Various Benefits for Line Surge Arrester

Application & Advantages of Externally Gapped Line Arresters

Florent Giraudet, Siemens AG, Germany Florent.giraudet@siemens.com

1. Introduction

The application of Line Surge Arresters (LSA’s) s is well known for its cost-effective performanceimprovementintheelectricitysupplyindustry.Althoughtheirreductionofoutagesfromlightningactivityand poor grounding make their application vital, there is frequent resistance to make their usagecommonplace. LSA’s are generally installed as a retrofit application, where conventional mitigation-measures have not shown significant improvement. Besides the lightning performance improvement,variousbenefitscanbederivedfromtheuseofLSA’s.Someusersapplythistechnologyforsafetyconcernssuchaspreventingpopulationandinjurydamage,switchingsurgecontroltooptimizestructures,reducingclearances, line uprating and compaction, and live-line working. Even though the implementation ofExternallyGappedLineArresters (EGLA’s) canachieveoutstanding resultsbyminimizing investment, thissmart protection device shows a mild growth, except for some specific countries. Users generallyunderestimatetheadvantagesandbenefitsoftheEGLAsolution,whilemostofthemfaceissueswithNon-GappedLineArresters(NGLA’s).TheEGLAapplicationnotonlysignificantlyimprovestheperformanceandoperation of the power system, but also enhances the design and lowers the cost of construction andmaintenance. The benefits and costs savings become even more important if properly considered andintegrated in the design stage of the transmission lines. Unlike with substations, line designers andengineersdonot consider theapplicationof linearresters as systematic inoptimizing their transmissionline systems. Surge Arresters are among the most reliable components on the grid. When designed,dimensioned and installed properly on transmission lines they must be as reliable as the insulator andpower linehardware.Thispaper intends topresentSiemensknowledge,experience, feedbackand latestinnovationsforcompactandcost-efficientlightning-prooftransmissionlines.

2. Mainuseasretrofitapplicationforlightningperformance

Acrosstheglobe,mostoftheutilitiesandtransmissionsystemoperators(TSOs)areconsideringLSA’swhenconventionalmethodsdonotprovidesatisfyingresults.LSA’sareoftenconsideredascurativemaintenancebut not proactively in the design stage of the transmission lines. Table 1 tries to describe the differentmitigationsmethodswhicharecommonlyusedtoimproved

Methodoflightningprotection

Feasibilitycheck Economicviability

Addorextendshieldingwire(s)

-Linesaregenerallyunshieldedforspecificreasons(rivercrossings/clerancesissues)-Stronglydependsonlinedesign/parameters-Noteffectiveforhighfootingresistanceandbadsoilresistivity

-Highmaterial&laborcosts-Powerinterruptionisrequired-Non-economicalsolution

IncreaseBIL(insulatorreplacementorextension)

-Stronglydependsontowerdesignandsystemclearances-Leadstotravelling/propagatingwavesonthelineforhighfootingresistances!

-Highmaterial&laborcosts-Powerinterruptionmightberequired-Non-economicalsolution-Experiencerequiredforinsulatorsreplacementinliveconditions

Improvedtowerfootingresistances

-Additionalcoppercounterpoisesandgroudingextensionmightbecompletelyinefficientwithhighsoilresistivity-Onlyefficientforshieldedlines-Eliminatesonlybackflashoversanddoesn‘tinfluenceshieldingfailures.

-Moderateinstallationcosts-Improvement&cost-efficiencyisnotguaranteed

InstallLineSurgeArresters -Versatile&Largefesability-Highestprotectiveeffectivenessevenforhighfootingresistancesinallterrains-Eliminatealltypesoflightningfailures.

-Lowmaterial&laborcosts-Severaloptionsforlive-lineinstallation.-Cost-efficientsolution.-PossibilitytoreplaceinsulatorsandEGLAasanintegratedsolution!

Table1:Comparisonofmitigationmethods

3. VariousBenefitsforLineArresterApplication Besides the lightning performance improvement, various benefits can be derived from the use of LSA’s.Someusersapplythistechnologyfordifferentpurposesas:

• switchingsurgecontroltooptimizestructuresandreducingclearances,• lineupratingandcompaction,• safetyconcernssuchaspreventingpopulationandinjurydamage,• andlivelineworkingtoreducetheminimumapproachdistance

LSA’s,especiallytheEGLA’s,haveanuntappedpotentialtolowerthecostsofthelineconstructionsandtheoperation.

a. SwitchingSurgeControlReducingsurgefactorshelpstolowerclearancesandthereforeoptimizetransmissionlinedesigns.Switching over-voltages are typically associated with high speed reclosing on EHV transmission lines.Strategicallyplaced,LSA’shavebeenusedinsteadofclosingresistorsand/orcontrolledswitchingschemesto control switching over-voltages along EHV transmission lines. Unlike lightning related applications,wherearrestersmaybeinstalledonconsecutivestructures,arresterstocontrolswitchingsurgesareonlyneededatspecificlocationsalongtheline.Ateachlocation,arrestersareusuallyinstalledinallphases[9].LSA’salongthelinemaytypicallyrequireoneenergyclasslowerthanwhatisneededforarrestersinstalledat the lineends in thesubstations.Transientsimulationsshouldbeperformed inorder todeterminetheamountofenergyabsorbedbythearresters.LSA’sforthisapplicationaretypicallyusedforsystemvoltagesof245kVandabove.However,withtheincreasinguseofcompactorupratedlinedesigns,thisapplicationisnolongerreservedjustforEHVlevels[4][5].Even though EGLA’s are the most suitable design for overhead line application, NGLA application isdedicated to control switching over-voltages. For the IEC standard 60099-8, the external gap of EGLA isdesignedandtestedtowithstandswitchingtransients.Inaddition,itisquitechallengingtodefineanEGLAspark gap distance to withstand power-frequency wet and guarantee operation in case of switchingimpulsewet. The tolerances in the gap adjustment are very narrow. Furthermore, for switching control,only few arresters at selected location are required, therefore the easiestway is obviously to useNGLAbecauseitmightbenotworthyofverificationinsulationcoordinationforonlyfewunits.Asanexampleofproject,Figure1ashowsthereducedswitchingsurgefactorfrom2.2(redcurve)to1.8(bluecurve).Ithelpedtheutilitytogettheirexistingclearancesacceptable.Figure1bshowstheinstallationoftheNGLAinliveconditionsforthesameproject.

Figure1a.SwitchingsurgefactorimprovementFigure1b.Liveinstallationforswitchingcontrol

b. LineUpratingandCompactionCompactandUpratedLinesarenotanewtopic forutilitiesand linedesigners.Significant improvementshavebeenmadeinthelastdecadeswithinsulatorscompositetechnology,buttheconcepthasnotreachedyet its final stage since line arresters have not been considered to reduce clearances and substantiallydecrease the arcing distances. Due to the necessity for the utilities to build discrete and aesthetic linestructuresandthedevelopmentof thecomposite linepostand longrod insulators,compact linedesignsare a realistic alternative to the standard line designs [9]. Even though the LSA’s offer outstandingpossibilitiestooptimizelinecompaction,mostofthedesignershavenotconsideredthetechnologyduetoalackofexperienceandcooperationwitharresters’manufacturers.Lineuprating involves the increaseof theoperatingvoltageandthecurrentcarryingcapacitybykeepingthe existing structure. Both conductors’ bundle and cross-arms/insulators stringsmust be redesigned. Ingeneral,forsuchamodification,severalissuesmustbeconsideredasphaseclearancesorinsulatorlength.LSA’sbecomeextremelyhelpfulandeconomicallyjustifiedtoconverttheexistinglinestohighervoltageswithout changing the clearances and insulator strings. It is certainly required that insulators strings andhardwarecomponentsmustbechangedduetoRIV/Coronastressdependingontheexistingcomponentsandperformance.For line uprating and compaction, the principle is the same. Line arresters are controlling overvoltagestresseson the line insulation. The conventional ratings,mainly lightning impulsewithstand level, of theinsulatorscanbereducedtotheprotection leveloftheEGLA.The lightning impulsesparkovervoltageoftheEGLAgapmustbelower(orequal)thanthelightningimpulsewithstandvoltageoftheinsulatorstring.TheprotectionleveloftheEGLAisthemaximumresidualvoltagewhenthelightningovervoltageinitiatesaflashoveron theEGLA.When the tests areperformed to verify the insulation coordinationbetween theEGLAsparkgapandtheinsulatorassembly,theflashovercanonlyoccurontheEGLA.TheSeriesVaristorsUnit (SVU)of theEGLAhas thecapabilityofextinguishingthearcwithout thenecessityofa linebreakeroperation.Uptoacertainvoltagelevel(330kV),switchingsurgesbecomeacriticalfactorforthedimensioningofthestring. EGLA’s are not generally designed to be capable of protecting the line insulation from switchingsurges. Depending on the system parameters, several mitigation methods can be used to reduce theswitchingsurgefactorsasimprovingstationclassarresters,installationclosingresistorsoncircuitbreakerorinstallationofshuntreactors(primarilyusedtostabilizethevoltageduringloadvariations).ButfewNGLA’salongthelinearedefinitelythemostconvenientandeffectivesolutiontoreduceswitchingsurgecontrol,minimizethegapdistanceofEGLAonEHVsystemsandthereforemaximizethecompactionortheupratingoftheline.

Figure2a.Conventionallineuprating245kV->420kVFigure2b.EGLALineuprating50kV->123kV

c. Safetyconcerns-Preventpopulationinjury&equipmentdamagesInFrance,sincethe60’s,growingurbansareasandextensionofHVtransmissionsystemshasresulted invicinityproblemsbetweensuburbanlivingareasandoverheadlines.ExternallyGapedLineArrestershavebeenusedinthosesensitiveareasor“hotzones”ashouses,factory,parking,etc[14].Themainpurposeisto increase safety by significantly reducing the risk of having dangerous touch or step voltages due topower frequency earth potential rise following the insulation flashover. Issues of touch potentialcoordinationbecomeespeciallyimportantwhensurgearrestersareusedtosubstituteshieldwiresastheonlyformoflightningprotectiononMVandHVlines[9].Suchgroundfaultscanresultinseveralkilovoltsforhundredsofmilliseconds.DuringanEGLAoperation, the followcurrent thatmustbeextinguishedbythe SVU (Series Varistors Unit) is limited to few amperes in the range of 2-3A with durations of 5-10milliseconds.As it isthecasefor lightningperformance improvement,aduediligenceprocessstartswiththeinvestigationofthegroundingqualityandtheconfigurationofshieldwires.Conventionalmethodsareoften costly and ineffective. French engineers have understood in the 90’s the advantages of EGLAapplicationas its compactdesignand reliability. FrenchutilitieshaveadoptedandexperiencedEGLA forsafetyconcernsandlightningperformanceimprovementforalmost20yearswithverypositiveresultswithrespecttolong-termreliability,reductionofoutagesandofcoursesafetyimprovement.Furthermore, there isapotentialuseof LSAs formitigating the riskofgaspipeline failure from lightningstrikes but also to prevent electro-magnetic dysfunction in IT buildings that can be caused insulatorsflashovers.

Figure3a.Sensitiveareaor“hotzone” Figure3b.Sensitiveareaor“hotzone”

d. Livelineworking-TemporarilyReduceMinimumApproachDistanceLive working is an established part ofmaintenance activities formany utilities. A continued concern is,however,toensurethatworksitewillnotbeexposedtounacceptablyhighovervoltagesduringsuchwork.SomeUtilitiesusePortableProtectiveAirGap (PPAG) to limitovervoltagesat theworksite forenergizedmaintenancework, but there are several disadvantages associatedwith it.As an alternative, EPRI in theUSA is investigating the use of LSA’s for this purpose. LSA’s may provide an enhanced protection ofworkers, provided that practical ways be found to ensure the arrester integrity prior and during theexecutionofthemaintenancetasks.Regularinsulationtestsontheactivepartarenotrelevant,asattheleast the MCOV should be applied to properly assess the condition. NGLA’s are preferable since thepurpose during live line working is to protect linemen against switching overvoltages. Energy ratingsrequirements,specificforeachlineandworksite,mustbeaccuratelydefinedtooptimizetheweightofthearresterwhichisanimportantfeatureforhandlingandinstallationontheline.TheNGLAisconnectedfromthephaseconductortothetower.Thecrestvalueoftheovervoltageswhichmightexistattheworksiteisdeterminedby the arrester rating, its residual voltageof protection level in relationwith the protectiondistance.Thearresterofferstheadvantagethat itsoperationisnotassociatedwithapowerarcas isthecase for a PPAG, thereby minimizing the risk of exposing workers to power-arcs. The installation ofarrestersonallphasesonstructuresadjacenttothework(worksitestructurenotbeingequippedwithLSA)sitemaybe sufficient toprotectworkers, dependingon surrounding grounding conditions. LineArrestershouldbeusedwithoutadisconnectiondeviceinthisapplicationtoensurebetterworkerprotection[9][3][16].

Figure4:InstallationofNGLAforlivelineworking

e. LoweringcostsandlossesinyoursystemRockysoilsand ingeneralareaswithveryhighsoil resistivitycanbeaverydifficultproblemofachievinggood lightning performance in the poor grounding conditions. EGLA’s can be used to replace costly andineffectiveadditionalcounterpoisesandgroundingextensions.SimulationscanbeperformedtoguaranteetheEGLAperformancebelowacertainmeasuredfootingresistancewithreasonablesafetymargin.EGLA’scanbeintegratedtotheinsulatorsstringstosimplymountingandworkonsite.Costsavingsaregenerallyunderestimated.A study conducted by Arresterworks has demonstrated the possibility to significantly reduce the capitalcostsoftheconstructionandlowerlossesontransmissionlinesbyinstallingEGLAoneachtowerandeachphase instead of using an Overhead Ground Wires (OHGW’s) [1]. Most of the utilities and EPCs arereluctantatthisstagebecauseshieldwiresandOPGWarerequired intheproject’sspecificationandthelightning strokes can physically damage the phase conductors. Anyhow, the costs savings are quiteimpressiveandlinedesignershavesolutionstoreinforcetheconductors’designandinstallopticalfibresindifferentlocations.

4. Comparisonof2differentapplications:NGLAvs.EGLA Non-Gapped Line Arresters (NGLA) is a basic adaptation of the substation’s arresters used to protectvaluable equipment like power transformers. The active part is directly connected between the phaseconductorandthegroundedstructure.TheresidualvoltageoftheMOVcolumnwilllimittheovervoltagesacrosstheinsulatorsandpreventflashoverwhenBILisexceeded.NGLArequiresaclampingsystemtotheconductor, a Ground LeadDisconnector (GLD) and inmost cases a grading ringwith a corona ring. Themechanicalandmountingconsideration isanessentialstepwhiledesigningNGLA.Theyare installedandusedunderharsherserviceconditionsthanothersurgearrestersatsubstations.Thecompleteassemblyispermanentlystressedoveritslife.NGLAinstallationismadeinsuchawaythattheymaymoveduetowindand/or line swinging and vibration. Users and manufactures must pay a special attention to avoidprematuremechanical failures[15].Togalvanically isolatethe linesurgearrester fromthe linevoltage intheunlikelyeventofafaultorthermaloverload,adisconnectoris installedinseries.Itautomaticallyandimmediatelydisconnects the line surgearrester from the line voltage. This allows theaffectedoverheadline to be reenergised and operated until convenient replacement can be scheduled [4]. Relevantstandards: IEC 60099-4 / IEEE C62.11. ApplicationGuide IEC 60099-5 / IEEE C62.22, IEEE 1243 (lightningperformanceimprovement).

Figure5:NGLA-Directlyconnectedtopowerline

Externally Gapped Line Arresters (EGLA) have an external spark gap placed in series that galvanicallyisolatestheactivepart (SVU–SeriesVaristorUnit)of the linesurgearrester fromthe linevoltageundernormal conditions. In caseof lightning, the sparkgap is ignited, and theovervoltage is safelydischargedthroughtheresultingarc.Theactivecomponent(SVU)limitsthesubsequentcurrenttoensurethatthearcis extinguished within the first half-cycle of the operating power-frequency voltage. After this, the linesurge arrester immediately returns to standby condition. No breaker operation required [2]. The activecomponent can have either one or two SVUs (on each side) depending on the system voltage level anduser’srequirements.Relevantstandards:IEC60099-8

Figure6:EGLA(1SVU)-Isolatedfrompowerlinethroughaseriesgap

5. Asmarterprotection-AdvantagesoftheEGLAapplicationa. Material

LessmaterialisneededtodesignanEGLA.MetalOxideVaristors(MOV’s)haveasmallerdiametersincetheenergy handling requirements are lower than NGLA. The EGLA must not handle TOVs and switchingovervoltages which is not relevant for lightning performance. The rated voltage (Ur) of the EGLA has adifferent definition than a typical rated voltage for NGLA application. Ur of the EGLA represents themaximumphase-to-ground voltage to be extinguished safely during the follow current interruption test.LessMOVsare required in comparison toaNGLAdesignwhereUr represent the temporaryovervoltage(TOV)withpriorduty.Figure7showstheVoltage-Currentcharacteristicsof192kVratedNGLAandEGLA.EGLA’sprotectionlevelhasalwayslowerresidualvoltages.Additional hardware (support, counterweights) might be required for retrofit application if the existingstringsdonotallowdifferently.Fornewtransmissionlines,the“smart”integrationoftheEGLAmakesthesolutioncompactandcost-effective.Thehardwarecanbeeliminated.EGLAarespecificallydesignedtoimprovelightningperformance.NGLA’sareaneasycopingofthestationclassarresterswhichisnotoptimized.NGLA’srequireaclampingsystemtotheconductor,aGroundLeadDisconnector(GLD)andagradingringinmostcases.

Figure7:V-IcurvesEGLAvs.NGLA192kVrated

b. Costs

Asdescribedabove, lessmaterialmeans lowercosts.NotonlymaterialasMOV,FRPandsiliconerubberbutalsohardwareandaccessoriescanbereducedandevensuppressed.Significantcostsavingsmustbeconsidered depending on the final design of themounting hardware. The higher the voltage, themoreimportantarethecostsreduction.Installation costs have been reported to be lower from different utilities. EGLA can be preassembledtogetherwiththeinsulator’sstrings.Operationandmaintenancecostsarealsolowerduetothereliabilityandlong-termstabilityofthedesign.

c. ElectricalPerformanceandlong-termstabilityTheprotectionlevel(residualvoltage)oftheEGLAisalwaysbetterthanNGLAsincelessMOVarerequired.The EGLA’s air gap isolates the SVU from the systemunder normal operation. Therefore,MOVs are notcontinuouslyundervoltageandthereisnoleakagecurrentandnoelectricalstress.Abetterageingandalonger life are expected. Japanese utilities have EGLA installed on their network formore than 25 yearswhicharestillingoodcondition.EGLAisasmartsolutionwhichisspecificallydesignedtoimprovelightningperformance.

d. InstallationTheweightofthecompleteEGLAsolutionshouldbelighterthanNGLA.TheSVUcanbepre-assembledonthegroundontheinsulatorstrings(ordirectlyonthetowerstructure)toeasetheinstallationonsite.Theinstallation generally does not require the use of helicopters and cranes as it is for NGLA installation.Immediatevicinity to the insulatorsimplifies the finalconfigurationandguaranteesaproper installation.Therearemoreoptionsforinstallationinliveconditionsforexistinglines.The pre-assembly of a complete set (EGLA + Insulators +Hardware) can be prepared by the supplier tooptimizethemountingtimeandbeliftedonthestructurewhilereducingtheriskoflosingcomponents.

e. Maintenance/OperationBasically,nomaintenance is required.The failure rateof theEGLA isextremely low. Ifproperlydesignedandinstalledinthesystem,itshouldbeconsideredasreliableasthestandardcomponentsintheinsulatorsstrings.There isnospecificmonitoring requiredsince regularoverhead line inspectionareperformedbyutilities.FailedSVUcanbeeasilyidentifiedvisuallyduringlineinspections.EGLA’sdesignmustberigidandstable.Nomovingpartsareacceptable.Thegroundleadandthemechanicalstress(vibration/galloping)oftheNGLAreducethereliabilityincomparisontoanEGLA[15].Highresistancetovibrationandmechanicalstressasseismicactivitiescanbedemonstrated.SVUsfailuresdonotinfluencecontinuousoperationofthelineduetothegapthatisolatesfromthesystem.Thereisnoneedofimmediatereplacement.EGLA cannot fail due to line fault. The gap is dimensioned towithstand power frequency and switchingovervoltages.

6. EGLAusageacrosstheglobeEGLA application is not a new development although it might be called new application since theoutstandingfeaturesarenotwidelyspreadandcommunicatedbytheleadingutilitiesandmanufacturers.Most of those countries below in Table 2,with some exceptions, only accept EGLA application on theirnetwork.NGLAisprohibitedduetoreliabilityandlong-termperformance.Theinformationbelowmightbenotallaccurate,buttheyarerepresentativeoftheglobalmarket.Country Systemvoltages Experience InstalledbasedacrossthecountryJapan fromdistributionto500kV ~25years Millionsofunits.Standardcomponents.

OnlyEGLAaccepted.China fromdistributionto500kVAC

±500kVand±800kVDC~15yearsAC~5yearsDC

Millionsofunits.MarketslowlydominatedbyEGLAonly.

Mexico fromdistributionto400kV 20years ~450.000units.OnlyEGLAaccepted[17].SouthKorea 154kVand345kV 13years ~150.000units.Standardcomponent.Only

EGLAaccepted[13].France 63/90kVand225kV ~20years Initially used for safety concerns. Today

used for lightning performance mainly inoverseasislands.OnlyEGLAaccepted[14].Morethan3000units.

Vietnam 110kV,220kVand500kV ~10years More than 3000 units. Market slowlydominated by EGLA only. Massiveinvestments.

Thailand 115kVand230kV ~10years MarketdominatedbyEGLA.Canada 230kV - FirstEGLAprojectin2019Malaysia 132kV,275kVand500kV 25years ~1200 units [18]. Market dominated by

EGLAHongKong for132kVand400kV 10years Lessthan1000unitsTaiwan 69kV ~10years Lessthan1000unitsCambodia 115kVand230kV - Firstprojectin2019Laos 115kV -

Table2:CompleteoverviewofEGLAcountries

Figure8:EGLAKEPCO–Dead-endinsulatorstring

7. Conclusion Theelectricity supply industry is a conservative sector.New technologiesand innovative solutionsare ingeneral experienced progressively. In the last decades, most of the activities were related to lightningoutagesreduction.LSA’sdoesnotonlyimprovetheperformanceandtheoperationofthepowersystemsbut also improve thedesignand lower the costsof the constructionand themaintenance. The financialbenefits are easily demonstrated. A better synergy between line designers and arrester manufacturersmightbean importantwayofenhancement towidely share theknowledgeandexperience.On theonehand,manufacturersshouldmaketheusersmoreconfidentaboutlong-termreliabilitysinceitisoneofthemain concerns for utilities and their customers.On the other hand, itwould be verywelcome from themain grid operators and utilities to better communicate the outstanding achievements that have beenreached through a mid-term assessment. For line uprating and compaction, the opportunities and theadvantagesaresignificant for the industry.Unfortunately, theapplicationsareunderutilizedbecause theengineershavetheresponsibilitytoredefinetheratingsthathaveappliedforthelastcentury[2].Currently, the IEC60099-4or IEEEC62.11 forNGLAapplicationdonotmakea cleardistinctionbetweenLineSurgeArrestersandStationClassArresters intermofenergyhandlingrequirementsandmechanicalconsiderations.ThereisnotanIEEEStandardthataddressesEGLA’s.ThelatestIEC60099-8EGLAStandardhastwoenergyclassifications,neitherofwhichissufficienttoclearlydefinetheenergyperformance.Theseissuesshouldbe resolvedwith the releaseof thenew IEC60099-11 thatwill coverbothEGLAandNGLAapplications.Each revolution passes through three stages. First, it is ridiculed as installing EGLA’s systematically tooptimizestructures,qualityandcosts.Second,therewillbesomeoppositionsfromthemajorplayersinthesector.Third, itwillbeacceptedasbeingself-evident. LSA’sapplicationwill certainlybecomeastandardcomponentforthedistributionandtransmissionlinesinanearfuture.

1. REFERENCES [1] J. Woodworth: “Lowering Losses on Transmission Lines Using Arresters”, CEATI’s 6th Annual Grounding & Lightning Workshop, Niagara Falls, October 2014. [2] J. Woodworth: “The Externally Gapped Line Arrester – A Design and Application” and “Guide for Condition Assessment of Lightning Arresters installed on Transmission Lines”, CEATI’s 8th Annual Grounding & Lightning Conference, Arlington, USA, November 2016. [3] J. Woodworth: “Benefits Justify More Use of Transmission Line Arresters”, INMR.com [4] U. Bauch: “Line Arresters – Live Insurance Of The Grid - Trends & Developments”, INMR World Congress, Munich, October 2015. [5] P. Bunov, L. Klingbeil, D. Udovcic and D. Biswas: “Externally Gapped Line Arresters – First experience with the new IEC 60099-8 standard and Line study analysis”, IEEE Power & Energy society transmission and distribution conference and exposition, Orlando, Florida, USA, May 2012. [6] P. Bunov, L. Klingbeil, B. Goßler and L. Montaz: “Design and Testing of a 225-kV Externally Gapped Line Arrester for installation under live conditions”, CIGRE C4 International Colloquium on Lightning & Power Systems, Lyon, May 2014. [7] R. Göhler, L. Klingbeil, M. Schubert: “Surge Arrester design and testing experiences according to the new IEC 60099-4”, CIGRÉ International Technical Colloquium, Rio de Janeiro, Brazil, September 2007. [8] L. Klingbeil, Z. Baus, S. Nikolovski, I. Ivankovic: “Case Study for Application of Transmission Line Arresters at Croatian 400kV Line along the Adriatic Coast Mountains”, CIGRÉ Colloquium on Application of Line Surge Arresters in Power Distribution and Transmission Systems, Cavtat, May 2008. [9] CIGRE 440 - Working Group C4.301: “Use of Surge Arresters for Lightning Protection of Transmission Lines”, December 2010. [10] F. Bologna and C. Engelbrecht: “Mitigating Transmission Line Arrester Lead Stresses”, INMR World Conference, Munich, October 2015. [11] V. Hinrichsen: “Overview of Recent Technological Developments for HV Line and Station Arresters & Future Tendencies, INMR World Conference on Insulators, Arresters and Bushings, Rio de Janeiro, May 2007 [12] S. Page: “A Comparison of Commercial Lightning Software”, CEATI’s 8th Annual Grounding & Lightning Conference, Arlington, USA, November 2016. [13] KEPCO: “Seminar for Transmission Line Arrester” [14] Mitigating Ground Potential Rise on Towers & Improving Electricity Quality Using Line Arresters, Frederic Maciela, INMR [15] CIGRE B2 TF 007, “Interaction of vibration dampers with surge arresters,” October, 2016. [16] Private discussion with EPRI. [17] 10 YEARS OF FIELD EXPERIENCE WITH EGLA TYPE LINE ARRESTERS AT THE COMISION FEDERAL DE ELECTRICIDAD (CFE), INMR 2018, M.I J. Gerardo Montoya Tena [18] Arrester Technology Improves Transmission Line Performance: Experience in Malaysia, INMR 2017, Iryani Mohamed Rawi