继电保护中英文翻译精.docx
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继电保护中英文翻译精.docx
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继电保护中英文翻译精
Fundamentalsofprotectionpractice
Thepurposeofanelectricalpowersystemistogenerateandsupplyelectricalenergytoconsumers.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,thechanceofafaultoccurringandthedisturbancethatafaultwouldbringarebothsogreatthatwithout
equipmenttoremovefaultsthesystemwillbecome,inpracticalterms,inoperable.Theobjectofthesystemwillbedefeatedifadequateprovisionforfaultclearanceisnotmade.Noristheinstallationofswitchgearalonesufficient;discriminativeprotectivegear,designedaccordingtothecharacteristicsandrequirementsofthepowersystem.mustbeprovidedtocontroltheswitchgear.Asystemisnotproperlydesignedandmanagedifitisnotadequatelyprotected.
Protectivegear
Thisisacollectivetermwhichcoversalltheequipmentusedfordetecting,locatingandinitiatingtheremovalofafaultfromthepowersystem.Relaysareextensivelyusedformajorprotectivefunctions,butthetermalsocoversdirect-actinga.c.tripsandfuses.Inadditiontorelaysthetermincludesallaccessoriessuchascurrentandvoltagetransformers,shunts,d.c.anda.c.wiringandanyotherdevicesrelatingtotheprotectiverelays.
Ingeneral,themainswitchgear,althoughfundamentallyprotectiveinitsfunction,isexcludedfromthetermprotectivegear,asarealsocommonservices,suchasthestationbatteryandanyotherequipmentrequiredtosecureopera-tionofthecircuitbreaker.Reliablity
Theperformanceoftheprotectionappliedtolargepowersystemsisfrequentlyassessednumerically.Forthispurposeeachsystemfaultisclassedasanincidentandthosewhichareclearedbythetrippingofthecorrectcircuitbreakersandonlythose,areclassedas'correct'.Thepercentageofcorrectclearancescanthenbedetermined.
Thisprincipleofassessmentgivesanaccurateevaluationoftheprotectionofthesystemasawhole,butitissevereinitsjudgementofrelayperformance,inthatmanyrelaysarecalledintooperationforeachsystemfault,andallmustbehavecorrectlyforacorrectclearancetoberecorded.Onthisbasis,aperformanceof94%isobtainablebystandardtechniques.
Completereliabilityisunlikelyevertobeachievedbyfurtherimprovementsinconstruction.Averybigstep,however,canbetakenbyprovidingduplicationofequipmentor'redundancy'.Twocompletesetsofequipmentareprovided,andarrangedsothateitherbyitselfcancarryouttherequiredfunction.Iftheriskofanequipmentfailingisx/unit.theresultantrisk,allowingforredundancy,isx2.Wherexissmalltheresultantrisk(x2may
benegligible.
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,willreducetheoverallperformance.SELECTIVITY
Protectionisarrangedinzones,whichshouldcoverthepowersystemcompletely,leavingnopartunprotected.Whenafaultoccurstheprotectionisrequiredtoselectandtriponlytheneareatcircuitbreakers.Thispropertyofselectivetrippingisalsocalled'discrimination'andisachievedbytwogeneralmethods:
aTimegradedsystems
Protectivesystemsinsuccessivezonesarearrangedtooperateintimeswhicharegradedthroughthesequenceofequipmentssothatupontheoccurrenceofafault,althoughanumberofprotectiveequipmentsrespond,onlythoserelevanttothefaultyzonecompletethetrippingfunctiopn.Theothersmakeincompleteoperationsandthenreset.
bUnitsystems
Itispossibletodesignprotectivesystemswhichrespondonlytofaultconditionslyingwithinaclearlydefinedzone.This'unitprotection'or'restrictedprotection'canbeappliedthroughoutapowersystemand,sinceitdoesnotinvolvetimegrading,canberelativelyfastinoperation.
Unitprotectionisusuallyachievedbymeansofacomparisonofquantitiesatthe
boundariesofthezone.Certainprotectivesystemsderivetheir'restricted'propertyfromtheconfigurationofthepowersystemandmayalsobeclassedasunitprotection.Whichevermethodisused,itmustbekeptinmindthatselectivityisnotmerelyamatterofrelaydesign.Italsodependsonthecorrectco-ordinationofcurrenttransformersandrelayswithasuitablechoiceofrelaysettings,takingintoaccountthepossiblerangeofsuchvariablesasfaultcurrents.maximumloadcurrent,systemimpedancesandotherrelatedfactors,whereappropriate.
STABILITY
Thisterm,appliedtoprotectionasdistinctfrompowernetworks,referstotheabilityofthesystemtoremaininerttoallloadconditionsandfaultsexternaltotherelevantzone.Itisessentiallyatermwhichisapplicabletounitsystems;theterm'discrimination'istheequivalentexpressionapplicabletonon-unitsystems.
SPEED
Thefunctionofautomaticprotectionistoisolatefaultsfromthepowersysteminaverymuchshortertimethancouldbeachievedmanually,evenwithagreatdealofpersonalsupervision.Theobjectistosafeguardcontinuityofsupplybyremovingeachdisturbancebeforeitleadstowidespreadlossofsynchronism,whichwouldnecessitatetheshuttingdownofplant.
Loadingthesystemproducesphasedisplacementsbetweenthevoltagesatdifferentpointsandthereforeincreasestheprobabilitythatsynchronismwillbelostwhenthesystemisdisturbedbyafault.Theshorterthetimeafaultisallowedtoremaininthesystem,thegreatercanbetheloadingofthesystem.Figure1.5showstypicalrelationsbetweensystemloadingandfaultclearancetimesforvarioustypesoffault.Itwillbenotedthatphasefaultshaveamoremarkedeffectonthestabilityofthesystemthandoesasimpleearthfaultandthereforerequirefasterclearance.
SENSITIVITY
Sensitivityisatermfrequentlyusedwhenreferringtotheminimumoperatingcurrentofacompleteprotectivesystem.Aprotectivesystemissaidtobesensitiveiftheprimaryoperatingcurrentislow.
Whenthetermisappliedtoanindividualrelay,itdoesnotretertoacurrentorvoltagesettingbuttothevolt-ampereconsumptionattheminimumoperatingcurrent.
Agiventypeofrelayelementcanusuallybewoundforawiderangeofsettingcurrents;thecoilwillhaveanimpedancewhichisinverselyproportionaltothesquareofthesettingcurrentvalue,sothatthevolt-ampereproductatanysettingisconstant.Thisisthetruemeasureoftheinputrequirementsoftherelay,andsoalsoofthesensitivity.Relaypowerfactorhassomesignificanceinthematteroftransientperformance.Ford.c.relaystheVAinputalsorepresentspowerconsumption,andtheburdenisthereforefrequentlyquotedinwatts.
PRIMARYANDBACK-UPPROTECTION
Thereliabilityofapowersystemhasbeendiscussedinearliersections.Many
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