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1)WhatisanImpulse?An Impulse is defined as aunidirectional timevaryingvoltage that rapidly rises to apeakvalue anddrops considerablyslowly to 0magnitudewith respect to time. It simulates an over voltage. Overvoltage is by definition a time varyingvoltagewhosepeakismorethanthepeakofthesystemhighestratedvoltage.ThemaindifferencebetweenanOvervoltageandanImpulse is that, theformer isanoutcomeofanaturalphenomenonandthelatter isamanmadepulseusuallyforsimulatinganaturalevent.

2)DefineaLightningImpulse.Lightningisanaturalphenomenonwhichisoneofthemajorcontributorsforfaults/disturbancesontransmissionlines.Alightning strokeon the transmission lineproduces travellingwaves atnear the speedof light,which superimposeon thealready travelling line voltage on the transmission line.Thus, the insulation of the equipments connected to the line arevulnerablefordamage.Lightningimpulsesimulatesalightningovervoltage.Astandardnotationforspecifyingalightningimpulseisawavecharacterizedbya1.2/50spulse.ThissimulatesLightningorAtmosphericovervoltages.Note:1.2sdenotesthetimetakenbythewavetoreachitspeak(0.9p.u)50sdenotesthetimetakenbythewavetoreachto50%ofitspeakvalue(0.5p.u)

3)Whylightningovervoltageisdangerous?Alightningstrokeisfollowedbyoneormoretravellingwaves,whoseamplitudeisoftenlimitedbythemaximuminsulationstrengthoftheconcernedoverheadtransmissionline.Therateofvoltageriseduetothesestrokesisdirectlyproportionaltothetransmissionlinecircuitparameters.Travellingwaveswithsteepwavefrontmaystresstheinsulationofthehighvoltageequipments.Ifthelightningovervoltageexceedsthecriticalwithstandovervoltageoftheinsulation(s),itmaypuncturethesame.

4)WhatcomprisesoftheLightningimpulsevoltageprotectionsystem?Basically a lightning pulse is characterized by a s pulse. So, the foremost consideration should be a protection systemwhichoperatesinthenanosecond(ns)scaleoftimeorfaster.MetalOxideVaristor(MOV)operatesonafewhundrednanosecondscale.Itsapproximateresponsetimeiscloseto50100ns.

5)WhatisSwitchingImpulse?AstandardSwitchingImpulseischaracterizedbya250/2500spulse.Thenotationsforthefronttimeandtailtimearethesameasmentionedinthecaseofalightningimpulse.Itsimulatesaswitchingovervoltage.

6)Whatarethemajordifferencesbetweenabovetwopulses?Aswitchingovervoltageisinitiatedbymeansofchangingsystemtopologies.Thebestexamplecanbeasuddenswitchingof a charged capacitor in and out of a transmission line which is charged. This gives rise to a switching pulse as theimpedanceofthenetworkchangesinafairlylessamountoftimespan.Switchingovervoltageisnotanaturalphenomenon.The peak voltage of the disturbance is dependent on the systemvoltage.ASwitching surge is initiated fromwithin thesystem.Itdoesnotdependonatmosphericconditions.On the other hand, a lightning overvoltage is a natural phenomenon and it depends on atmospheric conditions. Theprobabilityoflightningataspecifiedpointonthegroundcannotbepredicted.Themagnitudesofcurrentandthevoltagethat are induced into the system after a lightning strike are considerably very high ( voltages can be as high as severalmillionvolts,andthecurrentshavepeakamplitudesexceedingseveralkiloamperes,typicallyprobablepeakcurrentis40kA). Themagnitude of peak voltage or the peak current does not depend upon the system voltage or the current. Thedisturbanceisinitiatedbyanexternalsource.

Above all, both the impulse wave discussed above superimpose a high energy, high frequency pulse on the alreadypropagatingtravellingwavethroughthetransmission/distributionsystem.

7)WhatarethefactorsthatmakeImpulseVoltageGenerationanecessity?Transmission and distribution lines stretch to several thousand kms, housing some very costly equipments like powertransformers,relays,circuitbreakers.Alltheseequipmentsareunderaconstantthreatofbeingstruckbynaturalforcesofnaturelikethunderstorms,lightningetc.Foremostconsiderationistoprovidecontinuouspowersupplyforindustries/homesandprotectionofseveralcostlyequipments.Impulsevoltagegenerationisveryessentialtosimulatealightningpulsetouseitfortestingofequipmentsforthesame.

8)Whatdoyoumeanbyflashover?Whenaself restoring insulationbreaksdownthrough itsweakest lineof force, it iscalleda flashover. Inotherwords,a

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spontaneousshortcircuitofaconductingairgapiscalledaflashover.Itisduetothesimultaneoustransferofallthevalencebandelectronstotheconductionband.Inviewofanimpulsevoltagetest,aflashoverisaphenomenonwhichdischargesalltheenergyacquiredbymeansofashortcircuitbetweenthe2conductingtestspheres.

9)WhatisthedifferencebetweenaFlashoverandaPuncture?Usually, a self restoring insulation, recovers its insulating properties after a brief period of time, then its termed as aflashoveroratemporarydischarge.

Ontheotherhand,apuncture iswhenasolid insulationfails torecoverback to itsoriginal insulatingproperties.This isbecause the magnitude of the disturbance might have been high enough to cause a permanent damage through theinsulation.

10)Whatiscriticalflashovervoltage?Theimpulsebreakdownofairdoesnotresultinasinglevalueforaparticulartemperatureandpressure.Thisfluctuationinbreakdownstrengthscanbebetterexpressesintermsofastatisticaldistribution.Insimpleterms,thebreakdownofanairgapispurelyprobabilisticinnature.Thebestfittothiscanbeobtainedusingastatistical(Gaussian)distribution.

CriticalFlashovervoltage(CFO),inviewofphysics,isdefinedastheminimumvoltageabovewhichalltheelectronsintheenclosed gap acquire enough energy to cross the valence band into the conduction band. Gaussian distribution ischaracterized by mean and standard deviation. Mean value of 50 % value is termed Critical Flashover Voltage.Determinationofbothmeanandstandarddeviationistermedasdeterminationofwithstandstrength.

11)Whatissurgeimpedance?Whatisitssignificance?Surgeimpedanceorcharacteristicimpedance,generallyexpressedasZL.Itistheratioofvoltageandcurrentpropagatingalongthelineintheabsenceofelectricalreflections.Inotherwords,whenthetransmissionlineisterminatedbyaresistorequaltothesurgeimpedanceoftheline,itappearsasaninfinitelylongtransmissionlinetothesource.Essentially,thesurgeimpedanceresistancephysicallybringsdownthereflectivecoefficientsmagnitudetonear0.ItsSIunitisohms.

Thelineisterminatedwitharesistancewhosevalueisequaltothevalueofthesurgeimpedanceofthetransmissionline,soastogettheexactImpulsevoltagewavetappedintotheCRO.If,forinstance,thelineisnotterminated,apartofthewavewhichpropagatedtotheloadwillbelostduetoreflectionsbecauseofthedifferenceincoefficientsofreflectiveindices.

12)WhatarethevariousapplicationsofIVG?Impulsevoltagegenerationfindsitsplaceinavarietyoffields.Someofthemanyapplicationsarestatedbelow:

1.Generationofhighpowermicrowaveusingvirtualcathodeoscillatordevices2.Lightningtestingoncablesandinsulatorsat1.2/50sand8/20s.3.Materialanddielectrictesting.4.Breakingofrawdiamondsinmineralogy.5.Flashxraygeneration.6.Kiloamplinearaccelerators.7.Generationofplasmafocusing.8.TosimulateNuclearExplosions.