusedinthismethod canbeanyofthefollowing:barrel(cylindrical)ordiscorsectional.Thecastcoilisusedwith thebarrelorsectionalwindings.Thecoilsareplacedinacastingmould.Themouldandcoils arethenplacedinavacuumchamberandevacuated.Anepoxyresin(oflowviscosity)isput (injected)insidethemould(undervacuum).Themouldwithitscontentsisthenplacedinan oventosolidify(ovenbake)theresin.Theresultinginsulationcoatingis250mils, approximately.Thecoatedcoilisusedwithsimilarwindingsasusedwiththecastcoil insulatingtype.Afterthewindingswereprepared,thecoilsareplacedinavacuumchamberand evacuated,thenfloodedundervacuumwithalowviscosityepoxyresin.Thecoilsaredrained andbaked(tosettheresin).Thisprocessisrepeated,butthistimewithahighviscosityresin. thecoatingoverthewindingswillbeabout100mils. 32)Whenatransformerisenergized,whatarethelossesencounteredifitisnotloaded& ifitisloaded? Lossesindistributiontransformerscanbeclassifiedintoloadandnoloadlosses.Theload

lossesequalI2Randascanbeseenvarieswiththesquareoftheloadcurrent.Itis,also,referred toaswindinglosses.Thenoloadlossesaretheresultoftheelectriccurrentsandmagnetic fieldsnecessarytomagnetizethetransformerscore.Thenoloadlossesarepresentaslongas thetransformeriskeptenergized(itisindependentofthetransfomerloading).Auxilliarylosses (likefansenergyconsumptionthatischargedtothedemand)arenotavailablewiththe distributiontransformersbutarepresentinthesubstationtransformersabove5MVAratings. Thetotalowningcostofatransformerconstituteoftheinitialcost(purchasingprice),costof noloadlosses(overtheexpectedlifeofthetransformer)andloadlosses(overtheexpectedlife ofthetransformer)andmaybethemaintenancecost(overtheexpectedlifeofthetransfomer). Thelossescostandmaintenance(ifincluded)arepresentedasfirstcost(presentvalue)inorder tomaketheevaluationofthetotalowningcostofthedifferentavailabletransformersforeach sizeorrating(oftransformer)possible. 33)Whatarethecausesofthegeneratedgasesinanoilfilledtransformer? Thepropertiesofthenewoiltobeusedintransformers,thetestsperformedontheoil,the acceptablevaluesfromthedifferenttests,theinterpretationoftheusedoiltestresultsandthe instrumentsusedtodetectgasesinoilarecoveredfullyinASTMandIEEErelatedstandards. Gasesduetocompositionofoilandsolidinsulationresultfromconductortemperature(dueto loadlosses)andexposuretoarctemperature.Gasesunderlowenergydischargesandpartial discharge(corona)conditionsareformed,mainlybyionicbombardment.Theproductsthat resultfromthermaldecompositionofoilimpregnatedcellulosematerialarecarbonoxides(CO &CO2)andhydrogen(H2)ormethane(CH4). 34)Whatfactorsaffectthegenerationofgasinoilimmersedtransformers? Thevolumeofthegeneratedgasanditsratedependonthetemperatureandthevolumeofthe heatedmaterial.Thebreakingofcarbonhydrogenandcarboncarbonbondsresultfromthe thermalorelectricalfaultsunderoil.Thearcunderoilwillhaveahighpressuregasbubblewith thefollowingfluids,fromtheoutsidetowardthearc,inwardly(ofthebubble):oil,wetoil vapour,superheatedoilvapour,hydrocarbons(C2H2acetylene)andhydrogen.Thearcrunsin amixtureofhydrogenions,metalvapour,electrons.Thus,tousethepresenceofthegasinthe transformeroilasanindicationofthepresenceofafault,threedistinctivetypesoffaultshaveto bedefined.Thethreetypesare:thermal,electrical(lowintensitydischarges)andhighintensity electricalarcs.Thegasesthatmaybefoundintransformeroileitherundernormalorfaulty conditionsare:methane(CH4),ethylene(C2H4),ethane(C2H6),acetylene(C2H2),hydrogen (H2),carbonmonoxide(CO)andcarbondioxide(CO2).Thethermalfaultsthatwillcausethe oiltemperaturetorisetoupto500deg.CwillproduceH2,CH4andtracequantitiesofC2H4& C2H6.TemperaturesinthemiddlezonewillgeneratesignificantquantitiesofC2H6&C2H4. Attheupperendofthethermalfaultsandhighintensityarcing(electrical)faults,the temperaturerangewillbe700to1800deg.Candthegasesyieldedare:H2,C2H4andtracesto significantamountsofC2H2.Thermaldecompositionofcelluloseandothersolidinsulating materialswillproduceCO,CO2andwatervapour.TheratioofCO2/COortheppm(quantity) maybeusedasanindicationoftheinsulationdisentigration.Forelectricallowintensity

dischargesunderoil,H2,CH4andtracesofC2H2willbeproduced. 35)Whatarethemethodsoflayingundergroundcables?Whataretheessentialpartsofa cable? Undegroundcablescanbedirectlyburried,canbeput(pulled)inaconduitandburried(cable inaconduit)orcablesinconduitsencasedinconcrete,fig.1.7. Theyconsistofthreeessentialparts:theconductorfortransmittingelectricalpower,the insulationmediumrequiredtoinsulatetheconductorfromdirectcontactwithearthorother objectsandtheexternalprotectioncovertoprotectagainstmechanicaldamage,chemicalor electrochemicalattack,fig.1.7.Copperandaluminumconductorsarefoundinunderground distributioncables.Theconductorcanbesolidorstranded.Themostcommonlyusedinsulating materialsinthemediumvoltage(primary)rangearethecrosslinkedpolyethyleneandthe ethylenepropylenerubberwhichareratedforcontinuousoperationof90degC.Theconcentric neutralorshieldedtape(themetallicinsulationshield)isappliedontheinsulation semiconductingshield.Theconcentricneutraliswoundhelicallyandismadeofannealed uncoatedcopperwires,usually.Undertheconcentricneutral,anequalizingtape(annealed untinnedcoppertapes),eachisappliedinoppositedirectiontotheother.Thereareafewways andmaterialsthatareusedascablejackets.Briefly,theyaresleevedorencapsulated,the materialisPVCorlinearlowdensitypolyethylene(LLDPE).Certaincablesareusedwithout anyjackets.ThePVCcoveringofcablescomesasasleevedjacketandaseparatorbetween concentricneutralandthejacket.TheLLDPEcoveringcomeseitherasencapsulatedjacketor sleeved.WhenLLDPEsleevesareused,awaterblockingagentisusedtopreventthe longitudinaltravelofwaterinthespacebetwenthejacketandtheinsulationshield.Thesame agentcanbeusedwithencapsulatedjacketstofillthevoidsbetweenthejacketandtheneutral. Ingeneralforthesamesizecable,thesleevedcablesaremoreflexibleandiseasiertohandle thantheencapsulatedones.

36)Whatarerubbers?Whatarethegeneralpropertiesofanycableinsulatingmatrial? Rubbermaterialscanbeclassifiedintovulcanizedrubber&syntheticrubbermaterials (elatomers). Vulcanizedrubber:rubberinitsnaturalformitisconsideredaninsulatingmaterial.Thedraw backisitspropertyofabsorbingmoisture.Theresultofthisdrawbackwouldbethelossofits insulatingproperty.Hardorvulcanizedrubberisproducedbymixingrubberwith30%sulphur, othersoftenersandantioxidationorothercompoundingagents.Theendresultisaninsulating materialwhichisrigid,resilientanddoesnotabsorbmoisture. Whenitcomestosyntheticrubbermaterials(elastomers)knownasrubbers,theycanbe classifiedinto:generalpurposesyntheticswhichhaverubberlikepropertiesandspecialpurpose syntheticswhichhavebetterpropertiesthanrubberwithrespecttofireandoilresisting properties.Thefourmaintypesare:butylrubber,siliconerubber,neopreneandstyrene. Rubbersarehydrocarbonpolymericmaterialssimilarinstructuretoplasticresins.Anelastomer isdefined,perASTM,asapolymericmaterialwhichatroomtemperaturecanbestretchedtoat leasttwiceitsoriginallengthanduponimmediatereleaseofthestressitwillreturnquicklyto approximatelyitsoriginallength.Certaintypesofplasticscanapproachtherubberlikestate (polyethylenes).Othershaveelastomergrades,forexampleolefins,styrenes,fluorplasticsand

silicones. Butylrubber:Alsoreferredtoasisobutyleneisopreneelastomeriscopolymersofisobutylene andabout1to3%isoprene.Itissimilarinmanywaystonaturalrubber.Ithasexcellent resistance,butitresistsweathering,thesunlightandchemicals.Thistypeofinsulation,in general,haslowermechanicalproperties(tensilestrength,resilience,abrasionresistanceand compressionset)thanotherelastomers.Ithasexcellentdielectricstrength,thusitcanbeused forcableinsulation,encapsulatingcompoundsandavarietyofelectricalapplications. Siliconerubber:isonememberofthefamilyofsiliconeelastomers.Theelastomersare polymerscomposedbasicallyofsiliconandoxygenatoms.Theycanbeclassifiedintogeneral purpose,lowtemperature,hightemperature,lowcompressionset,hightensilehightear,fluid resistant.Theyarethemoststableofallelastomers,theyhavegoodresistancetohighandlow temperatures,oilsandchemicals.Thesiliconerubberisusuallyalongchaindimethylsilicone whichcanbevulcanizedbycrosslinkingthelinearchainsandcanflowunderheatand pressure.Basically,itconsistsofalternatesiliconandoxygenatomswithtwomethylgroups attachedtoeachsiliconatom.Itresistsheat,mostchemicals(exceptstrongacidsandalkalies). Thedielectricstrengthis500volt/mil(20KVpermeter). Neoprene:alsoknownaschloroprene,itisthefirstcommercialsyntheticrubber.Itis chemically,structurallyandmechanicallysimilartonaturalrubber.Itresistsoils,chemicals, sunlight,weatheringandaging.Itisconsumedbyfirebutitisnoncombustible.Itisrelatively lowindielectricstrength. Styrenebutadieneelastomers:sometimescalled,BunaSarecopolymersofbutadieneand styrene.Thegradeswithstyreneover50%areconsideredplastics.Awiderangeofproperty gradesexistsbyvaryingtherelativeamountsofstyreneandbutadiene.Styrenecontentvaries fromaslowas9%toupto40%.Theyaresimilarinmanywaystothenaturalrubbers. Theinsulatingmaterialsusedwithcableshavethefollowingproperties:highinsulation resistance,highdielectricstrength,goodmechanicalproperties,itshouldresistchemicals surroundingitanditshouldbenonhygroscopic,i.e.,moistureandwaterresistant. 37)WhatisPVC? PolyvinylChloride(PVC):itisapolymerderived,generally,fromacetylene.Itcanbeproduced indifferentgradesdependinguponthepolymerizationprocess.PVCisinferiortovulcanized rubberwithrespecttoelasticityandinsulationresistance.PVCwhenusedwithcableshastobe processedwithplasticizer.PVCcanbeclassifiedintogeneralpurpose,hardgradePVC(hasless amountofplacticizer)andheatresistingPVC. 38)Whatarepolyethylenes? Thesethermoplasticresinsincludelowdensitypolyethylene(LDPE),linearlowdensity polyethylene(LLDPE),highdensitypolyethylenes(HDPE)andethylenecopolymers.The advantagestobegainedwithpolyethylenearelightweight,outstandingchemicalresistance, mechanicalresistanceandexcellentdielectricproperties.Thebasicpropertiesofpolyethylenes canbemodifiedwithabroadrangeoffillers,reinforcementsandchemicalmodifiers. Polyethylenesareconsideredeasytoprocess:injectionmolding,sheetfilmextrusion,coating

extrusion,wireandcableextrusioncoating,blowmolding,rotationalmolding,pipeandtube extrusionandothers.Thebasicbuildingblocksofpolyethyleneare:hydrogenandcarbonatoms. Theseatomsarecombinedtoformethylenemonomer,C2H4i.e.twocarbonatomsandfour hydrogenones.Inthepolymerizationprocess,thedoublebondconnectingthecarbonatomsis brokenandthesebondsreformwithotherethylenemoleculestoformlongmolecularchains. Highdensitypolyethyleneresinshavemolecularchainswithcomparativelyfewsidechain branches.Itscrystallinityisupto95%.Lowdensitypolyethyleneresinhascrystallinityfrom60 to70%.Linearlowdensitypolyethyleneresinshasbetween60and75%.Thedegreeof cristallinityisameasureofthedensityoftheresin.Withthehigherdensities,theheatsoftening point,resistancetogasandmoisturevapourpermeatingandstiffnessarehigh.Ontheother hand,increaseddensitywillresultinreductionofstresscrackingresistanceandlowtemperature toughness.TherangeofdensityforLLDPEresinsis0.915to0.940g/cm3,forLDPEresins 0.910to0.930g/cm3andHDPE0.941to0.965g/cm3. 39)Whataretheelectriccharacteristicsofcables? Theelectricalcharacteristicsofcables: theresistanceisgivenbyRac=Rdc(Ys+Yp);whereRacistheacresistance,Rdcisthedc resistance,YsisthecorrectionforskineffectandYpcorrectionforproximityeffect.The inductanceisgivenby.460log(GMD/GMR)=0.2ln(GMD/GMR)mH/Km,wheregmdisthe distancebetweenthecabecentrecoreandthepointwheretheinductanceistobecalculatedat &GMRistheeffectiveradiusoftheconductor=r(.7788). Theinsulationresistanceforasinglecorecableisgivenbythefollowing(r/2pil)(lnD/2r) whereristheresistivityorspecificresistanceofthedielectric,ristheradiusoftheconductor,l isthelengthofthecableandDisthediameterofthesheathorconentricneutral. Thecapacitanceincm/cmlengthisgivenby:e/(2lnD/d),wheredistheconductordiameter. Thisequationcanbewrittenas0.03888e/logD/dFarad/milelengthore(109/18lnD/d) Farad/meter,whereeisthedielectricconstantofthecableinsulationmaterial.Thestressata distancexfromtheaxisisgivenbyE/(xlnD/d),thestressismaximumattheconductorandis equalE/(rlnD/d)or2E/(dlnD/d),thestressattheleadsheathis2E/(DlnD/d);whereEisthe peakvoltageoftheconductor(potentialdifferencebetweenthecoreandthesheath).Itcanbe seenfromtheabovethattheratioofthestressattheconductortothatatthesheathisD/d. Therearetwomainmethodsbywhichamoreuniformdistributionofstressmaybeachieved:by theintroductionofintersheathsandwithlayersofinsulatingmaterialwithdifferentdielectric constant(e).Thesemethodsareprimarilyusedinhighvoltagecables. Foranumericalexampleregardingthecalculationofcableconstants:resistance,inductance (inductivereactance),capacitance(capacitivereactance)&insulationresistance,pleasereferto level1/lesson2/question25. 40)Whatarethefailuremodesincables? Themostcommonwaysoffailureincablesare:coring(ortracking)andthermalinstability.The firsthastheprogressivecoringstartingattheconductororthesheathandultimatelybridgesthe electrodes(conductor&sheath).Thesecondoccurswhenthepowerfactorincreasessorapidly

withtheriseoftemperatureinsuchamannerthatasmallriseintemperatureincreasesthe dielectriclossesbyagreateramount.Thevoltagetobreakdownacertaininsulationdepends uponmanyfactorssuchasdurationofapplication,shapeofelectrodes,temperature,pressure, thepresenceofmoistureandgaseousspaces. Forcrosslinkingofpolyethylenethereareafewmethodsinustoday:peroxidesystems, radiationandsilanebridgesformation.Thecuringoftheextrudedcablestakesplaceinairat ambienttemperatures,inahotwaterbathorinasteamroom.Thefailureincableswiththis insulatingmaterialcanbeattributedtothewaterabsorptionpropertyofXLPE.Whenthecables aresubjectedtostress(i.e.undervoltage)andwaterisontheoutsideorintheconductor, transparenttreelikeimperfectionsareformed.Thesewatertreesareinitiatedinvoidsor contaminantsinthebodyoftheinsulation.Someofthefactorsthatcontributetowatertree growthinextrudedinsulationarevoltagestress,water,contaminantsandimperfections, temperaturegradientandaging.Thestrandseal(theliquidfillingthespacesbetweenthestrands intheconductorarea)characteristicsare:highviscosityatoverloadtemperaturetoensurethat thestrandsealwillnotflowfromtheconductor,goodlowtemperatureproperties(i.e.compound fractureunderlowtemperaturewhilebendingthecableshouldnothappen),compatiblewith metalsoftheconductors&conductorssemiconductingshieldsandadheretoconductorovera broadrangeoftemperatures. 41)Whataretherequirementsthatamanholemustfulfill? Whentheconcreteencaseddesignsareused,manholeshavetobeconstructedandshouldhave sufficientspacetocut,spliceandpullthecables.Theyshouldbestrongenoughtowithstandthe loadsabovethemwithoutcollapsing.Ingeneral,manholes,handholesandvaultsaretobe designedtosustainallexpectedloadswhichmaybeimposedonthestructure.Thevertical and/orhorizontaldesignloadsshallconsistofdeadload,liveloads,equipmentload,impact, loadduetowatertableorfrostandanyotherloadsexpectedtobeimposedonand/oradjacentto thestructure.Themanholesaregenerallybuiltofreinforcedconcreteorbrickandthecoversare madeofsteel.Theopeningleadingfromthestreettothemanholechamberiscalledthe chimneyorthroat.Anopeninghavingaminimumdiameterof32"isusuallyprovided.This openinghastobelargeenoughforamantoenteronaladderandalsotopasstheequipment neededforsplicingandtesting.Thepullingropeisattachedtothecablebymeansofawoven cablegrip,sometimescalledbasketgrip,orbymeansofaclevisoreye.Topreventinjurytothe cablebyscrapingonthemanholeframeorattheductopening,afeedingtube(guidingtube)is sometimesused.Toprotectthecablefromexcessivetensionduringpullingin,thecableis lubricatedwithacompatiblematerialtothejacket.Thecableisdrawnintotheductbymeansof awinchorcapstanThewinchisusuallymountedonatruckoraportablecablepullerlocated nearthemanholeortheriser(pole)conduit,atpullingend.Forcommonmanhole configurations,refertofig.1.8.

42)Whatarethedissipationfactor&powerfactorofacableinsulation? Thedissipationfactorortandisameasurementofthequalityoftheinsulation.Thelowerthe factorvalue(eg..001to.02),thebettertheinsulation.Startingat0.08&higherisanindication oftheinsulationdegradation.Notethatcosf=tand=wcr;wherecisthecapacitanceofthe insulationandristheresistanceoftheinsulation&w=2pif,fisthefrequencyoftheapplied voltage. Powerfactorofsinglecorecable:supposethatthedielectrichasaresistanceRwhichis independentofthestressandmaybeconsideredasconstantthroughoutthecableuponthe applicationofanalternatingvoltageoffrequencyf,therewillbeaninphasecurrentequalthe voltagedividedbytheresistanceoftheinsulationpercmlength.Theresistance=Rdx/(2pix 1)ohm/cm.Thelosseswithalternatingcurrentsarecausedbyabsorptionphenomenaandis usuallymuchlessthanthosecausedbyd.c.Thechargingcurrent=wCV;whereCisgivenby e/2lnD/dcm/cmandleadsthevoltageby90degrees.ThetotalcurrentIisthevectorsumof V/RandwCVandleadsthevoltagebyananglefi=1/wCR.Theconductance(reciprocalofR) ofthecablepercmlengthisG=cosfi(wC).Thismeasurementindicatesthequalityofthe insulation.Iftheanglefiis90(orcloseto)i.e.cosfiiszeroorcloseto(isequaltotand;where d=90fi)thecableisconsideredingoodcondition.ThedielectriclossisV2/R=V2G=wCV2 fi.Thepowerfactorofthedielectricmaterialsvarywithstressandtemperature.Itincreases withtheincreaseofanyofthesetwovariablesi.e.stressortemperature.

43)Howwouldtheinductance,capacitance&dissipationfactorofacablebemeasured? Bridgesareusedforthemeasurementofinductance,capacitanceandloss(dissipation)factor, fig.1.9.Thebridgesthatareusedinpracticeare:Maxwell'sinductance,Wien's&Schering's capacitancebridges(givealsotand)andcombinedMaxwell/WienbridgeforL,Candtand.

44)Whatarethedifferentmethodstolocatefaultsinundergroundcables? Themainmethodsthatmaybeusedare:Murraylooptest,fallofpotentialtest,dcchargeand dischargetest,inductiontest,impulsewaveechotestandarcreflection. TheMurraylooptestcanpreciselylocatethefaultifitscurrentismorethan10mAi.e.fora batterywith100V,thefaultresistancecanbeashighas10Kohm.Thesensitivityisfunctionof thedetectorused.Initssimplestform,thefaultycableisloopedtoanadjacentsoundconductor ofthesamecrosssectionalarea.Acrosstheopenends,agalvanometerisjoinedandparallel

withitaresistanceboxwithtwosetsofcoils.Thed.c.supplyisconnectedtothisarrangement. Whenthegalvanometerpointerisbalanced(becauseofadjustmentstotheresistancebox),the faultpositionisfoundby:distancetofault=(a/a+b).looplength;wherea:isthelengthofthe bridgearmjoinedtothefaultycore,b:isthelengthofthebridgearmjoinedtothesoundcore andthelooplengthisequaltotwicetheroutelength. Thefallofpotentialtest:isachievedbymeasuringthevoltageatbothends.Bycomparingthe measurements,thefaultlocationcanbeestimated.Theequipmentusedareabattery,rheostat, ammeterandlowrangemovingcoilammeter. Chargeanddischargetest:thismethodisvalidonlywhenlocatingabrokencorefault. Measuringtherelativecapacityfromeachendofthebrokencore,andusingtheformulad=(C1 /C1+C2).l;wheredisthedistanceofthefaultfromthemeasuredC1capacitance,listhe lengthofthecable,C1&C2aretheobservedcapacitancesfrombothendofthebrokencore. Allothercores,otherthantheonetestedaregroundedtoavoidfalsereadings. Theinductionmethod(fig.1.9a)orthethumpingmethod:thecableissuppliedwith intermittentpulsesofcurrentderivedfromimpulsegenerator,thecablerouteisthenexplored withasearchcoilconnectedtotelephonereceiver(acousticdetectiondevices).Thecoilisheld closetothegroundwithitsplaneparalleltotherunofthecable.Whenthefaultispassed,the cablewillcarrynocurrentandnothingwillbeheardontheearphone. Theimpulsewaveecho(Cableradar):thismethodisbasedontheprinciplethatapulse propagatingalongacablewillbereflectedwhenitmeetsanimpedancemismatch.Foracable ofuniformdielectric,thepulsereflectedatthemismatchisdisplayedonaCRTatatimedelay directlyproportiontothedistanceofthemismatchfromthetestend(irrespectiveofthe conductorsize)andisgivenbyX=(t1/t2).cableroutelength;wheret1isthepulsetimetofault (ormismatch)andt2isthepulsetimetofarendofcable.Thismethodcanbegeneralizedand throughtheuseofgeneratorsthatgivepulsesofshortdurationandlowvoltagesbeing transmittedthroughthecable,anydeviations(cablestart,splices,faultsandcableend)will producereflections.ThesereflectionswillbedisplayedontheCRTofthemeasuringdevice. Thelimitationofthismethodiswhenthefaultresistanceishigh(higherthantentimesthe cablecharacteristicsimpedancewhichvariesintherangeof30to50ohm).Withhigh impedancefaults,urningthefaultintoalowerresistancefaultandcableradarmethodisusedto obtainthefaultlocation. ArcReflection:thismethodisconsideredasacombinationofcableradarandsurgepulse methods.Thelatterbeingageneratorofhighvoltagepulsesthataresentthroughoutthecable andproducesarcingatthefaultlocation,partofthepulseenergyisreflectedtothecablestart whereitispartiallyreflectedintothefault.Thisprocesscontinuesuntilthecapacitivelystored energyisexhausted.TheobservationonaCRTofthespacingofthereflectionswillindicatethe faultlocation.

45)Whatistheneedforacabletermination? Theuseofcableterminations(fig.1.10)isdictatedbecauseofthefollowing: duetotheabruptchangeinthecontinuityofthecableshieldtheelectricalstressesincreases (theterminationwillreducethisstress). theneedtoincreasethecreepagedistancebetweentheliveconductorandtheground(or neutralorshield). theneedtopreventmoistureingressintothecable. overheadlinevibrationshavetobepreventedfrombeingtransmittedtotheundergroundcable. Thedifferenttypesofterminationsare:thefullytaped,moldedstressconeandtape,onepiece molded,porcelainterminators,potheadsandheatshrinkables.

46)Whywouldcablesplicesbeused? Cablesplices(fig.1.11)areusedforthefollowingreasons: continuationofallcablecomponentsistobemaintained. toprovideprotectionagainstentranceofwaterandothercontaminantsintothecable. toprovidemechanicalsupporttothecable. 47)Tomakeasplicethatwouldlastlong,whataretheconditionsthathavetobefulfilled whiledoingthesplice? Whenmakingasplice,thefollowingconditionshavetobefulfilled: voidsshouldnotbeintroduced. theinlineconnectorhastobeoftherightsize. therighttoolandcompressionforcehastobeusedtocrimptheconnectortotheconductorof thecable. theappliedinsulationthicknessshouldnotexceed1.5timesthecableinsulation,toavoid overheatingofsplice. Tappedsplices,heatshrinkablesandcoldshrinkablesarecommonlyused.

48)Whatarefaultedcircuitindicators?Wheretheyaremounted? Faultedcircuitindicatorswhichcanbeinstalledinpodmounttransformersorswitchgearcanbe classifiedaccordingly:manuallyresettable,highvoltage,currentresettableandtimedresettable. Thebasicideaofoperationisthatifafaulttooccurdownstreamthisdevicealltheindicators aheadofthefaultwilloperateorset(asthefaultcurrentisflowingthroughthem)andallthe onesdownstreamthefaultwillnotoperate.Forthemanualresettypes,theinterventionofthe operatorisrequiredtoresetthedevice.Forthehighvoltagetype,whenthesupplyisrestored i.e.thevoltageisavailableagainonthesection(circuit),thedeviceresets.Forthecurrenttype, afterrepairshavebeendone,thecurrentflowinginthecircuitreachesaminimumpreadjusted value(eg.2or3amp.)willcausethedevicetoreset.Forthetimedone,afterfourhours,let's say,itwillresetautomatically.Thesefourhoursareusuallyfactoryadjustable.Whenan indicatoroperates,itwillshowthesectionoftheburiedcablethatmaybefaulty.Certain designscomewithattachmentstoalleviatepotentialnuissanceresettingorsettinglikeinrush currentswhenenergizationoftransformersorduetoreclosingactionsofreclosuresorstation circuitbreakers. 49)Whatisroutetracing? Selfcontainedinstrumentsareavailablefortracingtheroutesandthedepthofhiddenorburied cables.Thelocationofundergroundcablesisbasedontheprincipleoftheconcentric electromagneticfieldsurroundingacurrentcarryingconductor.Toidentifyandlocateacable,a predeterminedfrequencycurrentfromageneratoristransmittedalongthecable.Theresulting magneticfieldisthenexploredbymeansofaninductiveprobeordetectorrodwiththeintegral searchcoilandreceiver.Theyareequippedwithtogiveaudioandvisualsignals.Ifthe searchingdevicescandetectpowerfrequency,thehighfrequencygenerator. 50)Drawasketchforanelbow&anelbowinsertshowingthemaincomponentsofsucha device?

51)Whatarethedifferenttypesofconductorsusedinoverheaddistributionsystems? Thewiresandcablesoverwhichelectricenergyistransmittedaremadeofcopper,aluminum, steeloracombinationofCuandsteelorALandsteel.Foroverheadlines,harddrawncopper canbeused.Itispreferredoversoftdrawnorannealedasthetreatmentofthelasttworeduces thetensilestrengthofthewirefromapproximately55,000to35,000lb/sq.in.Thisis,also,the reasonforeliminatingsolderingofharddrawncopper,asthiscausesthereductioninthe strengthoftheharddrawnwire.Jointsaremadeofsplicingsleeves.Annealedorsoftdrawn copperisusedforgroundsorspecialapplicationswherebendingandshapingtheconductoris necessary.Aluminumiswidelyusedfordistributionandtransmissionlineconductors.WhenAl conductorisstranded,thecentralstrandisoftenmadeofsteeltoreinforcethecable.Reinforced AlcablecalledACSR(aluminumconductorsteelreinforced)issuitableforlongspans. Copperweldconductorisacoatingofcoppersecurelyweldedtotheoutsideofthesteelwire. Thelayerofcopperincreasestheconductivityandgiveaprotectivecoatingtothesteelwire. Theconductivityofcopperweldconductorscanberaisedifthethicknessofcopperincreases. Theapplicationsforcopperweldare:rurallines,guywiresando/hgroundwires.Alumweld conductorsareconstructedfromsteelwirethatiscoveredwithaluminumtopreventthesteel

fromrustingaswellastoimproveitsconductivity. 52)Comparetheconductivity&strengthofaluminumconductorstothoseofcopper? Whenthesamesizesofcopperandaluminumconductorsarecompared,Alwillhave60%of thecopperconductorconductivity,45%ofcoppertensilestrengthand33%ofcopperweight. FortheAlconductortocarrythesamecurrentasthatofaCuconductoritscrosssectionalarea mustbe66%higherthanthatofcopperandinthiscaseitstensilestrengthwillbe75%andits weightwillbe55%ofthatofthecopperconductor. 53)Whatdoescoldflowmeans? Aluminumexpands36%morethancoper.IfAlconductorisinstalledinacopperconnector,the Al(whenheatedduetotheflowofthecurrent)tendstoflowoutoftheconnector.Whenthe connectorcoolsdown,theAlwillcontractwithadiameterinsidethecopperconnectorslightly smallerthanoriginallywas.Moreextrusionwilloccurduringsubsequentloadingcycles causingthecontactresistancetoincrease.Thecontactresistance&consequentlytheheat

generatedattheconnector(I2R)keepsincreasinguntilfailureoccurs.Materialsofconductor andconnectorshavetobecompatiblesoloadcycling(coldflow)wouldnotproducehotspots orfailures.Tominimizecoldflowanyorallofthefollowingshouldbeobserved:use compressiontypealuminumconnectors,useBellevillewashers,thealuminumconnectorsmust havesubstantialmasstoruncoolandthecontactareabetweentheconductorstrandsandthe connectoristobemaximized.Whenexposedtoair,aninvisibleoxidefilmisproduced(which iscorrosiveresistant).Ithasaninsulatingpropertyandhastoberemovedwhenconnectionsare made. 54)Whatarethedifferenttypesofconnectorsusedinpowerdistributionsystems? Mechanicalconnectors:arecommonlyusedwithcopperconductors.Whenthistypeof connectorisusedwithaluminumconductors,largespringloadedpadispartoftheinstallation toavoidcoldflow. Compressionconnectors:areusedforAlandCuconductors.Thelengthoftheconnectoris functionoftheampacityandthetension.Therighttoolsanddiesmustbeusedandtheright numberofcrimpsandpressurewillensureanefficient/properelectricconnection. Compressiontoolscanbeclassifiedintomanualandhydraulic.Connectorswhencompressed overaconductoraspecificrangeof%compactionmustbeattainedintherangeof5to15%of thhrconductorarea.Excessivecompactionwillresultinconductordeformationandlight compactionmaynotprovidesufficientpulloutstrength. Wedgeconnectors:aresuitableforwiderangeofmainandtapwiresizes.Aluminumshould physicallybeplacedabovecopperwhenbothmaterialsareusedinoneconnection. Stirrups:areusedtoprovideaconnectionzone(area)forthehotlineclampawayfromthe mainlinesothatarcingwillnotdamagethemainconductor.Theycanbeclassifiedintowedged andbolted,thefirstbeingmorereliable. Hotlineclamps:areusedtoconnectequipmentontothemainoverheadlines.Theymake connectinganddisconnectingeasier. 55)Whatarethefactorsaffectingtheselectionoflineconductors?

Themostimportantfactorsinsizingalineconductorare:thelinevoltage,theamountofpower tobetransmittedandthemechanicalstrengthrequired.Otherfactorsthatmaybecomerelevant, dependingontheapplicationare:voltageregulation(dropthroughtheline),powerloss,span, totallengthofline.Whenconductorsareconnectedtoeachotherbyconnectors,theconnection shouldprovideanadequatecurrentpathunderallexpectedoperatingconditions.The connectionshouldwithstandallthecombinedmechanicalandelectricalstresses(vibration, tension,shear,heat).Materialsofconductorandconnectorshavetobecompatiblesoload cycling(coldflow)wouldnotproducehotspotsorfailures.ProtectionagainstweatherLesson 2:Switchgear,circuitbreakers,MCCs&contactors.conditions,likewaterstopandcorrosion protection,isprovidedwheneverpossible. 56)Howareconductorsgradedwhenitcomestoflexibility? ConcentriclaystrandedcablescanbeclassifiedaccordingtotheirflexibilityclassAA,A,B,C andDwhereAAisthemostrigid,bareandusedinoverheadsystems,CandDbeingthemost flexible. 57)Whatisthefunctionofaninsulator? Thefunctionofaninsulatoristoseparatethelineconductorfromthepoleortower. 58)Whatarethetypesofinsulators? Insulatorsarefabricatedfromporcelain,glass,fiberglass,polymerorsilicone.Insulatorscanbe classifiedintopin,postandsuspension.Theycan,also,beclassifiedaccordingtothemethodof attachingtheconductortotheinsulatori.e.clampingortieing. 59)Whatarethepropertiesofaninsulator? Thepropertiesofanyinsulatorcanbeclassifiedinto:mechanical,electrical,environmentaland maintenance.Themechanicalcharacteristicscanfurtherbeclassifiedinto:everydayloads, exceptionalloads,designloads,cyclicloads,torsionandstaticloads,safetyfactors,singleor multipleinsulatorstrings,longtermstrength.Theelectricalcriteriaarefurtherdividedinto: clearances,BIL,powerfrequencyflashoverorwithstandvoltage(dryandwet),steepfront wave,powerarcs,leakagedistance,contaminationperformance.Fortheenvironmental characteristics,thefollowingareimportant:ageingunderUVanddryarcing,typeof contamination,corona,RIV,washing,corrosionofendfittingsandtemperaturerange.Thefinal propertyofaninsulatorismaintenance,itcomesdowntoeaseordifficultyofhandlingorthe needforspecialprecautions.Ingeneraltherearethreelinesofdefenceforaninsulator: hydrophobicity,selfcleaningandtrack/fireresistance. 60)Whatareporcelaininsulatorsmadeof? Porcelaininsulatorsaremadeofclay.Specialtypesofclayareselectedandmixedmechanically untilaplasticlikecompoundisproduced.Theclayisthenplacedinmouldstoformthe insulators.Themouldsareplacedinanoventodrytheclay.Whentheinsulatorisdry,itis dippedinaglazingsolutionandfiredinakiln.Theglossysurfaceproducedfromthisprocess makesthesurfaceselfcleaning.Cementingseveralshapescanmakeavailablelargeporcelain insulators.Cementgrowth,whichmayresultfromchemicalreactionbetweenthecementand themetalparts,cancausestressesthatcancracktheinsulator.

61)Whatareglassinsulatorsmadeof? Glassinsulatorsaremadefromsand,sodaashandlime.Thematerialsaremixedandmeltedin anovenuntilaclearplasticlikeproductisproduced.Thisproductisthenputinamouldandis allowedtocool.Thefinalstepisputtingglassinsulatorintheovenforannealing. 62)Whatarefiberglassinsulatorsmadeof? Fiberglassinsulators(fiberglassrodswithflexibleskirts)aremadeupoffiberglasstreatedwith polyesterresinormorecommonlywithepoxyresins.Rubberlikecompoundsareappliedtothe rodstofabricatesuspension,deadendandposttypeinsulators.Therubberlikecompoundcan beEPDM(EthylenePropyleneDiene,Modified)polymerorsiliconeelastomer.EPDMis appliedbyinjectionaswellassilicone.EPDMandsiliconecomeinmanydifferentfomulations. Siliconeisbasedonsiloxaneresin(Polydimethysiloxane).Thebasemoleculeconsistsofachain ofalternateoxygenandsilicaatomswithorganicmethylgroupsattachedtothesiliconeatoms. 63)Whatarethepropertiesofsiliconethatisusedininsulators? Thepropertiesofsiliconetobeusedasinsulatorsinpowerdistributionsystemsare:hightear strength,hightrackinganderosionresistance,hastobehighlyhydrophobic(waterrepellency property)orwaterrepellentandhastorecoverquicklyfromanytemporaryhydrophobicityloss andhastoresistUVaging.Continuouscoronaeffect,closeproximityofsiliconeinsulatorsto largequantitiesofwatervapour,sprayingtheinsulatorwithsaltwaterandrapidbuildupof depositonthesurfaceofthesiliconewillhavenegativeimpactontheperformanceofthe insulator.Thedegreeofpermanentlossofhydrophobicityisdifferentforeachoftheconditions aforementioned.Itbecameobviousthatcertaintestsshouldbeperformedonthematerials appliedtotherodstobeabletoanticipateitsperformanceoveritslongexpectedlifetime. 64)Whatarethetestsperformedonthebulkmaterial&thoseconductedonthecomplete insulator? Thefollowingpointsareworthinvestigatingforthebulkmaterialusedasinsulatorsfor installationoutdoors:thermalendurance,mechanicalcreep,longtermdielectricbreakdown, partialdischargeandasacomplementarytesttothepartialdischargexrayradiationtestfor largercavitiesandvoids. Thetestsconductedoninsulators: Surface,whichcanbeevaluatedfortrackingandarcresistance.Surfaceerosionismainly linkedtoUVradiation,coronaandisenhancedbyhumidityandsalt,thustestingforthiseffect foroutdoorinsulatorsisextremelyimportant.Surfaceerosionshouldbedifferentiatedfrom pittingerosion(damageindepthofinsulatoroverasmallarea). Trackingwheel(Merrygoroundtest)isthethetrackandfireresistancetestofthespecimen insulator.Trackingwheeltestproceduresvarybecauseofthefollowing:spraysolution ingredients,volumeofspray,testvoltage,maximumallowablecurrent(limitedbythefuseor C.B.),rotationspeed,testspecimenandorientation. Contamination(saltandfog)testisuseddirectlyoninsulators,theimportantfactorsinsuch testare:thesaltconcentration,testvoltage,shapeandcreepagedistanceoftheinsulator. 65)Whatarethedifferenttypesoffusesusedinpowersystems?

Indistributionsystems,threephasetransformersandthreephasebanks(i.e.3singlephase connectedtoprovideadeltaoraY3phaseconfiguration)arecommon.Ingeneral,the protectionofthepowertransformersisprovidedthroughtheuseofprotectiverelays(o/cor differentialandovercurrentground)andgasrelays.Thedistributiontransformersareprotected byfuses(currentlimitingandexpulsiontypes).Mediumvoltagefuses(2.4to72kV)canbe classifiedaccordingtothefollowing,theyeitherfallunderthedistributionfusecutoutsorpower fuses.Thepowerfusescanfurtherbeclassifiedintoexpulsiontypeandcurrentlimiting. Distributionfusecutoutsweredevelopedforuseinoverheaddistributioncircuits(aconnection todistributiontransformers,supplyingresidentialareasorsmallcommercial/industrialplants). 66)Howarefusesusedtoprotectpolemountedtransformers? Thepolemountedtransformershaveaheadofthemcurrentlimitingfusesanddistribution cutoutswithfuselinkswithspeedTorKasdefinedinANSIC37.100otherspeedsarealso availabletoachievepropercoordinationbetweenthefusesandupstream/downstreamprotective devices. 67)Whatarethecomponentsofafusecutout?Howdoesafusecutoutoperate? Adistributionfusecutoutconsistsofaspecialinsulatingsupportandfuseholder.The disconnectingfuseholderengagescontactssupportedontheinsulatingsupportandisfitted withafuselink(withspeedKfastorTslowasdefinedinANSI37.100).Thetypicalrefill constructionofthedistributionfusecutoutunit:currenttransferbridge(connectsthelower fusibleendtothelowerferrule),fusibleelement,auxiliaryarcingrod,auxiliarybore(wherethe arcisdrawnandisinterruptedforlowfaultcurrents),mainarcingrod,mainbore(wherethearc isdrawnandinterruptedformoderatetohighfaultcurrentsabove100A),solidmaterialarc extinguishingmedium(boricacidforexample),outertube(ofepoxy),fusetubeplug,upper terminal.Themajorcomponentsofthefuseholderofafuserefilltypefuse:thepullring,upper andlowerferrule,glassepoxyfusetube,blownfuseindicator,windowandsilencer.Thetypical partsthatconstituteapowerfuselinkare:theexhaustferrule,thecurrenttransferbridge,the fusibleelement,thearcingrod,theborewiththesolidarcextinguishingmaterial,drivespring, actuatingpin,glassepoxytube,theuppercontact,theupperseal,arcingrodretainer.The operationofthefuseisgoverenedbytwocurves:theminimummeltingandthetotalclearing. Thefuseholderislinedwithanorganicmaterial.Infusecutouts,theinterruptionofan overcurrenttakesplaceinsidetheholder.Thegasionized(liberated),whenthelinerisexposed totheheatofthearc(asaresultofthemeltingofthelink),isthendeionized(atcurrentzero). 68)Howdoesapowerfuseextinguishthearc? Powerfuseshavecharacteristicsthatdifferentiatethemfromdistributionfusecutouts,these characteristicsare:theyareavailableinhighervoltageratings,thecancarryhigherload currents,theycaninterrupthigherfaultcurentsandtheycanbeinstalledindoors.Powerfuses consistofafuseholder,whichacceptsarefillunitorfuselink.Thepowerfuse(expulsiontype) interruptscurrents,likethedistributioncutout. 69)Howdoesacurrentlimitingfuse(c.l.f.)interruptcurrents? Thecurrentlimitingtypeinterruptsovercurrentswhenthearcestablishedbythemeltingofthe

fusibleelement,issubjectedtothemechanicalrestrictionandcoolingactionofpowderorsand filler,surroundingthefusibleelement.Therearethreefeaturesforthemediumvoltagecurrent limitingfuse: 1.Interruptionofovercurrentsisaccomplishedquickly,withouttheexpulsionofarcproductsor gases,asallthearcenergyisabsorbedbythesandfillerand,subsequently,releasedasheat. 2.Currentlimitingactionthatoccursthroughthefuseissubstantial,iftheovercurrentexceeds, significantly,thecontinuouscurrentratingofthefuse. 3.Veryhighinterruptingratingsareachievedbyvirtueofthecurrentlimitingactionofthe fuse.Currentlimitingfusescanreducethemechanicalforcesexertedonthecomponents(in series)fromthesourceuptothefaultpointduetothepeakshortcircuitcurrent.Theycan,also, reducethethermaloverloadingduetotheintegrationoftheshortcircuitcurrentovertheperiod ofthefaultexistence.Theymayimposeanovervoltageconditionontheequipmentconnected duetothecurrentchoppingeffect(forcingcurrenttozerobefoenaturalcurrentzero). 70)Whatarethedefiningparametersoffuses? Thetypicalratingsforthefuse/fuseholdercombinationare:nominalvoltage,maximum voltage,BIL,continuous(load)currentrating,speedandinterruptingcapacity(rating). 71)Howarefusesusedtoprotectpadmountedtransformers? Padmountscanbeclassifiedintoradialfeedandloopfeed.Thepadmountedtransformerswill haveloadorfaultsensing(expulsion)typefusethatisaccessiblefromoutsidethetransformerto removeandreplaceandinserieswiththisfuseacurrentlimitingbackupfuseundertheoiland isinaccessiblewithoutdenergizingthetransformersandremovingthetransformerfromthesite andprobablybreakingtheweldsofthecover,dependingonthetransformerdesign.Thepartial rangecurrentlimitingfuseoperateswithoutdischargingflames,gasesoranyotherbyproducts ofexpulsivenature.Thisseriesoffusesprovidesthecurrenttimecharacteristicsofaco ordinatedfullrangefuse,C.L.fuseisselectedtooperateonlyoninternalfailuresofthe transformer(permanentshortcircuitfaults). 72)Howarefusesusedtoprotectvaultinstalledtransformers? Forvaultmountedtransformers,aseriesofcurrentlimitingandexpulsiontypewithpowerfuse orfuselinkmountedonthepoleorthewallofthevaultaremostprobablyusedasprimary protection.Forvaultmountedandpadmounted,theprimaryconnectionismadethroughthe useofelbows(wherethecablesareconnected)andinsertsinthetransformersconnectedtothe deepwell(cavity)bushings;thesecondarywindingsofthetransformersarebroughtoutthrough L.V.bushingsandspadeterminals. 73)Whatarethemaincomponentsofac.l.f.? TheconstructionofC.L.fuses:fiberglasshousing,itservestoholdthefusecomponentsintact andisolatethefuseinternalsfromoil;silicafiller,itabsorbstheheatofthearcandhelps extinguishingthearc;thesilverelement(currentlimitingportion)itservestoclearthehigh faultcurrent;thespidermadeofmicaorceramic,tomountthesilverelementon;endcaps, madeupofcopperandthetinelement(onlyforfullrangeclearinggeneralpurposefuse)which supposedlyclearslowcurrent(toprovidetheextendedrangeovertheC.L.).Thus,ithastheT

speedexpulsionlinkcharacteristics. 74)Whatisenergymanagement? Energymanagementcanbeappliedtothegeneration,transmission&distributionsideofthe electricnetwork(utilityside)aswellastotheutilizationside(userorconsumer).Thelattermay betermeddemandsideenergymanagement.Themainpurposeofpowersystemenergy managementistoachievethegoalofgenerating,transmitting&distributingelectricenergy efficiently&reliably.Itsmainfunctionsarethesupervision,thecontrolandmanagementofthe electricalnetworkinanintegratedmanner.ItincludesSCADA(systemsupervisorycontrol& dataacquisition)&DAC(distributionautomation&control).Refertolevel1/lesson2/questions 21to23inclusiveandquestion80belowformoredetails. 75)Whatisdemandsideenergymanagement? Thedemandsideenergymanagementmeanstheuseofelectricenergyefficiently,thus increasingtheprofitabilityoftheuserorpowerconsumeri.e.payingtheminimumpossiblefor theelectricalpowerutilization.Forindustrials,theelectricutilitybill,usually,includeaportion tothemaximumdemand(anaverageoveraperiodofletssay15minutes)inKWreachedfor thebillingperiodplustheenergyconsumptioninKWH.Toreducetheenergybillanyofthe followingcanbedone: monitoringthedemandsothatinbatchproductionlinestaggeringloadstomaintainthe demandinKWataminimumpossiblelevelcansaveintheenergybill, ifthetimeofdayortimeofyearisapplied,thentheprocessorequipmentthatutilizeshigh demandsorconsumesalotofenergycanbescheduledforthetimeoflowerrates, usuallytheutilitywillpenalizetheuserwhenpoorpowerfactorequipmentareuse,thus improvingshuntcapacitorsclosetotheloadswithlowp.f.canreducetheenergybills. Coordinationofcapacitorsdesign,rating&locationcanimprovep.f.aswellasreducelossesin linesandraisepowerqualityinaplant. turnningofftheloads(equipment&lights)thatarenotinusecanmakeadifferenceinthe energybill. 76)Whatarehighefficiencymotors? Highefficiencymotorsarebuiltwithbetterqualitymaterial(steel)toreducelosses&improve efficiency.Theyare3to8%moreefficientthanthestandardsone.Thelowerthemotor horsepowerthebiggertheefficiencygapbetweenthestandard&thehighefficiencyone. 77)Whataretheofficeequipmentcommonlyused&whataretheirapproximatepower consumption? Exampleforofficeequipment&theiraverageconsumption:desktopPC(130W),monochrome monitor(30W),colourmonitor(60W),laptopPC(10W),dotmatrixprinter(50Wwhile printing&25whileidle),laserprinter(300wwhileprinting&150Wwhileidle),inkjetprinter (10Wwhileprinting&3whileidle),photocopier(3001000W),fascimilemachine(100W whiletransmitting&15Wwhileidle),modem(20W)andelectrictypewriter(130W). 78)Whataretherequireddatatocomparethetotalcostofthedifferentlightingoptions ortocalculatethepaybackperiodofthedifferentinstallations?

Adetailedcostspreadsheetcanhelpincomparingthecostoftheavailablenewinstallationsor tocalculatethepaybackperiod,themainelementsofsuchsheet:totalwattage/fixture,burning hours/year,ratedlifeinhours,ratioofburninghourstoratedlifeinhours,numberoflamps, operatingcost(annualenergycost,annualdemandcost,labourcostforrelamping)&fixedcost (fixturecostincludingthelamppluslabourcostforinstallation). 79)Whatarethedatarequiredtobeabletocomparetherequiredwattageforthe availablealternativebulbs/fixtureoptions? Inordertocomparethewattagerequiredfordifferentalternativefixtures/bulbs,thefollowing parametersaretobeknown:wattage/bulb,actualwattage/fixture,meanlumens/fixture,the requiredlumensfortheareawherethefixtures/bulbswillbeinstalled,thenumberoffixtures required&thetotalwattage. 80)WhatarethemaintasksofaSCADAsystem? ThemaintasksthataSCADAsystemhastoprovideare:controllingtheplant/systemfield devices(obviously),alarmhandling,limitschanging,providingmorethan1operationmode, dataarchivingprovision,eventsloggingandtheproductionofreport&trendcharts(graphs). 81)WhatareGIS? GISisacomputertoolthatallowstheusertoposition,analyzeandverifyobjects&eventsof geographicnatureandproduceanoutputinageographicform(maps&tables).Itisasoftware thatlinksinformationaboutwherethingsarewithinformationaboutwhatthingsarelike.The componentsthatbuildthe(digital)mapsaredistributedondifferentlayers.Theuserenter differentgeographicfeaturesondifferentlayers(thischaracteristicissimilartoCADsoftware packages).Whentheuserretrievesthemap,tobedisplayedinfrontofhim/heronthescreen, he/shedecideswhichlayerstobeshownbasedontheinformationrequired.Thuscertainlayers (information)canbesuppressediftheyareirrelevanttothetaskathand.Adigitalmapcreated byGISwillhavepoints(dots)thatrepresentfeaturesonamaplikecities,polygons(smallareas) thatrepresentfeaturessuchaslakesandarc(lines)thatrepresentfeatureslikeroads.TheGIS softwarecanbeconsideredapackagemadeupofsubpackagesthatcancommunicateand understandeachother.Itcanaccessdatadirectlyfromothersoftwarepackageslikegeographic data&shapes,CADdrawings,databases,imagesorcanimport&exportdatafrom/toother programs.TheseintegratedcharacteristicsprovidethefullfunctionalityofaGISwhich includes:statisticalanalysis&research(similartocommonlyuseddatabasesoftwarepackages), entering,storing,manipulation&analysisofdataingeographicstyle(functionalityfora mappingorgeographicinformationsystem)anddisplayingoutputresultsorproducingof documentsforpresentationpurposesintablesand/ormapforms.Theoutputresultsarelaidout ingeographicformatthatprovidesamuchclearerandeasytounderstandpresentation.These featuresoftheGISdistinguishesitfromotherinformationtechnologysystemsandmakesita toolofgreatvaluetopublicaswellasprivateuserswhenitcomestopresentingwellexplained, informativedocuments,toprovidingtherequiredresultsofananalysisandshowingthe assumptionsuponwhichtheresultsarebased,toforecastingtheoutcomebasedontheavailable factors&variablesorinpreparingstrategicplans.Thepreparationofmapsandreportsbased

ongeographicanalysisarenotnewactivities,butwiththeGIStheoutcomeisproducedfaster, consistent&accurate.Before,onlyfewpersonshadtheknowledgeandwereabletoaccess thesegeographicdatainordertomakedecisionsorsolveproblems.Now,theGISsaretaughtin secondaryschoolsandpostsecondaryinstitutesworldwide.Theindustryisinthemillionsof dollarsandemployshundredsofthousandsworkers. 82)WhatarethecomponentsofaGIS? Thereare5maincomponentsthatbuildaGIS,theyare:hardware,software,data,users& methods. 83)HowdoesaGISwork? Thesystemstoresthegeographicfeatures(information)amongdifferentlayers.Suchfeatures willbedisplayedonthescreenasgeometricshapes,points,linesorareas.Forexamplealayer mayhavepointsthatrepresentthecitiesinaregion(orallthewellsinanarea),anotherlayerof linesmayrepresentallthestreetsinanarea(orwatercourseinaregion),alayerofareascan representconstructionareasorsimilaruseareasinaregion.Eachofthegeographicfeatures willhaveitsownsetofattributes(characteristicsofsuchfeature)thataredescribedbynumbers, characters,imagesandCADdrawings(typicallystoredintabularformatandlinkedtothe featurebyauserdefinedidentifier).Forinstanceawellmightincludedepthandgallonsper minuteasitsdescriptiveelement.Eachgeographicfeaturewillalsohaveitsexactgeographic positionexpressedascoordinates,eg.Cartesianplanar(x,y),3dimension(x,y,z),vector, whichisstoredwithitsattributes.Thoughthesystemisbuiltonsimpleideabutitispowerful& versatile.Itsolvesalotofreallifeproblemsfromsettingtherouteofadeliverytruck,to decidingonthebestwayforemergencyconvoystotake,tostoringalltheimportantdetailsofa municipalityoracitylikethelocationofPolice&Firestations,hospitals,parks,overhead& undergrounddistributionsystems,electricalserviceentrancepoints,meterslocations,water pipesroutes,sewage&stormsewagepaths&locations,telephonelines,electricalpower stationslocation&details,telephoneendoffices(localcentraloffices),tolloffices(tandem offices)&intermediateswitchingoffices,environmentaldataandweatherpatterns. 84)WhatarethetasksofaGIS? ThegeneralobjectiveofaGISistoessentiallyfulfillthefollowing5tasks:dataentering& saving,datamanipulation,managementofsystem,research&analysisonthedataentered& acquiredanddisplaying&viewingthesoughtinformation. Home page of VePi

Lesson2:Switchgear,circuitbreakers,MCCs&contactors. Switchgear: 1)Whatarethemainpartsthatbuildalowvoltage(l.v.)switchgear? 2)Whatarethepartsthatbuildthemediumvoltage(m.v.)switchgearassembly? 3)Whatarethepropertiesofinsulatingmaterialsusedinswitchgearassemblies?Whatarethe insulatingmaterialsthatareusedinbuildingsuchassemblies&howtheyareformed? 4)Whatistheeffectofanelectricfieldoninsulatingmaterials? 5)Whatisvibration? 6)Whatiskinetics? 7)Whatiskinematics? 8)Howcanvibrationbeclassified? 9)Howcanswitchgearassembliesbeverifiedforinstallationinseismic(earthquake)zones? 10)Howistheheatgeneratedinsideanenclosureduetothepassageofnormalloadcurrentis transferredtotheoutsideoftheenclosure? 11)Whataretheimportantfeaturesthatdistinguishmetalcladfrommetalenclosedconstruction? 12)Whatarethedifferenttypesofswitchgearcubicles? Circuitbreakers: 13)Whatisthedefinitionofacircuitbreaker(c.b.)? 14)Whatarethemaincomponents&whatisthefunctionofeachpart,ofamouldedcasecircuit breaker(m.c.c.b.)? 15)Whatisamouldedcaseswitch? 16)Whatarethedifferenttypesofsafetyswitches?Whatarethecomponentsofsuchswitches? 17)Whatarepanelboards&howtheyareclassified? 18)Whatismeantbytheratingofac.b.? 19)Whatdoesasymmetricalcurrentmean? 20)Whatismeantbyrestrikingvoltage&whatismeantbyrecoveryvoltage? 21)Whatistheclassificationofthetestsconductedonmediumvoltagecircuitbreakers&whatisth purposeofeachcategory? 22)Whatarethemaincomponentsofanoilc.b&howdoesitinterruptthearc? 23)Whatarethemajorcomponentsofanaircircuitbreaker&howdoesthearcgetinterruptedinair? 24)Whatarethemaincomponentsofavacuumc.b&howdoesthearcgetextinguishedinvacuum? 25)Whatismeantbycurrentchopping,multiplereignition&virtualchopping? 26)WhatarethedifferenttypesofSF6circuitbreakers,whatarethemaincomponentsofanSF6c.b. &howdoesthec.b.interruptthearc? Contactors&motorcontrolcenters: 27)Whatarecontactors&whatarethemajorelementsofacontactor?

28)Whatisamanualstarter&whatisanelectromagneticone? 29)Whatarethedifferentmethodsofstartingasquirrelcageinductionmotor(SCIM)? 30)Whatisacombinationmotorcontroller? 31)Fromwherethepowersourcetothecontrolcircuitcancomefrom? 32)Whatarethemajortestsconductedoncontactors? 33)Whatarethedifferenttypesofmediumvoltagecontactors? 34)Besidethecontactor&fuse,whataretheothermajorcomponentsusedtobuildthem.v.controller? 35)Whatarecontrolpanels? 36)Whatarethemajorcomponentsoflowvoltagemotorcontrolcenters? 1)Whatarethemainpartsthatbuildalowvoltage(l.v.)switchgear? Themainelementsforl.v.switchgearassemblieswithmouldedcaseorpower(airmagnetic)circuit breakers(drawoutorfixed)completewithorwithoutintegralovercurrentprotectiondevice(thermal magnetic,solidstateormicroprocessorbased)withorwithoutzoneinterlockingprovisionarethe cradlefordrawoutversiononly,fixedpotentialandcontrolpowertransformers,currenttransformers, interframeandbackbarriers,mainbusbars,runbacks,busrisers,cablelugs(compressionor mechanical),protectivefusesforbreakercontrolcircuits,instrumentcompartments,breaker accessories,breakerliftingdevices(integralwithgear),fusedorunfusedinterrupterswitches, protective/monitoring/auxiliaryrelays,meteringdevices(ammeters,voltmeters, watthourmeters/wattmetersanddigitalmeteringpackagesD.M.P.),controlswitchesandindicating lights.Forcommongeneralarrangements,refertofig.2.1below.

2) Whatarethepartsthatbuildthemediumvoltage(m.v.)switchgearassembly? Themainelementsformediumvoltageswitchgearassembliesaredrawoutcircuitbreakers(oil, air,SF6,vacuum),switchingdevice(loadbreakordisconnect)withorwithoutfuses(power currentlimitingorpowerexpulsiontype)withorwithoutblownfuseindicator/actuator, instrumenttransformers(current&potential),capacitortrippingdevice,relays(protective& auxilliary),meters&instruments,mainbus,control(switches&pushbuttons)&indicating devices(lights),jumpers,cablelugs,cablesupports,potheads,barriers,shutters,instrument compartments,testswitches,groundstirrupsandbreakeraccessories(springcharginglever, slowclosehandle&rackinglever).Fortypicalbreakercubicleincludingfrontelevation&3 wirediagram,refertofig.2.2.

3)Whatarethepropertiesofinsulatingmaterialsusedinswitchgearassemblies?What aretheinsulatingmaterialsthatareusedinbuildingsuchassemblies&howtheyare formed? Aninsulatingmaterialisamaterialthatoffersrelativelyhighresistancetothepassageofan electriccurrent.Thematerialusedforinsulationinelectricalapparatusandmachinesare numerousanddiverse:solid,liquid,gaseousorganicorinorganicnaturalormanmade (synthetic).Thecharacteristicsofanyinsulatingmaterialusedasbarriers,supportsorbushings ingearconstructionare:trackresistivity,highdielectricstrength,lowpowerfactor(or dissipationfactor),flameretardant(perCSAC22.2#31orANSIC37.20)andhighmechanical strength.Thematerialsusedinswitchgear/controlpanelsarepolyestere,epoxies,ceramics.The

formingmethodsarecasting,preformmolding,premix,laminatingandwrappedlaminating. 4)Whatistheeffectofanelectricfieldoninsulatingmaterials? Asthecasewithaconductoradielectricplacedinanelectrostaticfieldwillbesubjectedto electrostaticinduction.Inaconductorthefreeelectronsmovethroughtheentirevolumeofthe material,inadielectrictheelectricfieldwilltwistandstrainthemoleculestoorientthepositive chargesinthedirectionofthefieldandthenegativechargesintheoppositedirectioni.e. polarizationphenomena.Thepolarizationofthedielectricvanishesassoonastheelectricfield isremoved.Polarizationisanelasticshiftofelectricelementsinthedielectricmaterial,ifthe electricfieldistoohighthedielectricwillbreakdownandceasestoinsulate.Inadielectric, displacement(polarization)currentandconductioncurrentexist.Thismaterialcanbemodelled orrepresentedbyacapacitorinseries(orinparalleloracombinationofboth)witharesistor, fig.2.3.The2possibleleakagepathsinadielectricmaterialarethroughthebodyandoverits surface(creepagetracking).Thetotalconductingcurrentisthesumofthethroughandover thesurfacecurrents.Theresistivityofadielectricvarieswithitsstate(solid,liquid,gas), composition,humidityandambienttemperature.Theotherimportantpropertyisthepermitivity ofthedielectricdielectricconstantorspecificinductivecapacitywhichisdefinedasthe capacitanceofthecpacitorwithitsdielectricmaterial(itsinsulator)tothecapacitanceofthe samecapacitorwithvacuumasinsulator.Whenadielectricissubjectedtoachangingelectric fieldsomeoftheelectricenergyistransformedintoheatandislost.Inarealdielectricthe phaseangleislessthan90deg.duetotheconductioncurrent,fig.2.3.Thisangle,duetothe shiftofthecurrentvectorrelativetothevoltagevector,iscalledthedielectriclossangle(phase differenceorphasedeflectangle).Dielectriclossincreasesandthequalityofthedielectric decreaseswithincreasingtand(dissipationfactor)wheredisthelossangle.Scheringbridge (refertolesson1question43forasketchofScheringbridge)isusedtomeasurethecapacitance andtandofadeviceorequipment.Electricstrengthisthepropertyofofaninsulatingmaterial whichisameasureofitcapabilitytowithstandelectricstresswithoutinjury.Itistheminimum electricstressinKV/cmthatwillcausefailure(breakdown)ofthedielectricundercertain conditionsofservice.Breakdowninsoliddielectricisduetooneofthefollowing2main reasons:highelectricstressthatcausesdestructiontothestructureandtheotherduetothermal effectsthatcanleadtodecompositionormelting/burningofthematerial.Thedielectricstrength ofliquddielectrics(forexampleoil)depends,greatly,ontheamountofmoisture,gasesand impuritiespresent.Whenoverheatingoccursthemoleculesaredestroyedandfailureof dielectricisimminent.

5)Whatisvibration? Vibrationisanoscillatingmotion,periodic/transientinnature.Studyofvibrationrelyon dynamics(thefieldinmechanicsthatdealswithbodiesinmotionandtheforcescausingsuch motions). 6)Whatiskinetics? Kineticsisthestudyrelatingbetweenthemotionofthebodiesandtheforcesactinguponthem. 7)Whatiskinematics? Kinematicsisthestudyofthegeometryofmotion,withoutreferencetoforceormass. Kinematicsisprimarilyconcernedwithdisplacement,velocity,accelerationandtime. 8)Howcanvibrationbeclassified? Vibrationcanbeclassifiedinseveralways.Thefirstapproachtocategorizevibrationisas follows:freevibrationoccurswhenanelasticsystemisdisplacedorgivensomeinitialvelocity fromanimpact,forcedvibrationoccurswiththeapplicationofexternalforceforcedvibration areperiodic,aperiodicorrandom.Vibrationcanbeclassifiedbythenumberofdegreesof freedom(thenumberofindependentcoordinatesneededtocompletelydescribethemotion). Forharmonicmotion:themotioncompletedinanyoneperiodiscalledcycletheperiodisthe

timenecessarytocompleteonecycleofmotion:f=1/T,T=(m/k).5;wheremisthemassandkis thestiffnessfortherepresentedspringmassmodel.Themovementofthebaseorthefoundation mayresultinunwantedvibrationofanelasticstructurerestingonthebase.Themaximum response(displacement,velocity,acceleration)arequiteusefullinseismicstudies.Itis convenienttoplotthesemaximumvalues(responsespectracurves)asfunctionofthenatural frequenciesofaseriesofasingledegreeoffreedomsystemssubjectedtoanaccelerationtime historyrecord. Coordinatesselectioncanaffecttheeasinessbywhichadynamicsproblemissolved. Coordinates,generally,isdefinedasanindependentquantitythatwillspecifyposition. 9)Howcanswitchgearassembliesbeverifiedforinstallationinseismic(earthquake) zones? Anearthquakemaybedescribedasthevibratoryan