Recommended
More Related Content
What's hot
What's hot (20)
Similar to Surge current protection Seminar Report
Similar to Surge current protection Seminar Report (20)
More from Lakshminarayan Solanki
More from Lakshminarayan Solanki (13)
Recently uploaded
Recently uploaded (20)
Surge current protection Seminar Report
- 3. III ACKNOWLEDGEMENT FirstIwanttothankmy “ALLAH” who neverleavesmealonein bad circumstances,help me alwayswiththeirwellblessings. Thereareanumberofpeoplewhodeserverecognitionfortheir unwavering supportand guidance throughoutthis seminar presentation.Iwouldliketotakethisopportunitytothankthem. Firstandforemost,Iwanttothankmywell-wisheradvisorandas aguide,Mr.AsadZaiSirforhissupport,constructivecriticism, andnew technicalupdates.Thispresentationcouldhavenot beencompletedwithouthisenthusiasmanddirection. IagainwanttothankMr.AsadSirforgivingmethistypeof technicaltheorytopicbywhichIcanunderstandmycorebranch efficiently. IwillbealwaysthankfultoOurHODMr.ManishBhatiSirfor expandingmyunderstandingofglobalinequalitiesthroughout historyandherconstantmentorship. Iwanttothankmyfriendswhohaveencouragedmeoverthe studyofthispresentation. Finally,Iwanttothankmyfamily.Myparents,especially,have beenasourceofstrengthandsupportforme.Theycontinually pushmetothinkcriticallyandneversettleforanythinglessthan mybest.Theykeptmefocused,ontrackandfedinthefinal stretchofmycollegestudy. Fortheircontinualsupport,Iamforevergrateful.
- 4. IV ABSTRACT ModernPowerSystemsaregrowingfastwithmoregenerators, transformersandlargenetworkinthesystems.Theinstallation, runningandmaintenancecostsofthepowersystem equipment aremore.Wheneverafaultoccurs,thereisaneed forthe protection ofthese systems.PowerSystem Protection is a branch ofelectricalpowerengineering thatdeals with the protectionofelectricalpowersystemsfrom faultsthroughthe isolationoffaultedpartsfrom therestoftheelectricalnetwork. Superconductorsareoneofthelastgreatfrontiersofscientific discovery.Here,in thispaperwediscusstheuseofSuper ConductorsasprotectivedevicesforSurgeCurrentProtection. Superconductors conductelectricity offering zero resistance below certaintemperatures.Westudydifferenttypesofsuper conductorfaultcurrentlimitersandtheirworking.
- 9. 9 letus concentrateonwhatissurgecurrent?Whatitdoestothepowersystem?And whatare superconductors?MeissnerEffectanddifferenttypesofsuperconductor faultcurrent limiters. Electricpowerdistributionsystemsincludecircuitbreakerstodisconnect powerincaseofafault,buttomaximizereliability,theywishtodisconnect thesmallestpossibleportionofthenetwork.Thismeansthateventhe smallestcircuitbreakers,aswellasallwiringtothem,mustbeableto disconnectlargefaultcurrents. A problem arises ifthe electricitysupplyis upgraded,byadding new generationcapacityorbyaddingcross-connections.Becausetheseincrease theamountofpowerthatcanbesupplied,allofthebranchcircuitsmust havetheirbusbarsandcircuitbreakersupgradedtohandlethenewhigher faultcurrentlimit. Thisposesaparticularproblem whendistributedgeneration,suchaswind farmsandrooftopsolarpower,isaddedtoanexistingelectricgrid.Itis desirabletobeabletoaddadditionalpowersourceswithoutlargesystem- wideupgrades. Asimplesolutionistoaddelectricalimpedancetothecircuit.Thislimitsthe rateatwhichcurrentcanincrease,whichlimitsthelevelthefaultcurrentcan risetobeforethebreakerisopened.However,thisalsolimitstheabilityofthe circuittosatisfyrapidlychangingdemand,sotheadditionorremovaloflarge loadscausesunstablepower. Afaultcurrentlimiterisanonlinearelementwhichhasalowimpedanceat normalcurrentlevels,butpresentsahigherimpedanceatfaultcurrentlevels.
- 10. 10 Further,thischangeisextremelyrapid,beforeacircuitbreakercantripafew millisecond later.(High-powercircuitbreakers are synchronized to the alternatingcurrentzerocrossingtominimizearcing.) Whilethepowerisunstableduringthefault,itisnotcompletelydisconnected. After the faulting branch is disconnected,the fault current limiter automaticallyreturnstonormaloperation. 1.2SurgeCurrent Themaximum instantaneousinputcurrentdrawnbyanelectricaldevice whenfirstitisturnedonisdefinedassurgecurrent.Itisalsoknownas InrushcurrentorInputSurgeCurrentorSwitch-onSurge.Alternatingcurrent electricmotorsandtransformersmaydrawseveraltimestheirnormalfull- loadcurrentwhenfirstenergized,forafew cyclesoftheinputwaveform. Powerconvertersalsooftenhaveinrushcurrentsmuchhigherthantheir steadystatecurrents,duetothechargingcurrentoftheinputcapacitance. Theselectionofovercurrentprotectiondevicessuchasfusesandcircuit breakersismademorecomplicatedwhenhighinrushcurrentsmustbe tolerated.Theovercurrentprotectionmustreactquicklytooverloadorshort circuitbutmustnotinterruptthecircuitwhenthe(usuallyharmless)inrush currentflows.
- 12. 12 CHAPTER-2 SUPERCONDUCTOR 2.1Superconductor Anelement,inter-metallicalloyorcompoundthatwillconductelectricity without resistance below a certain temperature.The Dutch Physicist Heike KamerlinghOnnesofLeidenUniversitywasthefirstpersontoobserve superconductivityinmercury. Superconductivityisaphenomenonofexactlyzeroelectricalresistance certainmaterialswhencooledbelowacharacteristiccriticaltemperature.It isaquantummechanicalphenomenon. TypesofSuperconductors: LowTemperatureSuperconductor HightemperatureSuperconductors LTSarethesubstancesthatloseallresistivitycloseto4K,atemperature attainableonlybyliquid helium.HTS arethesubstancesthatloseall resistancebelowtemperaturemaintamablebyliquidnitrogen. ExamplesofLTS:LeadandMercury ExamplesofHTS:YBCO,BSCCO,LSCO,etc. 2.2MeissnerEffect The Meissner effectis the expulsion ofthe magnetic field from a superconductor duringitstransitiontothesuperconductingstate.TheGermanphysicists
- 13. 13 Walther MeissnerandRobertOchsenfelddiscoveredthephenomenonin1933by measuringthemagneticfielddistributionoutsidesuperconductingtinand leadsamples.Themagneticfluxisconservedbythesuperconductor,when theinteriorfielddecreasedtheexternalfieldisincreased. 2.3FaultCurrentLimiter AFaultCurrentLimiterisadevicewhichlimitstheprospectivefaultcurrent whenafaultoccurs.Generallyfaultcurrentlimitersaresuperconductorfault currentlimiters.SuperconductingFaultCurrentLimitersaredescribedas beinginoneofthetwomajorcategories: Resistive Inductive FirstapplicationsforFCLsarelikelytobeusedtohelpcontrolmedium- voltage electricity distribution systems,followed by electric-drive ships:naval vessels, submarinesandcruiseships.LargerFCLsmayeventuallybeemployedin highvoltagetransmissionsystems. Fault-currentlimiters using high temperature superconductors offera solution to controlling fault-current levels on utility distribution and transmissionnetworks.Thesefault-currentlimiters,unlikereactorsorhigh- impedancetransformers,willlimitfaultcurrentswithoutaddingimpedance tothecircuitduringnormaloperation.Developmentofsuperconductingfault -currentlimiters is being pursued by severalutilities and electrical manufacturersaroundtheworld,andcommercialequipmentisexpectedto beavailablebytheturnofthecentury.[1]
- 14. 14 2.4Fault-CurrentProblem Electricpowersystem designersoftenfacefault-currentproblemswhen expanding existingbuses.Largertransformersresultinhigherfault-dutylevels,forcing the replacementofexistingbusworkandswitchgearnotratedforthenewfault duty. Alternatively,theexistingbuscanbebrokenandservedbytwoormore smaller transformers.Anotheralternativeisuseofasingle,large,high-impedance transformer,resultingindegradedvoltageregulationforallthecustomerson thebus.Theclassictradeoffbetweenfaultcontrol,buscapacity,andsystem stiffnesshaspersistedfordecades. 2.5FaultControlProblem Insomeareas,suchastheUnitedStates,additionalgenerationfrom co generators andindependentpowerproducers(IPPs)raisesthefaultdutythroughouta system olderbutstilloperationalequipmentgraduallybecomesunderratedthrough system growth;someequipment,such astransformersin underground vaultsorcables,canbeveryexpensivetoreplacecustomersrequestparallel servicesthatenhancethereliabilityoftheirsupplybutraisethefaultduty. 2.6SuperconductiveFCL Superconductorsofferawaytobreakthroughsystem designconstraintsby presenting
- 15. 15 impedanceto theelectricalsystem thatvariesdepending on operating conditions. Superconductingfault-currentlimitersnormallyoperatewithlowimpedance andare "invisible"componentsintheelectricalsystem.Intheeventofafault,the limiterinsertsimpedanceintothecircuitandlimitsthefaultcurrent.With currentlimiters,theutilitycanprovidealow-impedance,stiffsystem witha lowfault-currentlevel. Fig.-(2.6)-Faultcontrolwithafault-currentlimiter Alarge,low-impedancetransformerisusedtofeedabus.Normally,theFCL doesnotaffectthecircuit.Intheeventofafault,thelimiterdevelopsan impedanceof0.2perunit(Z=20%),andthefaultcurrentISCisreducedto 7,400 A.Withoutthelimiter,thefaultcurrentwould be37,000 A.The developmentofhigh temperature superconductors (HTS)enables the developmentofeconomicalfault-currentlimiters.Superconductingfault- currentlimiterswerefirststudiedovertwentyyearsago.Theearliestdesigns used low temperature superconductors (LTS),materials thatlose all resistanceattemperaturesafewdegreesaboveabsolutezero.LTSmaterials aregenerallycooledwithliquidhelium,asubstancebothexpensiveand
- 16. 16 difficult to handle. The discovery in 1986 of high temperature superconductors,whichoperateathighertemperaturesandcanbecooledby relativelyinexpensiveliquidnitrogen,renewedinterestinsuperconducting fault-currentlimiters.[2] 2.7Fault-CurrentLimiterApplications Fault-currentlimiters can be applied in a numberofdistribution or transmissionareas. The fault-currentlimiterFCL protects an individualcircuiton the bus. Underratedequipmentcanbeselectivelyprotectedasneededinthismanner. Fig.-(2.7.1)-Fault-currentlimiterinthemainposition
- 17. 17 Fig.(2.7.2)-Fault-currentlimiterinthefeederposition Thetwobusesaretied,yetafaultedbusreceivesthefullfaultcurrentofonly onetransformer.Thetwobusesaretied,yetafaultedbusreceivesthefull faultcurrentofonlyonetransformer. Fig.-(2.7.3)-Fault-currentlimiterinthebus-tieposition. Themostdirectapplicationofafault-currentlimiterisinthemainpositionon abusBenefitsofanFCLinthisapplicationincludethefollowing: Alargertransformercanbeusedtomeetincreaseddemandonabuswithout breakerupgrades A large,low impedance transformercan be used to maintain voltage
- 18. 18 regulationatthenewpowerlevel I 2 tdamagetothetransformerislimited Reduced fault-currentflows in the high-voltage circuitthatfeeds the transformer,whichminimizesthevoltagedipontheupstream high-voltage busduringafaultonthemedium-voltagebus AnFCLcanalsobeusedtoprotectindividualloadsonthebus(Fig.4.7).The selectiveapplicationofsmallandlessexpensivelimiterscanbeusedto protectold oroverstressedequipmentthatisdifficulttoreplace,suchasunderground cablesor transformersinvaults.AnFCLcanbeusedinthebus-tieposition(Fig.4.8). Suchalimiterwouldrequireonly Asmallloadcurrentratingbutwoulddeliverthefollowingbenefits: separatebusescanbetiedtogetherwithoutalargeincreaseinthefaultduty on eitherbus. duringafault,alargevoltagedropacrossthelimitermaintainsvoltagelevel on theunfaultedbus. theparalleledtransformersresultinlowsystemimpedanceandgoodvoltage regulation;tap-changingtransformerscanbeavoided. excesscapacityofeachbusisavailabletobothbuses,thusmakingbetter useof thetransformerrating. 2.8SuperconductiveFault-CurrentLimiterConcepts 2.8.1TheSeriesResistiveLimiter
- 19. 19 Thesimplestsuperconductinglimiterconcept,theseriesresistivelimiter, exploitsthe nonlinearresistanceofsuperconductorsinadirectway.Asuperconductoris insertedinthecircuit.Forafull-loadcurrentofIFL,thesuperconductorwould bedesignedtohaveacriticalcurrentof2IFLor3IFL.Duringafault,thefault currentpushesthesuperconductorintoaresistivestateandresistanceR appearsinthecircuit. Superconductingfaultcurrentlimitersexploittheextremelyrapidlossof superconductivity (called "quenching)above a criticalcombination of temperature,currentdensity,andmagneticfield.Innormaloperation,current flows through the superconductor without resistance and negligible impedance. Ifafaultdevelops,thesuperconductorquenches,itsresistancerisessharply, andcurrentisdivertedtoaparallelcircuitwiththedesiredhigherimpedance. (Thestructureisnotusableasacircuitbreaker,becausethenormally- conducting superconductive materialdoes not have a high enough resistance.Itisonlyhighenoughtocausesufficientheatingtomeltthe material.) Superconductingfaultcurrentlimitersaredescribedasbeinginoneoftwo majorcategories:resistiveorinductive. InaresistiveFCL,thecurrentpassesdirectlythroughthesuperconductor. Whenitquenches,thesharpriseinresistancereducesthefaultcurrentfrom whatitwouldotherwisebe(theprospectivefaultcurrent).AresistiveFCLcan beeitherDCorAC.IfitisAC,thentherewillbeasteadypowerdissipation from AClosses(superconductinghysteresislosses)whichmustberemoved
- 20. 20 bythecryogenicsystem.AnACFCLisusuallymadefrom wirewoundnon- inductively;otherwisetheinductanceofthedevicewouldcreateanextra constantpowerlossonthesystem. InductiveFCLscomeinmanyvariants,butthebasicconceptisatransformer witharesistiveFCLasthesecondary.Inun-faultedoperation,thereisno resistanceinthesecondaryandsotheinductanceofthedeviceislow.Afault currentquenchesthesuperconductor,thesecondarybecomesresistiveand theinductanceofthewholedevicerises.Theadvantageofthisdesignisthat thereisnoheatingressthroughcurrentleadsintothesuperconductor,and sothecryogenicpowerloadmaybelower.However,thelargeamountofiron required meansthatinductiveFCLsaremuch biggerand heavierthan resistiveFCLs. Thequenchprocessisatwo-stepprocess.First,asmallregionquenches directlyinresponsetoahighcurrentdensity.Thissectionrapidlyheatsby Joule heating,and the increase in temperature quenches adjacent regions. [promotional language] GridON Ltd has developed the firstcommercial inductiveFCLfordistribution&transmissionnetworks.Usingauniqueand proprietary concept of magnetic-flux alteration - requiring no superconducting or cryogenic components - the self-triggered FCL instantaneouslyincreasesitsimpedancetenfold upon faultcondition.It limitsthefaultcurrentforitsentiredurationandrecoverstoitsnormal conditionimmediatelythereafter.ThisinductiveFCLisscalabletoextrahigh voltageratings. Thesuperconductorinitsresistivestatecanalsobeusedasatriggercoil, pushingthe bulkofthefaultcurrentthrougharesistororinductor.Theadvantageofthis
- 23. 23 consumeruseofelectricity.Inaddition,industrialuseofcomputersandother power-quality-sensitiveequipmenthasforcedtheutilitiestoprovidehigher qualityandmorereliablepower.Thequitesuccessfulapproachtoimproved powerqualityinJapanhasbeentoincreaseconnectionsbetweenvarious powersystems and to concentrate generation capacityin larger,more efficientunits. Increasing interconnection does,however,increase the maximum fault currentavailableatanypointinthesystem,andthisisrapidlyleadingtothe need forbreakerupgradesand system reconfigurations.Adding to the complexityofthesituationinJapanisthelimitedroom atsubstationsites, whichcanprecludebreakerupgrades. Theprimaryneed,asexpressedbymanagementoftheTokyoElectricPower Company (TEPCO),is fora limiterforthe nucleus ofthe Japanese transmissionsystem,the500kVtransmissiongrids.Inresponsetothisreal marketpulltherehasbeenaseriesofprogramstodevelopfaultcurrent limitersusingavarietyofmethods,withrecentfocusonsuperconducting limiters(Nakade1994).AlthoughFCLsarenotacomponentoftheNEDO budget,TEPCOhasreportedthatitspendsabout¥100millionperyear(~$1 million)onthisprogram,andsomeresistiveFCLworkisapparentlyincluded intheNEDObudgetunderthetopic"ResearchofSuperconductingMaterials andDevices”.Inthelate1980s,SeikeiUniversitymanufacturedasmall-scale three-phasecurrentlimitingreactoranddemonstratedsuccessfuloperation. Thisthree-phasesystem introducesalargeunbalancedreactanceinthe system tolimitcurrentsinthecaseofasingle-phaseshortandquenchesto introduceresistanceinthecircuitinthecaseofathree-phasefault.
- 24. 24 MitsubishiElectricCompany(MELCO)hasbeenparticipatinginaMITI/NEDO FCL programsince1990.ThisisaresistivelimiterapproachusingHTSfilmsona strontiumtitanatesubstratethathasdemonstratedlimitingof400Acurrents to11.3A.TheCentralResearchInstituteoftheElectricPowerIndustry (CRIEPI)hasdevelopedtheinductivelimitershowninFig.4.11(Ichikawaand Okazaki1995).Thisapproach,similartothoseofABBandSiemens-Hydro Quebec,usesacylinderofbulkBSCCO-2212orBSCCO-2223toseparatea normalcoppercoilfrom anironcore.Innormaloperation,thefieldfrom the coppercoildoesnotpenetratethesuperconductor;underfaultconditions, however,thecurrentinducedinthesuperconductorissufficienttodriveit normal,andthemagneticfieldlinkstheironyoke.Thisgreatlyincreasesthe inductanceofthecoppercoil,thusprovidingcurrentlimiting.CRIEPIwork hasfocusedonacmagneticshieldingperformanceofbulksuperconductors andtheirresponsestofaultcurrents. Inaddition,introductionofa"controlring"inthesystem toabsorbsomeof theenergydepositedduringafaulthasreducedthecooldowntimeofthe shieldfollowingafaultedstate.ThemostextensiveFCLprogram inJapan hasbeenthecollaborationbetweenTEPCOandToshiba.Thelong-term goal ofthisprogram isthedevelopmentofa500kVlimiterwitharatedcurrentof 8,000A.Initialdevelopmenthasbeenfocusedonadistributionlevellimiter designedfor6.6kV.
- 25. 25 Fig.(2.9.1)-SchematicdiagramoftheCRIEPIinductiveFCL AsshowninFig.4.12,theFCLisformedbyconnectingfoursuperconducting coilsinaseries-parallelconfigurationsothetotalinductanceisminimized. Onesetofcoilsisused foreach phaseofthedevice,and limiting is accomplishedbyquenchingthecoils. ThecurrentversionoftheFCLshowninFig.4.13usesaspeciallowacloss Nb-Ti conductor.Testsinacircuitwithanominalshortcircuitcurrentof25.8kA havesuccessfullydemonstratedlimitingtoabout4,000amps(Fig.4.14).
- 28. 28 discharge.Rodscanbefoundindifferentshapes,sizesanddesigns.Most aretoppedwithatall,pointedneedleorasmooth,polishedsphere.The funtionalityofdifferenttypesoflightningrods,andeventheneccessityof rodsaltogether,aresubjectsof manyscientificdebates. 3.1.2ConductorCables Heavycables(right)thatcarrylightningcurrentfrom therodstotheground. Cablesarerunalongthetopsandaroundtheedgesofroofs,thendownone ormorecorners ofabuildingtothegroundrod(s). 3.1.3GroundRods Long,thick,heavyrodsburied deep into theearth around a protected structure.The conductorcablesareconnectedtotheserodstocompletea safepathforalightningdischargearoundastructure. Theconductorcablesandgroundrodsarethemostimportantcomponents ofa lightning protection system,accomplishing the main objective of diverting lightning currentsafely pasta structure.The 'lightning rods' themselves,thatis,thepointyvertically-orientedterminalsalongtheedgesof roofs,donotplaymuchofaroleinthefunctionalityofthesystem.Afull protectionsetup,givengoodcablecoverageandgoodgrounding,wouldstill worksufficientlywithouttheairterminals.
- 29. 29 Fig.(3.1)-Lighteningsurge 3.2Lightningprotectionsystems-Whattheydoanddon'tdo Alightningprotectionsystem'sonlypurposeistoensuresafetytoabuilding anditsoccupantsiflightninghappenstohititdirectly,ataskaccomplished byprovidingagood,safepathtogroundforthelightningtofollow.Contrary tothemyths,lightningprotectionsystems: Don'tattractlightning Don'tandcannotdissipateorpreventlightningby'draining'astormofits charge Mostdon'toffersurgeprotectionforsensitiveelectronics Doofferfireprotectionandstructuraldamageprotectionbypreventingahot, explosivelightningchannelfrompassingthroughbuildingmaterials 3.2.1Howalightningprotectionsystemworks Withoutadesignatedpathtoreachground,alightningstrikemaychooseto insteadutilizeanyconductoravailableinsideahouseorbuilding.Thismay
- 30. 30 includethephone,cable,orelectricallines,thewaterorgaspipes,or(inthe caseofasteel-framedbuilding)thestructureitself.Lightningusuallywill followoneormoreofthesepathstoground,sometimesjumpingthroughthe airviaasideflashtoreachabetter-groundedconductor(watchanimation above).Asaresult,lightningpresentsseveralhazardstoanyhouseor building. Fire Firecanstartanywheretheexposedlightningchannelcontacts,penetrates orcomesnearflammablematerial(wood,paper,gaspipes,etc)inabuilding -includingstructurallumberorinsulationinsidewallsandroofs.When lightningfollowselectricalwiring,itwilloftenoverheatorevenvaporizethe wires,creatingafire. Sideflashes Sideflashescanjumpacrossrooms,possiblyinjuringanyonewhohappens tobeintheway.Theycanalsoignitematerialssuchasagasolinecanina garage. Damagetobuildingmaterials Theexplosiveshockwavecreatedbyalightningdischargecanblow out sectionsofwalls,fragmentconcreteandplaster,andshatternearbyglass. Damagetoappliances Televisions,VCRs,microwaves,phones,washers,lampsandjustabout
- 31. 31 anythingpluggedintoanaffectedcircuitmaybedamagedbeyondrepair. Electronicdevicesandcomputersareespeciallyvulnerable. Addingaprotectionsystem doesn'tpreventastrike,butgivesitabetter, saferpathtoground.Theairterminals,cablesandgroundrodsworktogether tocarrytheimmensecurrentsawayfrom thestructure,preventingfireand mostappliancedamage. 3.2.2Lightningprotectionfacts Rodsandprotectionsystemsdon'tattractlightning,nordotheyinfluence wherelightningwillstrike.Rodsorprotectionsystemsdonotandcannot preventlightning,norcanthey'discharge'thunderstorms. Lightning protection systems(including placementofrods,cables,and groundings)are custom-designed forindividualstructures and require complexengineeringtofunctionproperly.Theyshouldonlybeinstalledby qualifiedcontractors. Lightningprotectionsystemsdonotalwayspreventdamagetoelectronicsor computers.Youshouldstillunplugsuchdevicesduringthunderstormsto ensuresufficientprotection. 3.2.3Lightningdissipation/eliminationmyths Productscalled'lightningelimination'or'lightningdissipation'deviceshave arisenasaresultoftwomyths:one,thatathunderstorm'schargecanbe drainedorotherwiseaffectedbyobjectsontheground,andtwo,cloud-to- groundlightningdischargesbeginfrom theground.Theseproducts,thatare stillbeingsoldtoday,claim tobeabletopreventadirectlightningstriketo anyobjectonwhichtheyareinstalled.Thedeviceshavewidelyvarying
- 32. 32 appearances,butusuallyarecharacterizedbyametallicframewithhundreds ofsharp-pointedbristles,needlesorthinrods.Theframedesignsrangefrom comb-liketoumbrella-shaped. Thedevicesaresaidtopreventorreducedirectlightningstrikestoobjects onwhichtheyareinstalled,usingcoronadischargetoperformoneormoreof thefollowing:1.)todrainastorm ofitschargebeforelightningcanoccur,2.) tocreatealocalized'spacecharge'overtheprotectedareathatdiverts lightningstrikes,or3.)tomakeinitiationofupwardleadersfrom theobject moredifficult,therebyreducingthechancesofadirectsteppedleader- groundleaderconnection. Aswediscussedinourarticleaboutthunderstorm chargedissipation,the problemwiththesedevicesisthatwhiletheydocreatecoronadischarge,the rateofcharge'leakage'iscompletelyinsignificantincomparisontotherate of charge generation in the 10-mile-high,15 to 25 mile-diameter thunderstorm overhead!Noamountofman-madecoronadischargeonsuch asmallscalehastheslightestchanceofdrainingchargefasterthana gargantuanthunderstorm cloudisproducingit.Andalthoughsmall-scale coronadoeshelppreventtheinitiationoflaboratory-generatedsparks(such asfromVandeGraaffgenerators),thiscannotbeextrapolatedtoapplytofull -sizedlightningdischarges,whichareseveralthousandtimeslargerthanthe artificalcounterparts(seeourarticleoncomparingartificialandnatural lightning).Coronadischargefrom small'dissipators'isinsignificanttoafull- sizedthunderstorm andwilldonothingtoaltertheoccurenceorbehaviorof lightninginitsgeneeralvicinity. Cloud-to-groundlightningstrokesinitiatehighinthunderstorms,milesabove thesurfacewheregroundobjectshavenoeffect.Evenafterinitiationofthe
- 33. 33 discharge,thedownward-movingsteppedleaderis'blind'toobjectsonthe grounduntilitisveryclosetotheground,within50to100feet.Atthat distance,lightning willstrike within the very smallarea itis already descendingin,regardlessofanydevicesnearbythatclaim todivertor preventthestrike.Forexample,aphotographexistsofalightningstriketo theMerchandiseMartbuildingindowntownChicago.MerchandiseMartis veryclosetothe1,700foottallSearsTower,yetnoteventheSearsTower influencedthegroundconnectionofthisclosecloud-to-groundstroke. Inadditiontotheobviousscientificflawswiththeconceptoflightning 'dissipation'and'elimination'devices,theyhavebeenproventobeineffective inreal-worldinstallations.Many'lightningdissipation'devicesontowersand buildingshavebeenstruckdirectly.Despitetheevidence,theycontinuetobe sold,installedandpromoted. 3.3FuturePlans TEPCOwilldevelopathree-phaselimiteroverthenextthreetofouryearsand testitinthegridwithinthiscentury.Therearefew distribution-levelFCL applicationsseenintheTEPCO grid,however,andthecurrentplanisto introduce solid state breakers for distribution before installing superconductiveFCL.Thetrueapplicationforthe superconductingFCLisattransmissionvoltagesof500kV.Theview of TEPCOresearchersisthatthisvoltagerangewillrequiretheintroductionof HTScoils(rather thanLTS)toeliminatethehelium gasfrom thesystem.Introductionofa transmissionlevelFCLonthegridisanticipatedabout2010. 3.4Fault-CurrentLimitersInEurope
- 34. 34 By farthe mostcomprehensive FCL program in Europe is thatbeing conductedby collaboration between Electricité de France,GEC Alsthom,and Alcatel Alsthom Recherché.Theprogram'smaingoalistoprovideFCLsforthe225kVgridin France.ThegrouphaschosenaresistivelimiterbasedonLTSmaterialand hasdemonstratedeffectiveoperationat40kV (rms),withanindustrial demonstrationontheFrench63kVgridexpectedin1998.Evaluationofthe Frenchprogram isbeyondthescopeofthisWTECstudy,sonovisitwas madetothisproject.Verhaegeetal.(1996)provideanoverview ofthe technologyandprojectstatus. 3.5FCLPrograms TwositestheWTECpanelvisitedinEuropeaddressedFCL:ABBinBaden- Daetwil, Switzerland,andSiemensinErlangen,Germany.ABBispursuingafault- currentlimiterconceptverysimilartothatdescribedabovefortheCRIEPI program.Itisreferredtoasthe"shieldedironcoreconcept."Itusesawarm ironcoreenclosedbyasuperconductingshieldinafiberglassDewar.The copperprimarycoiliswoundexternaltothisDewar.ABBhasconstructed andtesteda100kWprototypeusingastackoffourBi-2212rings8cm long, and20cm indiameter.Operationwasat480Vwithfaultcurrentsof8kA.A newABBthree-phase1.2MW FCLisnowinoperationinapowerstationin Löntsch,Switzerland. SiemensisfollowingtworoutesforFCLinacollaborativeprogram with Hydro-QuebecCanada.AttheSiemenscorporatelabsinErlangen,thefocus hasbeenonresistivelimitersusingYBCOthinfilm meanderlinesonYSZor onYSZandsapphire(Gromolletal.1996).Theadvantageofthisapproachis
- 35. 35 thattheYBCOfilm hasahighnormalstateresistanceandisnotshuntedby normalmetal,as would be the case in a composite powder-in-tube conductor.Thefilm alsohasverylow heatcapacity,whichleadstorapid switchingtothenormalstate(<1ms)andthepossibilityofrapidcooldown. Analysisasof1996hasdeterminedthatbothpeaklet-throughcurrentand steadystatelimitingcurrentdecreaseasJcisraised.Inaddition,thedesign ofalimiterofusablesizedependsstronglyonJc--higherJcenablesamore compactdesign. Themajorfocusoftheprogram has,therefore,beenthefabricationof uniform high-Jc films ofYBCO.Techniques investigated have included pulsed laserdeposition (PLD),thermalco evaporation,and magnetron sputteringonbufferedp-YSZ,unbufferedp-YSZ,andsapphire.Biaxially textured YSZ bufferlayershavebeen fabricated on partofthep-YSZ substratesbyionbeam assisteddeposition.Currentdensitiesupto3x106 A/cm2havebeenachieved,ashavegoodlimitingperformanceandrecovery timeson theorderof1 second.Thenextmilestonefortheprojectis constructionofa100KVAlimiterusingacrycooler.Furtherdetailsofthis programaregivenintheSiemenssitevisitreport(AppendixD). TwoadditionalGermanFCLprojectsbeganinJanuary1997.Thefirstisa system studythatwillbefollowedbyconstructionofademonstrationFCL. ThisprojectisajointeffortbytheGermanutilitiesRWE,VEW,andBadenwerk, andbyEUSGmbHandFZK.Thesecondprojectinvolvingthedevelopmentof a smallinductive limiteris underthe auspices ofthe German Israel Foundation.TheGermanparticipantsareFZK,HoechstAG,andtheutility Badenwerk;theIsraeliparticipantsareTelAvivandBenGurionUniversities. TheworkatHydro-Quebechasresultedintheconstructionandtestofa
- 36. 36 numberofdevicessince1992(Fig.4.15).Thelatestsystem operatedat450 Vand95ampsforanominalpowerof43KVA.Twodifferentmaterialswere evaluatedforthesuperconductingshield:melt-castBi-2212from Hoechst, andcompositereactiontextured(CRT)materialfrom Cambridge.Although successfulcurrentlimiting wasdemonstrated,thelimiterthatused the Hoechstmaterialfailedduringa480Vshortcircuittestduetoafractureof thesuperconductor(Caveetal.1996).SubsequentanalysisbyHydro- Quebecindicatedthatthermalstressinthebulksuperconductorgaveriseto thefailure.Thenear-term futuredirectionofthisprogram willbeconcerned withimprovingthehomogeneity,criticalcurrentdensity,andresistivityofthe bulksuperconductor. CHAPTER-4 ADVANTAGES,DISADVANTAGESAND LIMITATIONS 4.1ADVANTAGES
- 37. 37 Fig.(4.1)-Powerratingoftheinductivelimitermodelsbuilt/testedatHydro-Quebec Relativelynarrow superconducting wirescan beused to carryhuge currents. Lessfuelrequiredtogenerateelectricitywhichwillleadtoareductionin costs Superconductingcableswillbesmallerandcanfitintoexistingconduits for expansionofthepowersupply. Environmentalbenefits from less pollution and more efficentpower production 4.1.1TransformingtheElectricityGrid
- 38. 38 Theelectricpowergridisamongthegreatestengineeringachievementsof the20thcentury.Demand,however,isabouttooverwhelm it.Forexample, thenorthAmericanblackoutof2003,whichlastedaboutfourdays,affected over50millionpersonsandcausedabout$6billionineconomicloss. Superconductor technology provides loss-less wires and cables and improvesthereliabilityandefficiencyofthepowergrid.Plansareunderway toreplaceby2030thepresentpowergridwithasuperconductingpowergrid. Asuperconductingpowersystem occupieslessrealestateandisburiedin theground,quitedifferentfrompresentdaygridlines.[13] 4.1.2ImprovingWide-BandTelecommunication Wide-bandtelecommunicationstechnology,whichoperatesbestatgigahertz frequencies,isveryusefulforimprovingtheefficiencyandreliabilityofcell phones.Suchfrequenciesareverydifficulttoachievewithsemiconductor- based circuitry.However,they have been easily achieved by Hypres's superconductor-basedreceiver,usingatechnologycalledrapidsingleflux quantum,orRSFQ,integratedcircuitreceiver.Itoperateswiththeaidofa4- kelvincryocooler.Thistechnologyisshowingupinmanycellphonereceiver transmittertowers. 4.1.3AidingMedicalDiagnosis Oneofthefirstlarge-scaleapplicationsofsuperconductivityisinmedical diagnosis. Magnetic resonance imaging, or MRI, uses powerful superconductingmagnetstoproducelargeanduniform magneticfields inside the patient's body.MRIscanners,which contain liquid helium refrigerationsystem,pickuphow thesemagneticfieldsarereflectedby organs in the body.The machine eventually produces an image.MRI machinesaresuperiortox-raytechnologyinproducingadiagnosis.Paul
- 39. 39 LeuterburandSirPeterMansfieldwereawardedthe2003Nobelprizein physiology or medicine,"for their discoveries concerning magnetic resonanceimaging,"underlyingthesignificanceofMRI,andbyimplication superconductors,tomedicine. 4.2LIMITATIONS Thereisamaximumcurrentthatsuperconductingmaterialscancarry. Costisprohibitiveforimmediatereplacementofexistingtechnologies. DevelopingcountrieswillnotbeabletoaffordthetechnologyAboveacritical currentdensity. superconductivitybreaksdownlimitingcurrent. Low criticaltemperaturesaredifficult,expensiveandenergyintensiveto maintain. Thematerialsareusuallybrittle,notductileandhardtoshape. Theyarealsochemicallyunstableinsomeenvironments. ItcannotfunctionwithACelectricity,astheswitchinginACdestroysCooper pairs. Thereisa"limit"tothecurrentpassingthroughthematerialbeforeitlosesits superconductingproperties. 4.3DISADVANTAGES Superconductingmaterialssuperconductonlywhenkeptbelow agiven temperaturecalledthetransitiontemperature. Forpresentlyknownpracticalsuperconductors,thetemperatureismuch below77Kelvin,thetemperatureofliquidnitrogen.Keepingthem belowthat
- 40. 40 temperatureinvolvesalotofexpensivecryogenictechnology.[13] Thus,superconductorsstilldonotshow upinmosteverydayelectronics. Scientistsareworkingondesigningsuperconductorsthatcanoperateat roomtemperature. Thegreatestdrawbackofsuperconductorsisthattheyonlyfunctionassuch attemperatureslowerthanitscriticaltemperature. Thistemperaturevariesbutistypicallyaround70Kelvinformostcommonly usedsuperconductors. Thereforeanysuperconductingapplicationisgenerallycoupledwithsome sortofactiveorpassivecryogeniccooling. Otherdrawbacksincludeprice,materialhandling,maximum currentcarrying capacityEtc.butthecryogeniclimitationmustbethebiggest. This is why the search for the near-mythical ‘room temperature superconductor’ is so important for the future of superconducting applications. 4.3.1Highertier Atthemoment,superconductorsonlyworkatverylowtemperatures.They havetobekeptverycoldwithliquidnitrogenorliquidhelium.Alotofworkis goingintodevelopingsuperconductorsthatwillworkatnormaltemperatures. Untilthishappens,theiruseswillbelimited. 4.3.2Highmeltingandboilingpoints Metallicbondsarestrongandalotofenergyisneededtobreakthem.Thisis whymetalshavehighmeltingpointsandboilingpoints. 4.3.3Conductingelectricity
- 42. 42 CHAPTER-5 APPLICATIONS 5.1BasicApplications Theproductionofsensitivemagnetometersbasedonsquids Fastdigitalcircuits(includingthosebasedonJosephsonjunctionsandrapid singlefluxquantumtechnology), Powerfulsuperconductingelectromagnetsusedinmaglevtrains,Magnetic ResonanceImaging(MRI)andNuclearmagneticresonance(NMR)machines, magnetic confinementfusion reactors (e.g.Tokamaks),and the beam- steeringandfocusingmagnetsusedinparticleaccelerators Low-losspowercables RFandmicrowavefilters(e.g.,formobilephonebasestations,aswellas militaryultra-sensitive/selectivereceivers) Fastfaultcurrentlimiters Highsensitivityparticledetectors,includingthetransitionedgesensor,the superconductingbolometer,thesuperconductingtunneljunctiondetector, thekineticinductancedetector,andthesuperconductingnanowiresingle-
- 43. 43 photondetector Railgunandcoilgunmagnets Electricmotorsandgenerators 5.1.1Lowtemperaturesuperconductivity Thebiggestapplication forsuperconductivityisin producing thelarge volume,stable,andhighmagneticfieldsrequiredforMRIandNMR.This represents a multi-billion US$ marketforcompanies such as Oxford Instruments and Siemens.The magnets typically use low temperature superconductors(LTS)becausehigh-temperaturesuperconductorsarenot yetcheapenoughtocost-effectivelydeliverthehigh,stableandlargevolume fieldsrequired,notwithstandingtheneedtocoolLTSinstrumentstoliquid helium temperatures.Superconductorsarealsousedinhighfieldscientific magnets. 5.1.2Particleacceleratorsandmagneticfusiondevices ParticleacceleratorssuchastheLargeHadronCollidercanincludemany highfieldelectromagnetsrequiringlargequantitiesofLTS.Toconstructthe LHCmagnetsrequiredmorethan28percentoftheworld’sniobium-titanium wireproductionforfiveyears,withlargequantitiesofNbTialsousedinthe magnetsfortheLHC’shugeexperimentdetectors. [2] Asmallnumberofmagneticfusiondevices(mostlytokamaks)haveusedSC coils.ThecurrentconstructionofITERhasrequiredunprecedentedamounts ofLTS(eg.500tonnes,causinga7foldincreaseinworldsannualproduction capacity). [3] 5.1.3High-temperaturesuperconductivity(HTS)
- 44. 44 Thecommercialapplicationssofarforhightemperaturesuperconductors (HTS)havebeenlimited. HTScansuperconductattemperaturesabovetheboilingpointofliquid nitrogen,which makes them cheaper to coolthan low temperature superconductors(LTS).However,theproblem withHTStechnologyisthat thecurrentlyknownhightemperaturesuperconductorsarebrittleceramics whichareexpensivetomanufactureandnoteasilyformedintowiresorother usefulshapes. [4] ThereforetheapplicationsforHTShavebeenwhereithas someotherintrinsicadvantage,e.g.in LowthermallosscurrentleadsforLTSdevices(lowthermalconductivity), RFandmicrowavefilters(lowresistancetoRF),and Increasinglyinspecialistscientificmagnets,particularlywheresizeand electricityconsumptionarecritical. WhileHTSwireismuchmoreexpensivethanLTSintheseapplications,this canbeoffsetbytherelativecostandconvenienceofcooling. Theabilitytorampfieldisdesired(thehigherandwiderrangeofHTS's operatingtemperaturemeansfasterchangesinfieldcanbemanaged);or cryogenfreeoperationisdesired(LTSgenerallyrequiresliquidhelium thatis becomingmorescarceandexpensive).[14] 5.1.4HTS-basedsystems HTShasapplicationinscientificandindustrialmagnets,includingusein NMR andMRIsystems.Commercialsystemsarenow availableineach category. [5] AlsooneintrinsicattributeofHTSisthatitcanwithstandmuchhigher magneticfieldsthanLTS,soHTSatliquidhelium temperaturesarebeing
- 45. 45 exploredforveryhigh-fieldinsertsinsideLTSmagnets.Promisingfuture industrialand commercialHTS applications include Induction heaters, transformers,faultcurrentlimiters,powerstorage,motorsandgenerators, fusionreactors(seeITER)andmagneticlevitationdevices. Earlyapplicationswillbewherethebenefitofsmallersize,lowerweightor theabilitytorapidlyswitchcurrent(faultcurrentlimiters)outweighsthe addedcost.Longer-term asconductorpricefallsHTSsystemsshouldbe competitiveinamuchwiderrangeofapplicationsonenergyefficiency groundsalone.(ForarelativelytechnicalandUS-centricviewofstateofplay ofHTS technology in powersystems and the developmentstatus of Generation2conductor. CHAPTER-6 FUTUREASPECTS 6.1SuperconductingTransmissionLines Since10%to15%ofgeneratedelectricityisdissipatedinresistivelossesin transmissionlines,theprospectofzerolosssuperconductingtransmission lines is appealing.In prototype superconducting transmission lines at BrookhavenNationalLaboratory,1000MW ofpowercanbetransported withinanenclosureofdiameter40cm.Thisamountstotransportingthe entireoutputofalargepowerplantononeenclosedtransmissionline.This couldbeafairlylowvoltageDCtransmissioncomparedtolargetransformer banksandmultiplehighvoltageAC transmissionlinesontowersinthe
- 46. 46 conventionalsystems. The superconductor used in these prototype applications is usually niobium-titanium,and liquid helium cooling is required. Current experiments with power applications of high-temperature superconductorsfocusonusesofBSCCOintapeformsandYBCOinthinfilm forms.Currentdensitiesabove10,000amperespersquarecentimeterare considerednecessaryforpracticalpowerapplications,andthisthresholdhas beenexceededinseveralconfigurations. 6.2PowerApplications,HighTc Powerapplicationsofhightemperaturesuperconductorswouldhavethe majoradvantageofbeingabletooperateatliquidnitrogentemperature.The biggestbarriertotheirapplicationhasbeenthedifficultyoffabricatingthe materialsintowiresandcoils.CurrentdevelopmentfocusesonBSCCOand YBCOmaterials. 6.3Fault-CurrentLimiters High fault-currents caused by lightning strikes are a troublesome and expensivenuisanceinelectricpowergrids.Oneofthenear-term applications forhigh temperaturesuperconductorsmaybetheconstruction offault- currentlimiterswhichoperateat77K.Theneedistoreducethefaultcurrent toafractionofitspeakvalueinlessthanacycle(1/60sec). Arecentlytestedfault-currentlimitercanoperateat2.4kVandcarryacurrent of2200amperes.ItwasconstructedfromBSCCOmaterial.
- 47. 47 6.4SuperconductingMotors Superconductingmotorsandgeneratorscouldbemadewithaweightof aboutonetenththatofconventionaldevicesforthesameoutput.Thisisthe appealofmaking such devicesforspecialized applications.Motorsand generatorsarealreadyveryefficient,so thereisnotthepowersavings associatedwithsuperconductingmagnets.Itmaybepossibletobuildvery large capacity generators for power plants where structuralstrength considerationsplacelimitsonconventionalgenerators. In1995theNavalResearchLaboratorydemonstrateda167hpmotorwith high-Tcsuperconductingcoilsmadefrom Bi-2223.Itwastestedat4.2Kand atliquid neon temperature,28K with 112 hp produced atthe higher temperature. 6.5SuperconductingMaglevTrains Whileitisnotpracticaltolaydownsuperconductingrails,itispossibleto constructasuperconductingsystem onboardatraintorepelconventional railsbelowit.Thetrainwouldhavetobemovingtocreatetherepulsion,but oncemovingwouldbesupportedwithverylittlefriction.Therewouldbe resistivelossofenergyinthecurrentsintherails.Ohanianreportsan engineeringassessmentthatsuchsuperconductingtrainswouldbemuch saferthanconventionalrailsystemsat200km/h. AJapanesemagneticallylevitatedtrainsetaspeedrecordof321mi/hin 1979 using superconducting magnetson board thetrain.Themagnets inducecurrentsintherailsbelowthem,causingarepulsionwhichsuspends thetrainabovethetrack.
- 48. 48 FutureApplicationsofSuperconductivity With such features as zero resistivity and high current density, superconductivity provides low-loss operation and high magnetic field, featuresinconceivablewithnormalconductivity.Accordingly,expectations arehighthatsuperconductivitywillimprovetheperformanceofelectrical appliances. Thesuperconductingstateoccurswithinlimitedtemperature,magneticfield and current density ranges. Thanks to the discovery of oxide superconductorsofhighcriticaltemperatures*1andtheincreasedcritical current density*2 of superconducting wires made from them, superconductivityisexpectedtobeusedinabroaderrangeofcommercial fields. Futuristicideasfortheuseofsuperconductors,materialsthatallowelectric currenttoflow withoutresistance,aremyriad:long-distance,low-voltage electricgridswithnotransmissionloss;fast,magneticallylevitatedtrains; ultra-high-speed supercomputers;superefficientmotors and generators; inexhaustiblefusionenergy–andmanyothers,someintheexperimentalor demonstrationstages. Butsuperconductors,especiallysuperconductingelectromagnets,havebeen around for a long time.Indeed the first large-scale application of superconductivity was in particle-physics accelerators,where strong magnetic fields steerbeams ofcharged particles toward high-energy collisionpoints.
- 49. 49 Accelerators created the superconductorindustry,and superconducting magnetshavebecomethenaturalchoiceforanyapplicationwherestrong magneticfieldsareneeded– formagneticresonanceimaging(MRI)in hospitals,forexample,orformagneticseparationofmineralsinindustry. Otherscientificusesarenumerous,from nuclearmagneticresonancetoion sourcesforcyclotrons. Someofthestrongestandmostcomplexsuperconductingmagnetsarestill builtforparticleacceleratorslikeCERN’sLargeHadronCollider(LHC).The LHC uses over1,200 dipole magnets,whose two adjacentcoils of superconducting cable create magnetic fields thatbend proton beams traveling in opposite directions around a tunnel 27 kilometers in circumference;theLHCalsohasalmost400quadrupolemagnets,whose coilscreateafieldwithfourmagneticpolestofocustheprotonbeamswithin thevacuumchamberandguidethemintotheexperiments. TheseLHCmagnetsusecablesmadeofsuperconductingniobium titanium (NbTi),andforfiveyearsduringitsconstructiontheLHCcontractedformore than 28 percentofthe world’s niobium titanium wire production,with significantquantitiesofNbTialsousedinthemagnetsfortheLHC’sgiant experiments. What’smore,althoughtheLHCisstillworkingtoreachtheenergyforwhichit wasdesigned,theprogram toimproveitsfutureperformanceisalreadywell underway. Designingthefuture “Enablingtheacceleratorsofthefuturedependsondevelopingmagnetswith muchgreaterfieldstrengthsthanarenowpossible,”saysGianLucaSabbiof
- 50. 50 BerkeleyLab’sAcceleratorandFusionResearchDivision(AFRD).“Todothat, we’llhavetousedifferentmaterials.” Fieldstrengthislimitedbytheamountofcurrentamagnetcoilcancarry, whichinturndependsonphysicalpropertiesofthesuperconductingmaterial suchasitscriticaltemperatureandcriticalfield.Mostsuperconducting magnetsbuilttodatearebasedonNbTi,whichisaductilealloy;theLHC dipolesaredesignedtooperateatmagneticfieldsofabouteighttesla,or8 T —hundredsofthousandsoftimeshigherthanEarth’smagneticfield. TheLHCAcceleratorResearchProgram (LARP)isacollaborationamong DOElaboratoriesthat’sanimportantpartofU.S.participationintheLHC. SabbiheadsboththeMagnetSystemscomponentofLARPandBerkeley Lab’s Superconducting MagnetProgram.These programs are currently developing acceleratormagnetsbuiltwith niobium tin (Nb3Sn),abrittle materialrequiringspecialfabricationprocessesbutabletogenerateabout twicethefieldofniobium titanium.Yetthegoalformagnetsofthefutureis alreadysetmuchhigher. “Amongthemostpromisingnewmaterialsforfuturemagnetsaresomeof thehigh-temperaturesuperconductors,”saysSabbi.“Unfortunatelythey’re verydifficult
- 51. 51 CONCLUSION Thepurposeofthispresentationwasthestudyofsurgecurrentprotection usingSuperconductors.TheSuperconductorFaultCurrentLimitersoffers efficientadvantagestopowersystemsandopensupamajorapplicationfor superconductingmaterials. Surge suppressors should be used as a mailer of habit with all semiconductorbased electronicandcomputerhardwareincludingperipheralssuchasprinters monitors, externaldiskdriversandothers.Butitcan’tbereliedupontoprovide protectionagainstlightninginducedtransients.Sothesafestprocedureto ensurethatallsusceptiblehardwareistounplugthesuppressormainpower cordduringlightning.
- 52. 52 References [1] KEGray,DEflower–superconductingfaultcurrentlimiters [2] IEEEtransactiononappliedsuperconductivity,march1997 [3] HMRosenberg–TheSolidState.http://www.amazon.co.uk/Solid-State Introduction-MaterialsEngineering [4] CPPoole,HAFarachandRJCreswick,Superconductivity(Academic PressInc, [5] SanDiego,California,1995(~ £40)http://www.amazon.co.uk/SuperconductivityCharles-PPoole/ [6] SuperconductivitybyW.Buckel,ReinholdKleiner [7] Superconductivity:PhysicsandApplicationsbyKristianFossheim,Asle Sudboe [8] Superconductivity:fundamentalsandapplicationsbyWernerBuckel [9] ieeexplore.ieee.org›...›Spectrum,IEE [10]http://www.scribd.com/doc/115890153/surge-current-protection-using- superconductors [11]http://jntuhome.com/surge-current-protection-using-superconductors- seminardownload-full-paper-eee-seminar-topics/