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FundamentalsofprotectionpracticeThepurposeofanelectricalpowersystemistogenerateandsupplyelectricalenergytoconsumers.Thesystemshouldbedesignedandmanagedtodeliverthisenergytotheutilizationpointswithbothreliabilityandeconomy.Asthesetworequirementsarelargelyopposed,itisinstructivetolookatthereliabilityofasystemanditscostandvaluetotheconsumer.Onehand,Thediagrammastmakesurethereliabilityinsystemdesign,.Ontheotherhand,highreliabilityshouldnotbepursuedasanendinitself,regardlessofcost,butshouldratherbebalancedagainsteconomy,taking.Securityofsupplycanbebetteredbyimprovingplantdesign,increasingthesparecapacitymarginandarrangingalternativecircuitstosupplyloads.Sub-divisionofthesystemintozones.eachcontrolledbyswitchgearinassociationwithprotectivegear.providesflexibilityduringnormaloperationandensuresaminimumofdislocationfollowingabreakdown.Thegreatestthreattothesecurityofasupplysystemistheshortcircuit,whichimposesasuddenandsometimesviolentchangeonsystemoperation.Thelargecurrentwhichthenflows,accompaniedbythelocalizedreleaseofaconsiderablequantityofenergy,cancausefireatthefaultlocation,andmechanicaldamagethroughoutthesystem,particularlytomachineandtransformerwindings.Rapidisolationofthefaultbythenearestswitchgearwillminimizethedamageanddisruptioncausedtothesystem.Apowersystemrepresentsaverylargecapitalinvestment.Tomaximizethereturnonthisoutlay.thesystemmustbeloadedasmuchaspossible.Forthisreasonitisnecessarynotonlytoprovideasupplyofenergywhichisattractivetoprospectiveusersbyoperatingthesystem,butalsotokeepthesysteminfulloperationasfaraspossiblecontinuously,sothatitmaygivethebestservicetotheconsumer,andearnthemostrevenueforthesupplyauthority.Absolutefreedomfromfailureoftheplantandsystemnetworkcannotbeguaran-teed.Theriskofafaultoccurring,howeverslightforeachitem,ismultipliedbythenumberofsuchitemswhicharecloselyassociatedinanextensivesystem,asanyfaultproducesrepercussionsthroughoutthenetwork.Whenthesystemislarge,thechanceofafaultoccurringandthedisturbancethatafaultwouldbringarebothsogreatthatwithoutequipmenttoremovefaultsthesystemwillbecome,inpracticalterms,inoperable.Theobjectofthesystemwillbedefeatedifadequateprovisionforfaultclearanceisnotmade.Noristheinstallationofswitchgearalonesufficient;discriminativeprotectivegear,designedaccordingtothecharacteristicsandrequirementsofthepowersystem.mustbeprovidedtocontroltheswitchgear.Asystemisnotproperlydesignedandmanagedifitisnotadequatelyprotected.ProtectivegearThisisacollectivetermwhichcoversalltheequipmentusedfordetecting,locatingandinitiatingtheremovalofafaultfromthepowersystem.Relaysareextensivelyusedformajorprotectivefunctions,butthetermalsocoversdirect-actinga.c.tripsandfuses.Inadditiontorelaysthetermincludesallaccessoriessuchascurrentandvoltagetransformers,shunts,d.c.anda.c.wiringandanyotherdevicesrelatingtotheprotectiverelays.Ingeneral,themainswitchgear,althoughfundamentallyprotectiveinitsfunction,isexcludedfromthetermprotectivegear,asarealsocommonservices,suchasthestationbatteryandanyotherequipmentrequiredtosecureopera-tionofthecircuitbreaker.ReliablityTheperformanceoftheprotectionappliedtolargepowersystemsisfrequentlyassessednumerically.Forthispurposeeachsystemfaultisclassedasanincidentandthosewhichareclearedbythetrippingofthecorrectcircuitbreakersandonlythose,areclassedas'correct'.Thepercentageofcorrectclearancescanthenbedetermined.Thisprincipleofassessmentgivesanaccurateevaluationoftheprotectionofthesystemasawhole,butitissevereinitsjudgementofrelayperformance,inthatmanyrelaysarecalledintooperationforeachsystemfault,andallmustbehavecorrectlyforacorrectclearancetoberecorded.Onthisbasis,aperformanceof94%isobtainablebystandardtechniques.Completereliabilityisunlikelyevertobeachievedbyfurtherimprovementsinconstruction.Averybigstep,however,canbetakenbyprovidingduplicationofequipmentor'redundancy'.Twocompletesetsofequipmentareprovided,andarrangedsothateitherbyitselfcancarryouttherequiredfunction.Iftheriskofanequipmentfailingisx/unit.theresultantrisk,allowingforredundancy,isx2.Wherexissmalltheresultantrisk(x2maybenegligible.Ithaslongbeenthepracticetoapplyduplicateprotectivesystemstobusbars,bothbeingrequiredtooperatetocompleteatrippingoperation,thatis,a'two-out-of-two'arrangement.Inothercases,importantcircuitshavebeenprovidedwithduplicatemainprotectionschemes,eitherbeingabletotripindependently,thatis,a'one-out-of-two'arrangement.Theformerarrangementguardsagainstunwantedoperation,thelatteragainstfailuretooperate.Thesetwofeaturescanbeobtainedtogetherbyadoptinga'two-out-of-three'arrangementinwhichthreebasicsystemsareusedandareinterconnectedsothattheoperationofanytwowillcompletethetrippingfunction.Suchschemeshavealreadybeenusedtoalimitedextentandapplicationoftheprinciplewillundoubtedlyincrease.Probabilitytheorysuggeststhatifapowernetworkwereprotectedthroughoutonthisbasis,aprotectionperformanceof99.98%shouldbeattainable.Thisperformancefigurerequiresthattheseparateprotectionsystemsbecompletelyindependent;anycommonfactors,suchascommoncurrenttransformersortrippingbatteries,willreducethe