Power-System-Protections

PowerSystemProtectionsIntroductionThesteady-stateoperationofapowersystemisfrequentlydisturbedbyvariousfaultsonelectricalequipment.Tomaintaintheproperoperationofthepowersystem,aneffiective,efficientandreliableprotectionschemeisrequired.Powersystemcomponentsaredesignedtooperateundernormaloperatingconditions.However,ifduetoanyreason,sayafault,thereisanabnormality,itisnecessarythatthereshouldbeadevicewhichsensestheseabnormalconditionsandifso,theelementorcomponentwheresuchanabnormalityhastakenplaceisremoved,i.e.deletedfromtherestofthesystemassoonaspossible.Thisisnecessarybecausethepowersystemcomponentcanneverbedesignedtowithstandtheworstpossibleconditionsduetothefactthatthiswillmakethewholesystemhighlyuneconomical.Andtherefore,ifsuchanabnormalitytakesplaceinanyelementorcomponentofthepowersystemnetwork,itisdesirablethattheaffectedelement/componentisremovedfromtherestofthesystemreliablyandquicklyinordertorestorepowerintheremainingsystemunderthenormalconditionassoonaspossible.Theprotectionschemeincludesboththeprotectiverelaysandswitchingcircuits,i.e.circuitbreakers.Theprotectiverelaywhichfunctionsasabrainisaveryimportantcomponent.Theprotectiverelayisasensingdevice,whichsensesthefault,determinesitslocationandthensendcommandtothepropercircuitbreakerbyclosingitstripcoil.Thecircuitbreakeraftergettingcommandfromtheprotectiverelay,disconnectsonlythefaultedelement.Thisiswhytheprotectiverelaymustbereliable,maintainableandfastinoperation.Inearlydays,thereusedtobeelectromechanicalrelayofinductiondisk-type.However,verysoonthediskwasreplacedbyinvertedcup,i.e.hollowcylinderandthenewrelayobtainedwasknownasaninductioncuporinductioncylinderrelay.Thisrelay,whichisstillinuse,possessesseveralimportantfeaturessuchashigherspeed,highertorqueforagivenpowerinputandmoreuniformtorque.However,withtheadventofelectronictubes,electronicrelayshavingdistinctfeaturesweredevelopedduring1940s.Withthediscoveryofsolidstatecomponentsduring1950s,staticrelayswithnumerousadvantagesweredeveloped.Theuseofdigitalcomputersforprotectiverelayingpurposeshasbeenengagingtheattentionofresearchandpracticingengagingtheattentionofresearchandpracticingengineerssincelate1960sand1980s.Now,themicroprocessor/minicomputer-basedrelayingscheme,becauseofitsnumerousadvantagessuchasself-checkingfeatureandflexibility,hasbeenwidelyusedinpowersystemsallovertheworld.Theoverallsystemprotectionisdividedintofollowingsections:(i)Generatorprotection,(ii)Transformerprotection,(iii)Busprotaection,(iv)Feederprotection,(v)Transmissionlineprotection.BasicRequirementstoProtectiveRelaysAnyprotectionscheme,whichisrequiredtosafeguardthepowersystemcomponentsagainstabnormalconditionssuchasfaults,consistsbasicallyoftwoelements:(i)Protectiverelayand(ii)Circuitbreaker.Theprotectiverelaywhichisprimarilythebrainbehindthewholeschemeplaysaveryimportantrole.Thereforepropercareshouldbetakeninselectinganappropriateprotectiverelaywhichisreliable,efficientandfastinoperation.Theprotectiverelaymustsatisfythefollowingrequirements:(1)Sincefaultsonawelldesignedandhealthysystemarenormallyrare,therelaysarecalledupontooperateonlyoccasionally.Thismeansthattherelayingschemeisnormallyidleandmustoperatewheneverfaultoccurs.Inotherwords,itmustbereliable.(2)Sincethereliabilitypartlydependsuponthemaintenance,therelaymustbeeasilymaintainable.(3)Themaloperationoftherelaycanbeintwoways.Oneisthefailuretooperateincaseafaultoccursandsecondistherelayoperationwhenthereisnofault.Asamatteroffact,relaymustoperateifthereisafaultandmustnotoperateifthereisnofault.(4)Relayingschememustbesensitiveenoughtodistinguishbetweennormalandthefaultysystem.ProtectiveRelaysThefunctionoftheprotectiverelaysistosensethefaultandenergizethetripcoilofthecircuitbreaker.Thefollowingtypesofprotectiverelaysareusedfortheapparatussuchassynchronousmachines,busbar,transformerandtheotherapparatusandtransmissionlineprotection.(1)Overcurrentrelays.(2)Undervoltagerelays.(3)Underfrequencyrelays.(4)Directionalrelays.(5)Thermalrelays.(6)Phasesequencerelayssuchas(i)negativesequencerelaysand,(ii)zerosequencerelays.(7)Differentialrelaysandpercentagedifferentialrelays.(8)Distancerelayssuchas(i)planeimpedancerelays,(ii)angleimpedancerelays,i.e.Ohmorreactancerelays,(iii)angleadmittancerelays,i.e.Mhorelaysand,(iv)offsetandrestrictedrelays.(9)Pilotrelayssuchas(i)wirepilotrelays,(ii)carrierchannelpilotrelays,(iii)microwavepilotrelays.Therearedifferenttypesoftherelayingschemebasedonconstruction.Theyare:(i)electromechanicaltype,(ii)thermalrelays,(iii)transductorrelays,(iv)rectifierbridgerelay,(v)electronicrelays,(vi)staticrelays,(vii)digitalrelayingschemes.FaultsandTheirDamagesonPowerSystemsFaultsonTransmissionLinesBecausetransmissionlinesareexposedtolightningandotheratmospherichazards,faultsonthemoccurmorefrequentlythanthoseinapparatus.Thetypesoffaultstakingplaceonatransmissionlinearelisted,intheorderofseverity,asfollowing:(1)3-φfault(LLLfault)or3-φtogroundfault(LLLGfault)withorwithoutfaultimpedance.Thisfaultwhichismostseverebutleastcommonisonlyoneinnumber.(2)Doublelinetoground(LLG)