voltage upgrading of overhead lines.pdf

8/10/2019 Voltage Upgrading of Overhead Lines.pdf

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June 2010

Magne Eystein Runde, ELKRAFT

Master of Science in Electric Power EngineeringSubmission date:

Supervisor:

Norwegian University of Science and Technology

Department of Electric Power Engineering

Voltage Upgrading of Overhead Lines

Anders Tuhus Olsen


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Problem DescriptionStatnett wants to increase the transmission capacity in their 300 kV overhead lines by upgrading

the operating voltage to 420 kV. To make this possible some modifications must be done. Insulator

strings have to be elongated or replaced and the air clearances must be increased. EN standards

provide guidelines for how to calculate the air clearances adequately to provide required safety

margins.

It turns out that the formulas given by the standards provide greater safety margin than

appropriate for upgraded transmission lines. Proper minimum air clearances represent a great

potential for saving money when upgrading the voltage on overhead lines. It is therefore desirable

to calculate the air clearances on the basis of smaller safety margins than described in the

standard.

Assignment given: 15. January 2010

Supervisor: Magne Eystein Runde, ELKRAFT


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III

SummaryStatnettwantstoincreasethetransmissioncapacityintheir300kVoverheadlinesbyupgradingthe

operatingvoltageto420kV.Tomakethispossiblesomemodificationsmustbedone.Insulator

stringshavetobeelongatedbytwotofourinsulatorsandtheairclearancesmustbechecked.EN

standardsprovideguidelinesforhowtocalculatetheairclearancesadequatelytoproviderequired

safetymargins.

Itturnsoutthattheformulasgivenbythestandardsprovidegreatersafetymarginthanappropriate

forupgradedtransmissionlines.Byfindingnewpropersafetymargins,severaltowerswhich

otherwisewouldhavetoberebuilttofulfilltherequirementsforclearances,canstayunmodified.

Whenconsideringthenumberoftowersinanaveragetransmissionline,thereisobviouslyagreat

potentialforsavingmoneybyputtingsomeeffortlookingintoproperminimumairclearances.By

reducetheairclearancebyapproximately10cm,6.5mill.NOKweresparedina65kmtransmission

line.Itisthereforedesirabletocalculatetheairclearancesonthebasisofsmallersafetymargins

than

described

in

the

standard,

but

which

is

still

within

acceptable

safety

limits.

In

the

formulas

for

minimumdistances,thestatisticalwithstandvoltageU50%,gapfactorsandaltitudefactorsare

examinedforthecasesofoperatingvoltage,switchingimpulseandlightningimpulse.

DiscrepanciesbetweentestresultsfromalaboratoryworkconductedbySTRIandcalculationsbased

ontheENstandardofU50%,havebeendiscovered.TestedU50forswitchingimpulsesare59%

higherthanU50fromthestandard.Thesameappliesforlightningimpulseswherethetestedvalueis

12%higherthanthestandard.Thisgivesreasontoassumethestandardtobesomewhat

conservative.

Further,discrepanciesarefoundbetweenthestandardEN50341thatsaysthatthegapfactorwhen

aninsulatorispresentisthesameasifnoinsulatorispresent,andCigrreport72,whichsaysthat

thegapfactorshouldbecorrectedforthepresenceofinsulators.Correctionforinsulatorswilllead

toalowergapfactori.e.lowerbreakdownstrengthalongtheinsulatorstringthanintherestofthe

airgap.Itturnsoutthatthecombinationofrainandinsulatorstringreducethegapfactorandthus,

thewithstandstrengthinthecasesofswitchingimpulsesintheorderof613%forVstring

insulatorsand2034%forIstringinsulatorsandforcontinuouspowerfrequencyvoltageinthe

orderof25%forVstringinsulatorsand3340%forIstringinsulators.

RainhasnoinfluenceonthewithstandstrengthofIstringsorVstringsexposedtolightningimpulses.

Severalpreviousresearches[1][2]showsthesametendenciesoflackofcorrelationbetweenU50andgapfactorswhenairgapswithinsulatorstringsareexposedtolightningimpulses.Thus,thegap

factorisnotsufficienttodescribethedischargecharacteristicsofairgapswithinsulatorstrings

exposedtolightningimpulses.

Itisfoundthattheairgapbetweenphaseandguywirehasapproximately7%greaterwithstand

strengththanovertheinsulatorstringinatowerwindow.Thisadditionalsafetymarginisadesirable

propertyintermsthattheguywiresaretheweakestpointofatower.Thisshouldhoweverbe

verifiedbyfullscalelaboratorytestsasthisismainlyvalidforthecaseofonlytheconductorguy

wiregapwithoutthepresenceoftheotherairgapsthatrepresentthetowerwindow.


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IV

PrefaceThisprojectisdoneincooperationwithStatnettandGrazUniversityofTechnologyandaddressesa

realproblemconcerningvoltageupgradingofoverheadlinesintheNorwegiangrid.Theprojectis

stronglylinkedtothestandardsofinsulationcoordination.Alargepartoftheworkhastherefore

beenlookingintowhatthestandardscontainandwhattheyarebasedon.

IwanttothankmyacademicsupervisorsMagneRundefromNTNUandSonjaMonicaBerlijnfrom

Statnettfortheircontributiontothisprojectreport.

IwouldalsoliketothankmybrotherHvardTuhusOlsenforlanguagerevisionofthisreport.

AndersTuhusOlsen

Trondheim11.06.2010


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V

TableofContentsSummary................................................................................................................................................III

Preface....................................................................................................................................................IV

Tablelist................................................................................................................................................VII

Tableoffigures.....................................................................................................................................VIII

Symbollist..............................................................................................................................................IX

Definitions..............................................................................................................................................IX

Gapfactor...........................................................................................................................................IX

Operatingconditions..........................................................................................................................IX

Fastfrontovervoltage.........................................................................................................................X

Slowfrontovervoltage........................................................................................................................X

Continuouspowerfrequencyvoltage.................................................................................................X

Minimumelectricalclearances...........................................................................................................X

1 Introduction.....................................................................................................................................1

2 Scopeandlimitations......................................................................................................................3

3 Societyandeconomy......................................................................................................................3

4 Gapfactors......................................................................................................................................3

4.1 Rodplanegap..........................................................................................................................3

4.2 Actualairgaps.........................................................................................................................4

4.2.1 ParisandCortina.............................................................................................................5

4.2.2 Cigr72technicalbulletin...............................................................................................7

4.3 Influenceofinsulators...........................................................................................................12

4.4 Influenceofrain....................................................................................................................13

4.5 Conclusionofthechapter.....................................................................................................13

5 Minimumairclearances................................................................................................................14

6 Wind..............................................................................................................................................15

6.1 EffectofwindonIstrings.....................................................................................................15

7 Towers...........................................................................................................................................18

7.1 Modificationoftowers..........................................................................................................18

7.2 Alternativemeasuresfornarrowtowers..............................................................................19

8 Airgapinsulatorconfigurations....................................................................................................21

8.1 Fourdifferentalternativeconfigurations..............................................................................21

9 Theoryvs.laboratorytesting........................................................................................................25

9.1 Researchreport1:STRI.........................................................................................................25


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VI

9.1.1 Lightningimpulse..........................................................................................................26

9.1.2 Switchingimpulse..........................................................................................................28

9.2 Researhreport2:EFI.............................................................................................................30

9.2.1 Gapfactors....................................................................................................................32

9.2.2 U50disruptivedischargetest.........................................................................................38


10 Acasestudy...............................................................................................................................43

10.1 KristiansandArendal.............................................................................................................43

10.2 Findingsofthecasestudy.....................................................................................................47

10.3 Savingpotential.....................................................................................................................47


11 Newlaboratorytest...................................................................................................................48

11.1 Testproposal.........................................................................................................................48

12 Discussion..................................................................................................................................49

13 Conclusion.................................................................................................................................51

14 Bibliography...............................................................................................................................52


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VII

TablelistTable1AirgapconfigurationsandtheirrespectivegapfactorsproposedbyL.Paris[3]......................6

Table2Insulator andairgapconfigurations.......................................................................................21

Table3Airclearancesforconfiguration14andminimumrequiredairclearances............................23

Table4Conductorcrossarmcalculatedfora250/2500spositiveswitchingimpulse,(SI)underdry

andwetconditions.Gapfactorsaregiveninparentheses.Altitudeof500m,towerheight=25m,

guywireangleof40.............................................................................................................................24

Table5U50,50%flashoverprobabilityforlightningimpulse(LIdry)...................................................26

Table6U50,50%flashoverprobabilityforswitchingimpulse(SIwet)................................................28

Table7U10,10%flashoverprobabilityforswitchingimpulse(SIwet)................................................30

Table8Gapfactorsforouterphaseatlightningimpulse.....................................................................35

Table9Gapfactorsformidphaseatlightningimpulse........................................................................35

Table10Gapfactorsforouterphaseatswitchingimpulse..................................................................36

Table11Gapfactorsformidphaseatswitchingimpulse....................................................................36

Table12Gapfactorsforouterphaseatpowerfrequencyvoltage......................................................37Table13Gapfactorsformidphaseatpowerfrequencyvoltage.........................................................37

Table14Conductorcrossarmexposedtoa1.2/50spositivelightningimpulse,(LI)underdry

conditions..............................................................................................................................................38

Table15Conductorcrossarmcalculatedfora1.2/50spositivelightningimpulse,(LI)underdry

conditions.Samegeometryasabove...................................................................................................38

Table16Towerwindowexposedtoa1.2/50spositivelightningimpulse,(LI)underdryandwet

conditions..............................................................................................................................................39

Table17Towerwindowcalculatedfora1.2/50spositivelightningimpulse,(LI)underdryandwet

conditions..............................................................................................................................................39

Table18Conductorcrossarmexposedtoa200/3000spositiveswitchingimpulse,(SI)underdry

conditions..............................................................................................................................................40

Table19Conductorcrossarmcalculatedfora250/2500spositiveswitchingimpulse,(SI)under

dryconditions.Samegeometryasabove.............................................................................................40

Table20Towerwindowexposedtoa200/3000spositiveswitchingimpulse,(SI)underdryandwet

conditions..............................................................................................................................................40

Table21Towerwindowcalculatedfora250/2500spositiveswitchingimpulse,(SI)underdryand

wetconditions.......................................................................................................................................41

Table22Towerwindowcalculatedfora250/2500spositiveswitchingimpulse,(SI)underdryand

wetconditions

and

for

different

swing

angles.

The

air

gap

between

phase

and

guy

wire

and

the

lengthoftheinsulatorstringisboth2.56m.........................................................................................41

Table23SwinganglesforaselectionoftowerofthetransmissionlineKristiansandArendal...........45

Table24MeasuredminimumdistancesfortowermidphaseconductedbyStatnett.The

correspondingswinganglesaregiveninparenthesis...........................................................................46

Table25MinimumrequireddistancesfortowermidphasecalculatedaccordingtoEN50341.........46

Table26Comparisonoftable24(measuredminimumclearances)andtable25(requiredminimum

clearancesaccordingtoEN50341).......................................................................................................46


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VIII

TableoffiguresFigure1Theprojectpresentedbyaflowchart......................................................................................2

Figure2InfluenceoftheradiusRofsphericalelectrodesonU50underpositivepolarity[2]................4

Figure3Conductorcrossarm[2]............................................................................................................7

Figure4Towerwindow[2].....................................................................................................................8

Figure5Probabilityforflashoverasafunctionofvoltageamplitudegivenasstandarddeviations..10

Figure6Minimumairclearancesforthreedifferentoperatingconditions.........................................14

Figure7Windmovestheconductortowardthetower........................................................................16

Figure8Verticalandhorizontalspanlengths.Visthelengthbetweenthelowerpointsoftheline

withintwospanswhileHisthelengthbetweenthemiddleoftwospans..........................................17

Figure9CalculationsofswinganglesoftheinsulatorstringsinPLSCADData)nowind,b)3year

windandc)50yearwind......................................................................................................................17

Figure10ExtensionofanIstringinsulator...........................................................................................18

Figure11ExtensionofaVstringinsulator...........................................................................................19

Figure12Fittingequipmentbetweenphaseandinsulator..................................................................19Figure13Towerwindowofatensiontowerwithsupportinginsulator..............................................20

Figure14Supportinginsulator..............................................................................................................20

Figure15Armourrod............................................................................................................................21

Figure16Testobjectsimulatingthetowerwindow.............................................................................25

Figure17Cumulativenormaldistributionprobabilitycurvesforflashoverforlightningimpulse(LIdry)

and15insulators(1.96m).....................................................................................................................27

Figure18Cumulativenormaldistributionprobabilitycurvesforflashoverforswitchingimpulse(SI

wet)and15insulators(1.96m)............................................................................................................29

Figure19Testobjectusedtosimulatethetowerarrangement.Figurea)andb)representstheouter

phase.Swingangleissimulatedaccordingtofigureb).Figurec)representsthemidphase[7].........31

Figure20Voltageimpulse.Xaxis:T1=timetothepeakvalueoftheimpulseisobtained,T2=timeto

halfofthepeakvalueoftheimpulseremains.Td=timewheretheimpulsehasavoltagelevel

between0.9and1.0PU.Yaxis:ValueofthevoltageoftheimpulseinPU.........................................32

Figure21Gapfactorforpositivepolarityswitchingimpulsetowerwindow(midphase)asafunction

ofthestrikedistance.............................................................................................................................33

Figure22Gapfactorforpositivepolarityswitchingimpulsetowerwindow(midphase)asafunction

ofthestrikedistance[7].......................................................................................................................34

Figure23TestobjectproposedbyMichaelHintereggerattheGrazUniversityofTechnology..........48


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IX

Symbollist

Symbol Unit Explanation

Kg gapfactor

Kg_sf gapfactorslowfrontwaveforswitchingimpulse

Kg_ff gapfactorfastfrontwaveforlightningimpulse

Kg_pf gapfactorforpowerfrequencyoperatingvoltage

Ka atmosphericcorrectionfactor

Kcs statisticalcoordinationfactor. Kcscomesfromchoosingariskoffailure

oftheinsulationthathasbeenprovenfromexperiencetobeacceptable

H m heightfromphasetoground

d1 m lengthoftheinsulatorstring

d2 m distancefromphasetotowerpole

d m distancefromphasetotowerconstruction

m withofthetowerpole

U50 kV thevoltagethatgivesaprobabilityof50%foraflashovertooccurfor

selfrestoringinsulation

U10 kV thevoltagethatgivesaprobabilityof10%foraflashovertooccurfor

selfrestoringinsulation

kg/m3

airdensity

Dpp m minimumrequiredairclearancephasetophase

Del m minimumrequiredairclearancephasetoearth

Definitions

Gapfactor

Gapfactoristherelationshipbetweentheflashovervoltageforarodplanegapandtheflashover

voltageofapracticalairgapofidenticalsizeandforapositivevoltageimpulse.

ThegapfactorisgivendirectlyforswitchingovervoltageasKg_sf.Thegapfactorforlightningimpulse

isderivedfromtheswitchingovervoltagegapfactorasKg_ff=0.74+0.26Kg_sfandthegapfactorfor

powerfrequencyvoltageisderivedfromswitchingimpulsegapfactorasKg_pf=1.35Kg_sf0.35Kg2_sf

Operatingconditions

Thestandardshavedefinedthreedifferentoperatingconditionsthatdescribethewindconditions

andtheworstandmostlikelycorrespondingelectricalstress.

Nowind:Lightningimpulse(LI).

3yearsreturntime:Switchingimpulse(SI).

50yearsreturntime:Powerfrequency(PF).


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X

Fastfrontovervoltage

Fastfrontovervoltagesofimportanceforoverheadlinesaremainlylightningovervoltagesduetoa

directstriketothephaseconductor.Therepresentativevoltagestressischaracterizedbythe

standardlightningimpulsewaveshape(1.2/50s).Fastfrontovervoltagesarealsoreferredtoas

lightningimpulse(LI)inthestandardsforinsulationcoordinationandareusedasdimensioncriteria

fordeterminingnecessaryairclearanceatnowindconditions.

Slowfrontovervoltage

Slowfrontovervoltagecanoriginatefromfaults,switchingoperationsordistantdirectlightning

strikestooverheadlines.Slowfrontovervoltagesofimportanceforoverheadlinesareovervoltages

causedbyearthfault,energizationandreenergization.Thestandardswitchingimpulsewaveshape

is(250/2500s).Slowfrontovervoltagesarealsoreferredtoasswitchingimpulses(SI)inthe

standardsforinsulationcoordinationandareusedasadimensioncriteriafordeterminingnecessary

airclearanceatwindspeedwith3yearsreturntime.

Continuouspowerfrequencyvoltage

Thecontinuouspowerfrequencyvoltage(PF)isconsideredasconstantandequaltothepeakvalue

ofthehighestsystemvoltage( 2 Us)whichisthehighestvalueofoperatingvoltagethatoccurs

undernormaloperatingconditionsatanytimeandanypointinthesystem.Inthestandardsfor

insulationcoordination,powerfrequencyvoltageisusedasdimensioncriteriafordetermining

necessaryairclearanceatextremewindconditionswith50yearsreturntime.

Minimumelectricalclearances

FivetypesofelectricalclearancesareconsideredinthepresentstandardEN50341:

Del Minimumairclearancerequiredtopreventadisruptivedischargebetweenphase

conductorsandobjectsatearthpotentialduringfastfrontorslowfrontovervoltages.

Delmaybeeitherinternalwhenconsideringconductortotowerstructureclearance,

orexternalwhenconsideringaconductortoobstacleclearance.

Dpp Minimumairclearancerequiredtopreventadisruptivedischargebetweenphase

conductorsduringfastfrontorslowfrontovervoltages.Dppisaninternalclearance.

D50Hz_p_e Minimumairclearancerequiredtopreventadisruptivedischargeatpower

frequencyvoltagebetweenaphaseconductorandobjectsatearthpotential.D50Hz_p_e

isaninternalclearance.

Thismasterthesismainlydealswiththeissuesrelatedtominimumairclearanceswithinthetowers

relatedtovoltageupgrading.TheairclearancesthataretreatedinthisthesisarethereforeonlyDel

andD50Hz_p_e.


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1 IntroductionThemaingridinNorwaywasbuiltduringthetimeperiodfrom1960to1990.Sincethenthedemand

forelectricityhasconstantlyincreased,causinghigherrequirementstothegridsabilitytotransmit

electricity.Inordertomeettoadysandfuturerequirementsfortransmissioncapacityandsecurity

ofsupply,measuresmustbetaken.Oneoptionistobuildnewtransmissionlines.Thisishowever

ratherexpensiveandtimeconsumingasthisrequirestheacquisitionoflicensesfordevelopmentof

newlines.Anotherpossibleoptiontomeettherequirementforhighertransmissioncapacityis

increasingthecapacityoftheexistingtransmissionlines.Anincreaseofcapacityintransmissionlines

canbedoneeitherbyincreasingthecurrentorincreasingthevoltage.Increasingthecapacityby

increasingthecurrentlevelmeansthetemperatureoftheconductors,thustheactivelosseswill

increase.Toallowahigherconductortemperatureonemustmakesuretomakeuseoftheproper

conductorthatisdesignedforhighoperatingtemperatures.Alternativelyonecanreplacethe

existingconductorswithconductorsofgreatercrosssection.Thisprojectexclusivelydealswiththe

otheralternativeforincreasingthetransmissioncapacity,whichistoincreasetheoperatingvoltage.

Whenupgradingtheoperatingvoltage,insulatingcoordinationhastobedoneoveragaininorderto

obtainsufficientinsulationstrengthforthenewvoltagelevel.Insulatingcoordinationistheselection

oftheinsulatingstrengthconsistentwiththeexpectedovervoltagetoobtainanacceptableriskof

failure.

Thetowergeometryanddimensionsareoriginallydesignedforanoperatingvoltageof300kV.To

allowthesetowerstobeexposedtohigherelectricalstressthantheyareoriginallydesignedfor,

somemeasuresmustbedone.Theinsulatorstringshavetobeelongatedorreplacedandtheair

clearanceshavetobeincreasedinordertoachievesufficientdielectricstrength.Thetighter

dimensionsina300kVtower,comparetoa420kVtower,limitstheextensionoftheinsulatorsandthepossibleminimumdistances.

Thestandardsforinsulationcoordinationprovideguidelinesforminimumairclearances.Ithas

provendifficulttomaintaintheminimumairclearanceswithinthestandardsrequirementswithout

doingmajormodificationstothetowers.However,theregulationssaythatitisnotanabsolute

requirementtofollowthemethodforinsulatingcoordinationdescribedbythestandards.The

standardsareprovidedasarecommendationtohowthingsshouldbeperformedtoensuresufficient

safetyandsecurityofsupply.Theregulationsrequirethatalldeviationsfromthestandardsmustbe

documentedtocomplywithapplicablelawsandregulationstoensurethesafety.Thismasterthesis

willexaminethepossibilitiesforvoltageupgradingoftightdimensionedtowerswhichlimitsthe

possibilitiesforvoltageupgradingaccordingtostandards.Whendifferentsolutionsareconsidered,

economy,safetysecurityofsupplymustbeconsidered.


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Aflowchartismadetogivethereaderabetteroverviewofthedifferentissuesthatareinvestigated

andhowtheinvestigationisperformed.

Figure1Theprojectpresentedbyaflowchart


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3

2 ScopeandlimitationsThisreportexaminesminimumairclearancesthatmaybeallowedinatower.Itisknownthat

currentrequirementsforairclearancesthataregiveninthestandardsaresomewhatconservative

andcannotbemetwithoutdoingmajormodificationstothetowers,whichinvolvesextensivecosts.

Amajortaskistofindtheappropriateminimumclearancesthatcanallowalargernumberoftowers

tostayunmodifiedwithoutlackofthesecurityofsupply.Thiswillalwaysbeanassessmentofcost

andreliability.

Gapfactorswillbeexaminedandthegapfactorsrecommendedbythestandardsarecomparedto

gapfactorsproposedotherresearchwork.Theimpactoftheswingangleoftheinsulatorandthe

insulatoritselftothegapfactorisexamined.ThestatisticalwithstandvoltageU50willbeexamined

forlightningimpulses,switchingimpulsesandcontinuous50Hzpowerfrequencyvoltage.

Otheraspectsofinterestofvoltageupgradingthatarenotincludedinthisreportarelocationof

surgearrestorsandcoronanoise.

3 SocietyandeconomyTheideaofupgradingtransmissionlinesistogetagridwithhighercapacityatlowerinvestmentcost

andwithlessenvironmentalimpactthanbuildingnewtransmissionlines.Accordingly,when

upgradingtransmissionlines,oneshouldtothegreatestextentpossiblemakeuseoftheexisting

lines,insulators,towersandotherequipment.Theenvironmentalaspectofvoltageupgradingisof

greatimportanceintodayssocietywherethereisalotoffocusonenvironmentfriendlyenergy

production.Environmentfriendlypowertransmissionshouldbeanaturalpartofthis.

4 GapfactorsDeterminingtheelectricalwithstandstrengthofairgapsisdonebydisruptivedischargetestsin

laboratories.Onedesiredoutcomeofsuchtestsistofindthevoltageamplitudethatgivesa50%

probabilityforflashover,U50.Thevoltageamplituderequiredtogiveaflashoverisstrongly

dependentontheshapeoftheelectrodes.Onthebasisofthisphenomenon,thegapfactorwas

introducedinordertocorrectthediscrepancybetweenareferencegapwhereitselectricproperties

wasknownandtheshapeoftheactualelectrode.Thegapfactorkgisamultiplyingfactorwhich

characterizestheshapeoftheelectrodesofanairgap,thusdischargecharacteristicsofanyairgaps

canbedeterminedbymultiplyingthegapfactorwiththedischargecharacteristicsofareferencegap.

Amongthedifferentairgapsofspacingd,thepositivepolarityrodplanegaphasthelowest

withstandstrengthandwasthereforeusedasareferencegapwithagapfactorkg=1.

4.1 Rod-planegap

Therodplanegapisawellknowngapconfigurationusedinlaboratorytesting.Theradiusof

curvatureoftherod(anode)isdecisiveforthevalueoftheflashovervoltage.Forpositivepolarity

impulse,themorepointedtheanode,thelowertheflashovervoltageofalargeairgap.Forthe

cathodeapplies:themorepointedthecathode,thegreatertheflashovervoltageofalargeairgap.


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TheU50valueremainsconstantwhentheanoderadiusislessthanacertaincriticalvalueRcriticalwhich

isdependentonairgapspacingasshowninfig.2.

Figure2InfluenceoftheradiusRofsphericalelectrodesonU50underpositivepolarity[2].

Fromfig.2itcanbeseenthatforairgapsof2metresandupthecriticalradiusisR>0.1metresi.e.

forthemajorityofpracticalproblems,theanoderadiusislessthanthecriticalradius.Inthiscasethedielectricwithstandstrengthonlydependsonthelengthoftheairgap,whichmeansthatformost

practicalairgapstherodplanegapstaysvalidasareferencegapwhendeterminingthegapfactor.

4.2 Actualairgaps

Thegapfactorkg,originallyproposedbyL.ParisandR.Cortina[1]in1968,istherelationofflashover

voltageforarodplanegapandapracticalairgapofidenticalgaplength,d.Theynotedthatthatall

curvesofU50asafunctionofgapspacingdhadessentiallythesameshapeforatowerconfiguration

andarodplanegap.Aconductorplanegap,whichhasshowntohavethesametendencyasarodplanegap,providedagoodbasisfordeterminingtheelectricalpropertiesofactualairgapswitha

referencetothewellknownrodplaneconfiguration.In1967LuigiParispublishedtheIEEEresearch

articleInfluenceofAirGapCharacteristicsofLinetoGroundSwitchingSurgeStrength[3].The

researchworkwasperformedbyapplyingimpulsewavessimulatingswitchingsurges.Themain

purposeoftheresearchwastoinvestigatethedependenceofairgapswitchingsurgeperformance

upongapgeometry.Mostofthetestsweremadewitha120/4000simpulsewavesincethis

particularwaveshapeisrecognizedforhavingthelowestpositivepolaritywithstandvoltageforrod

rodandrodplanegaps.Onthebasisofthetestresultstherewasdrawnsomeconclusionsaboutthe

influenceoftheelectrodeshapetotheelectricwithstandstrengthU50ofairgaps.Onthebasisofthe

testresultstheauthorproposedthegapconfigurationswiththecorrespondinggapfactors,kggiven

intable1.


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4.2.1 ParisandCortina

In1968LuigiParisandRosarioCortinapublishedtheIEEEresearcharticleSwitchingandLightning

ImpulseDischargeCharacteristicsofLargeAirgapsandLongInsulatorStrings[1].Thisarticlewas

publishedasasecondpartofthearticlewrittenbyL.Paristhepreviousyear,andalsoincludesthe

behaviourofairgapswhenexposedtothe1.2/50slightningimpulse.Oneofthediscoveriesthey

madewasthattheimpactoftheshapeoftheelectrodestothedischargevoltageforlightning

impulsesissimilarinbehaviourtothatseenforswitchingimpulses,whenthereisnoinsulatorstring

betweentheelectrodes.However,theinfluenceofelectrodeshapeislesssignificantforlightning

impulsesinairgapswithoutaninsulatorstringthroughit.Theyalsofoundthattheshapeofthe

insulatorsinaninsulatorstringhasaveryslightinfluenceonthebehaviouroftheairgap,meaning

thatintroducinganinsulatorstringbetweentheelectrodescanbeconsideredingeneral,leavingthe

typeofinsulatoroutofconsideration.Forlightningimpulsesappliesthattheinfluenceofthe

electrodeshapeismuchgreaterinthecaseofairgapswithaninsulatorstringbetweenthe

electrodes.Thearticleconcludesthatthegapfactorkgisnotsufficientfordefinitionofthebehaviour

ofair

gaps

with

an

insulator

string

through

itwhen

the

air

gap

isexposed

tolightning

impulses

with

positiveornegativepolarityunderdryandwetconditions,andindryconditionsfornegativepolarity

switchingimpulses.Negativepolarityimpulsesarehowevernotinterestinginthiscontextsincethe

flashovervoltageinanairgapisconsiderablyhigherinthiscase. Onthebasisoftheresearchwork

theyproposedthesemiempiricalformula:

0.650 500U d S kV (4.1)

fordeterminingthedischargevoltageU50ofanrodplaneairgapoftwotosevenmetresfora

positivepolarityswitchingimpulsewhereU50isinkVanddinmeters.

Byapplyingthegapfactor,kgtotheformula,itisvalidforallairgapsthatarecharacterizedbyagap

factor:

0.650 500 gU d k S kV (4.2)

Fordevelopingequation4.1theauthorsusedapositiveswitchingimpulsewithawaveshape

120/4000swhichisconsideredasthemostcriticalwaveshapeforrodrodandrodplanegaps.

Thus,theequationisnotbasedonthesamewaveshapeastheoneusedtodefinetheswitching

impulseintheENstandardof250/2500s.

IntheIEEEresearchreportInfluenceofAirGapCharacteristicsonLine toGroundSwitchingSurge

Strength[3],aselectionoftypicalgapconfigurationswaspresentedasshownintable1.Thegap

factorsforthedifferentairgapconfigurationssuggestedintable1weredevelopedthroughseveral

disruptivedischargetestswhereaswitchingimpulsewaveof120/4000s,recognizedforhavingthe

lowestpositivepolaritywithstandvoltageforrodrodandrodplanegaps,wasused.


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Table1AirgapconfigurationsandtheirrespectivegapfactorsproposedbyL.Paris[3].

Electrodes Testarrangement Gapfactor,kg

Energized Grounded Without

insulatorstring

WithI andV

insulatorstring

Rod Plane 1.00 1.00

Rod Structure(under) 1.05

Conductor Plane 1.15

Conductor Window 1.20 1.15

Conductor Structure(under) 1.30

Rod Rod(h=3m

under)

1.30

Conductor Structure(over

andlaterally)

1.35 1.30

Rod Rod(h=6m

under)

1.40

Conductor Rope 1.40

Conductor Rod(h=3m

under)

1.65

Conductor Crossarmend 1.50

Conductor Rod(h=6m

under)

1.90

Conductor Rod(over) 1.90 1.75


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4.2.2 Cigr72technicalbulletin

AsitappearsfromtheIEEEresearcharticleSwitchingandLightningImpulseDischarge

CharacteristicsofLargeAirgapsandLongInsulatorStrings[1],thegapfactordependsonthetype

ofelectricalstresstheairgapisexposedto.Inairgapswithoutinsulatorstring,bothswitching and

lightningimpulsescanbedescribedbyagapfactor.TheCigr72technicalbulletinsuggestsgap

factorsforlightningimpulsesandcontinuouspowerfrequencyvoltagenamedkg_ff(ff=fastfront

wave)andkg_pf(pf=powerfrequency)respectively.Thegapfactorskg_ffandkg_pfarederivedfrom

theswitchingimpulsegapfactorasfollows:

Whentheairgapisexposedtoalightningimpulse,thegapfactorkg_ffisexpressedintermsofkgas:

_ 0.74 0.26g ff gk k

(4.3)

Whentheairgapisexposedtopowerfrequencyvoltage,thegapfactorkg_pfisexpressedintermsof

kgas:

2

_ 1.35 0.35g pf g gk k k (4.4)

InsomeliteratureincludingtheENstandards,thegapfactorforswitchingimpulseisnamedkg_sf(sf=

slowfrontwave).Thisishoweverthesamegapfactorasthekggivenintablesforgapfactorssuchas

table1andthetablesforgapfactorsfoundintheENstandards,i.e.thegapfactorsareprimarily

givenforswitchingimpulsessothatkg_sf=kg.

Conductorcrossarmistheairspacethatrepresentstheinsulationoftheouterphaseinatower.The

insulationconsistsoftwoairgaps,d1whichisconductorcrossarmendandd2whichisconductor

structure(laterally)ref.table1.

Figure3Conductorcrossarm[2].


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8

TheCigr72technicalbulletinsuggeststhefollowingformulaforthegapfactoroftheconductor

crossarmconfiguration:

18 2

1 1

1.45 0.015( 6) 0.35( 0.2) 0.135( 1.5)dg dHk ed d

(4.5)

Applicableinrange:

d1=210m

d2/d1=12

/d1=0.11

H/d1=210

where

Kggapfactor

Hheightfromphasetoground

d1lengthoftheinsulatorstring

d2distancefromphasetotowerpole

withofthetowerpole

Thisformulawillinmostcasesgiveagapfactorcloseto1.45.Comparingthiswithtable1,thetwo

airgaps,d1conductorcrossarmendandd2conductorstructure(laterally),havethegapfactors1.50

and1.35respectively.Inpracticetheairgapconductorcrossarmendwithkg=1.50willbethe

determinantairgapsinceinmostcasesd1


21/64

9

TheCigr72technicalbulletinsuggeststhefollowingformulaforthegapfactoroftheconductor

towerwindowconfiguration:

81.25 0.005( 6) 0.25( 0.2)dg Hk ed

(4.6)

Applicableinrange:

d=210m

/d=0.11

H/d=210

where

Kggapfactor

Hheightfromphasetoground

ddistancefromphasetotowerconstruction

withofthetowerpole

Thisformulawillinmostcasesgiveagapfactorcloseto1.25.Ifinsteadusingtable1todetermine

thegapfactorofthetowerwindow,oneobtainthethreeairgaps,conductorstructure(overwith

insulatorstring),conductorstructure(laterally)andconductorrope,havingthegapfactors1.30,

1.35and1.40respectively.Unliketheconductorcrossarmconfigurationoftheouterphase,the

distancesfromconductortostructureareapproximatelythesameintheairspaceofthetower

window.Inthiscaseallofthethreementionedairgapswillbedeterminantfortheinsulating

strengthandmightbetreatedindividuallyratherthanasasingleairgap.

Inthiscasetheformula3.6compliesonlypartlywiththesuggestedgapfactorsintable1,asit

appearslowerthanthelowestsinglegapfactorofthetreeairgapsfromtable1.

Asafirstapproximation,asthelowestsinglegapfactorofthethreementionedgapsis1.3,itmakes

sensetouseacommongapfactorforthetowerwindowof1.25asaconservativevaluewhen

designingnewtransmissionlines.Whenitcomestovoltageupgrading,theairclearanceswithinthe

towerislimited,makingitinterestingtoexamineallrelevantairgapswithinthetowerwindow.

Asstatedearlier,itisdesirablethatincaseofaflashover,thestrikeshouldgotothecrossarm

ratherthantothefragileguywires.Accordingtothegapfactorsintable1,theconductorguywire

airgaphasanelectricalwithstandstrengthof78%higherthanconductorcrossarm,giventhatthe

airgapsareofidenticallength.Thismightbeexplainedbythestatementthataconductorandthusa

guywireorarope,hasapproximatelythesamepropertiesasarodinanairgap.Theconductor

structuregapwillthushavethesametendencyasarodplanegap,whiletheconductorguywiregap

willhavethesametendencyasarodrodgapwhichisknownforhavinghigherbreakdownstrength

thantherodplanegap.


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10

Asalreadymentioned,thestandardsforinsulatingcoordinationrecommendagapfactorofkg=1.25.

Ifinsteaddividingthetowerwindowintothethreementionedairgapswithkg=1.3,1.35and1.4

onemightgetamoreaccuratedescriptionofthecharacteristicsoftheairgapandhencetowhich

partofthetowerconstructionthestrikeismostlikelytogo.

Inthestandardsforinsulationcoordination,therequiredwithstandvoltage,Urwforlightning andswitchingimpulsesissettobethevoltagethatgivesa10%probabilityforflashovertooccurinthe

airgapi.e.Urw=U10.Thisvalueisobtainedbymoving1.3standarddeviationsdownfromU50onthe

probabilitycurveinfig.5,endingupat10%probabilityforflashover.Requiredwithstandvoltageis

calculatedbytheformula:

10 50 1.3rwU U U Z kV (4.7)

where

Zisthestandarddeviation

forlightningimpulsesZ=0.03U50

forswitchingimpulsesZ=0.06U50

Figure5Probabilityforflashoverasafunctionofvoltageamplitudegivenasstandarddeviations.

0

0,10,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

4 3 2 1 0 1 2 3 4

Probability

Standarddeviation


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11

Iftheconductortraverseairgap,whichhasthelowestgapfactor,kg=1.3isusedasareferencegap,

thisairgapshouldgivea10%probabilityforflashover.Onewillthenobtainthefollowing

probabilitiesforthetwootherairgapstohaveaflashover:

Conductortowerpole,kg=1.35

50 _ 1.35 50 _ 1.30 50 _ 1.301.35

1.041.30g g g

k k kU U U kV

(4.8)

Knowingthatthestandarddeviation,Z=0.06U50forswitchingimpulses,thegapfactorofthe

conductortowerpolecorrespondsto2/3ofthestandarddeviationforswitchingimpulses.Adding

the2/3tothe1.3gives1.97standarddeviations,whichcorrespondtoaprobabilityforflashoverof

2.5%ontheprobabilitycurveinfig.5.

Conductorguywire,kg=1.40

50 _ 1.40 50 _ 1.30 50 _ 1.301.40

1.081.30g g gk k kU U U kV

(4.9)

correspondsto4/3ofthestandarddeviationforswitchingimpulses.Addingthe4/3tothe1.3gives

2.63standarddeviations,whichcorrespondtoaprobabilityforflashoverof0.5%ontheprobability

curveinfig.5.

Usingthemethodfordeterminerequiredwithstandstrengthdescribedinthestandardforinsulating

coordinationincombinationwiththegapfactorsproposedintable1,therearea10%probabilityfor

theflashovertooccurovertheinsulatorstringtotraverse,2.5%probabilityfortheflashovertooccurtowardsthetowerpoleand0.5%probabilityfortheflashovertooccurtowardstheguywire.

Towhatextenttheseresultsarevalidforactualcasesoftowerwindowshastobedeterminedby

laboratorytesting.Itisalsoimportanttopointoutthatthisisonlyvalidincasesofswitching

impulses.Thestandardsusethesamemethodfordeterminerequiredwithstandstrengthoflightning

impulses.However,thegreatuncertaintyonthebehaviouroflightningstrikesinairgapshavingan

insulatorstringthroughitmakesitimpossibletorelatetheprobabilityforflashovertogapthe

factors.Thisisdiscussedinthenextchapter;Influenceofinsulators.Asthelightningstrikesseem

toacthighlyunpredictableinsuchcases ,theguywireshouldbeprotectedincaseswherethe

dimensionsinthetowerwindowaretightandthedistancefromtheconductortotraverseisequalto

conductorguywire.This,amongothersolutions,isdiscussedinthechapterAlternativemeasures

fortighttowers.


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12

4.3 Influenceofinsulators

Towhatextenttheinsulatorstringaffectstheelectricpropertiesofanairgapdependsonthewave

shapeofthevoltagetheairgapisexposedto.Differentliteratureagreeonthatinthecaseof

positivepolarityswitchingimpulses,theintroductionofaninsulatingmediumbetweenthe

electrodescausesonlyaslightchangeinthebehaviouroftheairgapandthusthegapfactor.Asfor

lightningimpulse,thepresenceofinsulatorsbetweentheelectrodesmayplayanimportantroleon

thedischargeprocess,thusalsoheavilyaffectingthestatisticalwithstandvoltageU50.

Therearemainlytwodifferentconfigurationsofcapandpininsulatorstringsthatareusedin

overheadlines,IstringandVstring.Asthenameindicates,theIstringconfigurationconsistsofa

singleverticallystringofcapandpininsulators,whiletheVstringconfigurationconsiststwostrings

withanangleof45,formingaVshape.Inthecaseofcapandpininsulatorstrings,thefield

distributionisevenlydistributedbythemetalliccapsandpins.

Givenexperimentalerror,thestatisticaluncertaintyandtheimprecisenatureoftheevaluationof

geometricalcharacteristicsoftheinsulatorlessairgap,thepresenceofdrycapandpininsulatorsin

theairgapcanbesaidtonothaveasignificantimpactwithrespecttothedielectricstrengthwhen

exposedtoslowfrontimpulsewavessuchasaswitchingimpulse.Cigrreport72indicatesan

influencelessthan3%[2].Foranairgapwherecapandpininsulatorsarepresent,thegapfactoris

givenby:

1

0.85 0.15 gk

t gk e k

(4.10)

wherekgisthegapfactorforanairgapwithnoinsulatorstringgoingthroughit.

Thisformulaindicatesaslightdecreaseofthewithstandstrengthofairgapswhereinsulatorsare

present.

TheParis/CortinaresearcharticleSwitchingandLightningImpulseDischargeCharacteristicsofLarge

AirgapsandLongInsulatorStrings[1]concludesthatthefactorkgisnotsufficienttodefinethe

behaviourofairgapswithinsulatorstringswhentheairgapisexposedtolightningimpulseswith

positiveornegativepolarityunderdryandwetconditions,andindryconditionsfornegativepolarity

switchingimpulses.Fortheseconditionstheauthorsnotedthatthebehaviourofthedischargeinthe

airgapdidnotseeminanywaytobeconnectedwiththegapfactor.Sinceinsulatingcoordinationis

basedonthemostseverecaseofswitching andlightningimpulses,onlythepositivepolarity

impulsesareconsidered.Thus,forallpracticalpurposes,thegapfactorisnotsufficienttodescribe

thedischargecharacteristicsofairgapswithinsulatorstringsexposedtolightningimpulses.


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13

TestresultsfromtheresearchworkpresentedinCigrreport72showthesametendencieswhere

thereisalackofcorrelationbetweenU50andgapfactorsundertheseconditions.Furthermore,the

testresultsshowedthattheinfluenceofcapandpininsulatorsisreducedwhenthestressonthe

firstinsulatoratbothextremitiesofthestringisreducedusingshieldingrings.

4.4 Influenceofrain

Testswhichhavebeencarriedoutshowthatrain,asaperturbationoftheinsulationmedium,hasno

significanteffectonthedielectricstrengthofairgaps.However,duetostreamingofwater,therain

canmodifytheshapeoftheelectrodesbyformationofcascadesofwaterdropletsalongthe

insulatorstringandaccordinglydecreasethedielectricstrengthofanairgap[2].Theinfluenceof

raintendstobemorepresentinIinsulatorsthantoVinsulatorsastheangleoftheVinsulator

makestheraindrainawaymoreefficiently.Foranairgapwherethecombinationofcapandpin

insulatorsandrainarepresent,thegapfactorisgivenby:

1

11 0.54 tkwet t

k e k

(4.11)

wherektisthegapfactorforanairgapwherecapandpininsulatorsarepresent.

Formula3.11indicatesaslightdecreaseofelectricwithstandstrengthinwetconditions.

4.5 Conclusionofthechapter

Anairgapinatransmissiontowercanbeseenasacomplexairgapwithmultipleelectrodes.This

appliesespeciallyforthetowerwindow(seepart4.2.2fig.4).Hence,thetowerwindowmightbe

describedmoreaccuratelybythethreegapfactorskg=1.3,1.35and1.4foundintable1.

UsingthemethodfordeterminerequiredwithstandstrengthUrw=U10willresultina10%

probabilityfortheflashovertooccurovertheinsulatorstringtotraverse,2.5%probabilityfortheflashovertooccurtowardsthetowerpoleand0.5%probabilityfortheflashovertooccurtowards

theguywire.

Thereisgreatuncertaintyaboutthebehaviourofaflashoverinairgapswithinsulatorsexposedto

lightningimpulses.Thus,therelationshipbetweenelectrodeshapesandgapfactorsishardtodefine

forthiswaveshape(seepart4.2.1).


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14

5 MinimumairclearancesWhenupgradingthevoltage,newrequirementsaresettotheinsulationstrength.Whenspecifying

theserequirements,thegoalisnotonlytoselectthenewinsulationstrength,butalsotoselectthe

minimuminsulationstrengthorminimumairclearance.Minimumairclearanceisdirectlyconnected

tothecost,sincetheminimumairclearanceiswhatdetermineswhetheratowerhastobemodified

ornot.Figure6showsatowerwithclearancecirclesthatillustratestheminimumairclearancesfor

thethreeoperationconditions:

1.

Nowind.Minimumairclearancedeterminedbylightningimpulse.

2.

3yearswind.Minimumairdeterminedbyswitchingimpulse.

3.

50yearswind.Minimumairclearancedeterminedby50Hzsystemvoltagepeakvalue

( 2sU ).

Figure6Minimumairclearancesforthreedifferentoperatingconditions.

Thecalculationsoftherequiredairclearancesforswitchingandlightningovervoltages,phaseto

earthandphasetophase,DelandDpp,andfortheoperatingvoltagephasetoearthandphaseto

phase,D50Hz_p_eandD50Hz_p_pinthestandardEN50341[4]arebasedonENV50196supportedbyEN

6500711,EN600712[5]andCigrreport72"Guidelinesfortheevaluationofthedielectricstrength

ofexternalinsulation[2].

Thedielectricstrengthinanairgapdependsonsuchfactorsaselectrodegeometry,spacingandthe

shapeandpolarityofthevoltageimpulse.Thebreakdownvoltageislowerforanimpulsewith

positivepolaritythanforanimpulsewithnegativepolarity.Thebreakdownvoltagewithlightning

impulsesincreasesapproximatelyproportionallytothespacing,whileitincreasesconsiderablymore

slowlywithpositiveswitchingimpulsesandlargespacing.Consequentlyathighersystemvoltages

300kVwheretherearelargeairgapspacinginthetowers,switchingimpulseplaysagreaterrolein

insulationdesignthanlightningimpulse[6].


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15

TheformulasforU50%forswitching andlightningimpulseandpowerfrequencyvoltageinairgaps

giveninEN50341[4]arederivedfromexperimentsofarodplanegapastheelectrodeconfiguration.

Theformulasaregivenby:

U50forarodplanegapconfigurationforswitchingimpulsesorslowfrontwaveimpulses:

50 _ 1080 ln 0.46 1 ;rp sf U d kV d m (5.1)

U50forarodplanegapconfigurationforlightningimpulsesorfastfrontwaveimpulses:

50 _ 530 ;rp ff U d kV d m

(5.2)

U50forarodplanegapconfigurationfor50Hzpowerfrequencyvoltage:

1.250 _ 50 750 2 ln 1 0.55 ;rp HzU d kV d m (5.3)

Inahighvoltagetowertheelectrodesarerepresentedbylinesandtowerconstruction.Gapfactors

giveninthestandardEN50341correctforthediscrepancybetweenthegeometryofthevarious

"electrodes"attheappropriateairgapsintowersandtherodplanegap.Consequentlytheformulas

forU50rphavetobemultipliedbyacertaingapfactortodeterminethewithstandstrengthina

certainpartofatower.

6 Wind

6.1

EffectofwindonI-strings

Istringsarenormallynotconstrainedfrommovementcausedbywind.Windcanmovethe

conductorclosertothetower,thusdecreasingboththestrikedistancetothetowerpole,guywire

andtraverse.ReducedstrikedistanceleadstodecreaseofU50,thusincreasingofSSFOR(Switching

SurgeFlashoverRate).AresearchworkconductedbyEFIin1971[7]foundthatthewithstand

strengthofanairgaptendstoreducegraduallywithincreasedinsulatorswing.TheU50isreducedby

4%at10swingand17%at30swing.Furthertheyfoundthatat10swingtheflashoveroccurred

alongtheinsulator,whilefor20swing,theflashovermostlyoccurredtothetraversethroughthe

air.

IntheeasternpartofNorwaythe3yearwindisnormally25m/sandthe50yearwindis32m/s.This

windspeednormallycorrespondstoswinganglesofapproximately30and50respectively.


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16

TheSSFORforanIstringcanbecalculatedbyconsideringeachwindspeedanditsprobabilityof

occurrence.

Figure7Windmovestheconductortowardthetower.

Forawindspeedvimpingingontheconductor,theswingangleSis

1 1.61tanS k v (6.1)

where

41 1.138 10 D W

kV H

(6.2)

and

D=conductordiameterincm,W=conductorweightinkg/m,V=verticalspanlength(fig.8),H=

horizontalspanlength(fig.8)andv=windspeed.


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17

Figure8Verticalandhorizontalspanlengths.Visthelengthbetweenthelowerpointsofthelinewithintwospanswhile

Histhelengthbetweenthemiddleoftwospans.

ForcalculationofswinganglesoftheinsulatorstringsaprogramnamedPLSCADDwhichisa

computerprogramwithgraphicaluserinterfaceusedfordesigningofoverheadpowerlines.Figure9

showstheanglesoftheinsulatorstringsinatoweratnowindconditions,3yearswindconditions

and50yearswindconditions.

Figure9CalculationsofswinganglesoftheinsulatorstringsinPLSCADData)nowind,b)3yearwindandc)50year

wind.


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18

7 Towers

7.1

Modificationoftowers

Toallow420kVin300kVtowerstheinsulatorstringshavetobeextendedbytwotofourinsulators,

dependingonthespaceavailableinthetowerwindowandwhetheritisaIstringasinfig.10oraV

stringasinfig.11.Inanycaseitappliesthatextensionofinsulatorstringsreducesthedistanceand

thusthesafetymarginstowardtheguywires.Eachofthetowersinatransmissionlineisuniquewith

respecttoitsdimensions,meaningthatthesecuritymarginswillvaryfromonetowertoanother.

Sometowersfulfilltherequirementswithouttheneedforfurtherinvestigationofthesafetymargins,

whileothertowerswilldefinitelyhavetobemodifiedtobeabletobeupgraded.Foreveryother

towerbetweenthesetwocases,onewillhavetoevaluateeachtowerparticularly.

Thetowerscanthusbedividedintothreecategories:

1. Ok

2. Caseofdoubt

3. Notok

Figure10ExtensionofanIstringinsulator.


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19

Figure11ExtensionofaVstringinsulator.

7.2 Alternativemeasuresfornarrowtowers

Fortowerscoveredbycategory1nomeasuresareneededotherthanextensionoftheinsulator

strings.

Towerscoveredbycategory2requireacloserinvestigationofthetowertoidentifythemostcritical

airgap.Whenthisisidentifiedonecanstartevaluatingpossiblemeasures.Insuchcasesitmightbe

enoughtojustdosomeminorchanges,suchasreplacingexistingfittingequipmentlocatedbetween

phaseandinsulatorwithmorecompactequipment,asillustratedinfig.12willgainalimiteddistance

intheairgap.

Figure12Fittingequipmentbetweenphaseandinsulator.


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20

Towerscoveredbycategory3requiremoreextensivemodificationofthetowertobeupgraded.

SupportinginsulatorisasolutionwheretwocompositeinsulatorsformingaVasshowninfig.13and

14.ItlockstheIstringinsulator,preventingitfromswingingtowardsthetowersideatwindy

conditions.Thissolutionisusedontensiontowersaswelltosupportthelooppreventingitfrom

movingtowardsthetowerconstructionasshowninfig.13.AnalternativeistoreplacetheIstring

insulatorwithanordinaryVstringinsulatorseeninfig.11.

Figure13

Tower

window

of

atension

tower

with

supporting

insulator.

Figure14

Supporting

insulator.


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21

Armourrod(fig.15)isahelicalsteelprotectionthatiswoundaroundtheguywiretoprotectitfrom

damagecausedbyarcs.Thissolutionisusedwhenthereisanuncertaintyonwhereinthetower

windowaflashoverwilltakeplace.Inpracticethisisdoneintowerswheretheairclearance

betweenphaseandtraverseisequaltotheairclearancebetweenphaseandguywireinthetower

window.

Figure15Armourrod.

8 Airgap-insulatorconfigurationsThischapterinvestigatesfourdifferentinsulator/airgapconfigurations.Theminimumairclearances

requiredbytheENstandard,U50andgapfactorsindryandwetconditionsareinvestigatedfor

differenttypeofelectricalstressandswingangles.

8.1

FourdifferentalternativeconfigurationsTransmissionlinesdesignedforvoltagelevelsof420kVwillnormallyhaveinsulatorstringsof18

insulators.Duetolackofspaceinthevoltageupgradedtransmissionlines,itisfoundexpedientto

useinsulatorstringsof17insulatorstoobtainsufficientreliability.However,dependingonthesizeof

thetowersonemightevennothaveenoughspaceforaninsulatorstringof17insulators.Intowers

withtightdimensions,onemustgodownto16insulatorstoprovidesufficientdistancebetween

phaseandguywire.

Inthecasesoftightdimensioning,onehastoacceptthatthereisahigherprobabilityforaflashover

tooccurincasesofovervoltages.Itisthereforeofgreatimportancetodimensiontheinsulator/air

gaprelationinawaythatanyflashoverfindsitswaytothetraverse,ratherthantotheguywire

whichmightburnoff.Fourdifferentinsulator/airgaprelationsforthemidphasetowerwindowhave

beenproposedbyStatnett:

Table2Insulator andairgapconfigurations.

Configuration No.ofinsulators Lengthofinsulator

stringinmetres

Distancetoguywirein

metres

1 16 2.55 2.55

2 16 2.55 2.7

3 17 2.72 2.72

4 17 2.72 2.9


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22

Thechoiceofinsulatorlengthversusdistancetoguywirewillbecompromisesoftheelectrical

withstandperformanceofthetoweratdifferentoperatingconditions.Theinsulatorstringconsisting

of16insulatorswillgivepoorerperformanceincaseofalightningstrikeandnowindthanan

insulatorstringof17insulators.However,ashorterinsulatorstringmightperformbetterinwindy

conditionsastheswingradiusislessforashorterstring.Thisisinvestigatedfurtherbydetermining

U50foraselectionofdifferentswinganglesforthefourbeforementionedinsulator /airgap

configurations.Itcanbeseenfromthetablethateachswinganglecorrespondstoaspecific

operatingsituation,i.e.typeofelectricalstress.BasedoninformationdatafromtheKristiansand

Arendaltransmissionline,thefollowingassumptionsforthemostseverecaseofelectricstresstothe

towersarefoundmostappropriate:

Stresscausedbylightningimpulseismostlikelytooccuratnowindconditions.However,

calculationsaredoneforstaticlineanglesupto10whichisthecaseforsomeofthetowers

inline.

Stresscausedbylineswitchingimpulseismostlikelytooccurforswinganglesupto30.

Stresscausedbypowerfrequencyoperatingvoltageismostlikelyforswinganglesupto40.

Themaximumamplitudeofthepowerfrequencyvoltageisthesystemvoltage.

Thecalculationsofminimumrequiredairclearances,statisticalwithstandstrengthvoltageU50and

gapfactorsaredonebyanexcelsheetmadeforthisproject.Theformulasusedforcalculationofthe

airclearancesandthestatisticalwithstandstrengthvoltageU50aretheformulasthataregiveninEN

50341[5],whiletheformulasusedforcalculationofgapfactorsaretakenfromtheCigrreport72[2]whichtheENstandardisbasedon.Airclearances,U50andgapfactorsareinvestigatedforvarious

swinganglesoftheinsulatorstringforthefourbeforementionedtowerconfigurations.Calculations

aredonefortheairgapbetweenphaseandguywireandtheairgapbetweenphaseandtraverse.

U50andgapfactorsarealsoinvestigatedovertheinsulatorstringindryandwetconditions,whilethe

airgapsarecalculatedfordryconditions.Calculationsaredoneforlightningimpulses(LI),switching

impulses(SI)andpowerfrequencyvoltage(PF).Theswitchingovervoltageisassumedtobe1.83PU

andtheoperatingvoltageis420kV.Thetowerisassumed25metreshigh,hasaguywireangleof

40*andislocatedatanaltitudeof500metresabovesealevel.

*Theguywireangleistheanglebetweenguywireandtowerpoleasshownonfig.7.Theairclearanceinthemidphaseasafunctionoftheinsulatorswingangleisalsodependentontheangle

oftheguywire.

Table3showstherelationshipbetweenswinganglesandairclearanceforconfiguration14,witha

guywireangleof40.Theminimumairclearancesisbetweenphaseandguywire,hencetheseair

clearancesarecomparedtominimumrequiredairclearancesaccordingtoENstandard.The

comparedclearancesaremarkedwithboldfront.


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23

Table3Airclearancesforconfiguration14andminimumrequiredairclearances.

Configuration

Ref.table3

Typeof

impulse

Min.requiredair

clearanceaccordingtoEN

Swing

angle

Distance

Phase guy

wire

Distance

Phase

traverse

1 LI 2.687m 0 2.55m 2.55m

1 LI

10 2.236m

2.511m

1 SI 1.907m 20 1.981m 2.396m

1 SI 30 1.793m 2.208m

1 PF 0.834m 40 1.678m 1.953m

2 LI 2.687m 0 2.7m 2.55m

2 LI 10 2.360m 2.511m

2 SI 1.913m 20 2.106m 2.396m

2 SI 30 1.916m 2.208m

2 PF 0.835m 40 1.799m 1.953m

3 LI 2.866m 0 2.72m 2.72m

3 LI 10 2.358m 2.679m

3 SI 1.914m 20 2.113m 2.556m

3 SI 30 1.912m 2.356m

3 PF 0.835m 40 1.790m 2.084m

4 LI 2.866m 0 2.9m 2.72

4 LI 10 2.531m 2.679m

4 SI 1.920m 20 2.262m 2.556m

4 SI 30 2.060m 2.356m

4 PF 0.837m

40 1.935

m

2.084m

Table3showsthat:

MinimumrequiredairclearanceaccordingtoENstandardforallofthethreeimpulsetypes,

LI,SIandPF,isachievedwithconfiguration2and4wheretheairgapbetweenphaseand

guywireisgreaterthanthelengthoftheinsulatorstring.

Configuration1and3doesnotfulfillENstandardrequirementforminimumairclearance

whenexposedtolightningimpulses(LI).

Forallconfigurations,thedistancebetweenphaseandguywire,whichistheleastdurable

airgap,isalsotheshortestoneatswinganglesexceeding10.


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Table4Conductorcrossarmcalculatedfora250/2500spositiveswitchingimpulse,(SI)underdryandwetconditions.

Gapfactorsaregiveninparentheses.Altitudeof500m,towerheight=25m,guywireangleof40.

Configuration

Ref.table3

Typeof

impulse

Swing

angle

U50Phase

guywire

U50Phase

traverse

U50Dry

insulator

U50Wet

insulator

1 LI 0 1428(1.057) 1416(1.048) 1416(1.048) 1416(1.048)

1 LI 10 1253(1.058)

1407(1.057)

1 SI 20 859(1.228) 979(1.220) 991(1.182) 989(1.180)

1 SI 30 801(1.233) 926(1.223)

1 PF 40 848(1.135) 961(1.130) 1082(1.030) 1082(1.030)

2 LI 0 1448(1.057) 1416(1.048) 1416(1.048) 1416(1.048)

2 LI 10 1322(1.057) 1407(1.057)

2 SI 20 896(1.225) 979(1.220) 991(1.182) 989(1.180)

2 SI 30 839(1.230) 926(1,223)

2 PF 40 899(1.133) 961(1.130) 1082(1.030) 1082(1.030)

3 LI 0 1523(1.056) 1510(1.048) 1510(1.048) 1510(1.048)

3 LI 10 1336(1.057) 1500(1.056)

3 SI 20 898(1.225) 1022(1.218) 1035(1.181) 1033(1.178)

3 SI 30 838(1.229) 967(1.220)

3 PF 40 895(1.132) 1011(1.129) 1136(1.030) 1136(1.030)

4 LI 0 1546(1.056) 1510(1.048) 1510(1.048) 1510(1.048)

4 LI 10 1418(1.057) 1500(1.056)

4 SI 20 941(1.222) 1022(1.218) 1035(1.181) 1033(1.178)

4 SI 30 882(1.226) 967(1.220)

4 PF 40 953(1.130)

1011(1.129) 1136(1.030) 1136(1.030

Table4showsthat:

Thesametrendisrepeatedforallofthefourinsulators /airgapconfigurations:U50is

decreasingwithincreasingswinganglewhilethegapfactorisincreasingwithincreasing

swingangle.

Thevariationsofthevalueofthegapfactorsareinsignificantcomparetothevariationsof

U50.Intheory

this

results

indicate

that

achange

inthe

geometry

ofthe

tower

has

negligible

impacttothegapfactorandthustothevalueofU50.

ReductionofU50isratherduetoreducedclearancecausedbyincreasinginsulatorswing

angle.Theairgapbetweenphaseandguywirehasthegreatestlossofelectricalwithstand

strengthsincethisistheairgapthatreducedthemostasafunctionofincreasedswingangle.

Configuration2and4performbetteratinsulatorswingingthanconfiguration1and3.

However,thedesiredpropertyofhavingagreaterU50betweenphaseandguywirethanthe

U50betweenphaseandtraverseisonlyachievedwhenthereisnoswingangle.


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25

9 Theoryvs.laboratorytestingAsaconsequenceofvoltageupgrading,severaltowerswillendinguphavingrathertightdimensions

andhencelowersecuritymargins.Someofthesetowersmightnothavesufficientsecuritymargin

accordingtostandards.Asmentionedearlier,itisnotanabsoluterequirementtofollowthe

standardsaslongasonecanprovethatthetowersmeettherequirementsforsafetysetbythe

regulations.Asapartofthedocumentationprocesslaboratoryexperimentsshouldbeperformed.A

laboratorytestshouldbedoneonafullscaletestobject,whichinthiscaseisa300kVtransmission

tower,andwiththeexpectedvoltagelevelsandimpulsetypesthatatowermightbeexposedto.

Thiskindoflaboratorytestsareexpensive,complicatedandtimeconsumingtoperformandwillnot

beincludedinthisreport. However,afullscalelaboratorytestunderthedirectionofStatnettwillbe

performedatalaterstage.Forthisreport,twoexistinglaboratoryreportshavebeenusedfor

investigationoftowerinsulation:

1. ConductedbySTRIwiththetitle:Experimentaldielectrictestsonaporcelaininsulatorstring

with

cap

and

pin

insulators

type

NGK

CA500

[8].

2. ConductedbyEFI(formerSINTEF)withthetitle:Luftisolasjon.Underskelseavelektrisk

holdfasthetforlinjeisolasjon[7].

InthischapterstatisticalwithstandvoltageU50andgapfactorsfordifferentairgapconfigurations

areinvestigated.Theresultsfromthetwoabovementionedreportsareusedasabasisfor

comparisontothestandards.

9.1 Researchreport1:STRI

In2009STRI[8],aSwedishaccreditedtestinglaboratory,didaresearchonthedielectricproperties

ofinsulatorstringsbeingexposedtodifferentelectricalstresses.Experimentaldielectrictestswere

performedonaporcelaininsulatorstringwithcap andpininsulatorsidenticaltotheoneslocatedin

thetransmissionlineNeaHjrpstrmmen.TheresearchwasperformedonrequestofSvenska

Kraftnt.Thetestobjectusedforthisresearchworkwassimulatingatowerwindowasshowninfig.

16.

Figure16Testobjectsimulatingthetowerwindow.


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26

Thetestobjectconsistedof:

Aporcelaininsulatorstringconsistingofy=14(1.8m),15(1.96m)and16(2.1m)capand

pininsulatorstypeNGKCA500fromthelineNeaHjrpstrmmen.Theinsulatorstringwas

fittedintothecrossarmwithoriginalfittingdetails.

Theinsulatorstringwasmountedonacrossarmequippedwithtwoverticalmemberswitha

distanceofx=7.2mfromeachother,simulatingthepolesofthetower.

Thelinewassimulatedbya6meteraluminiumtubewithdiameter30mm,mountedonthe

originalarchinghornanddetails,fittedonthelowerendoftheinsulatorstring.

ThedistancefromthepowerlinetothegroundisH=6metres.

Thetestwasperformedwiththethreeimpulsetypes

Lightningimpulsedisruptivedischargetestwithpositivepolarityanddryconditions(LIdry).

Switchingimpulsedisruptivedischargetestwithpositivepolarityandwetconditions(SIwet).

Powerfrequencydisruptivedischargetestforwetconditions(PFwet).

TestvaluesofU50fromtheSTRItestreportarecomparedwithvaluescalculatedwiththeformulas

forU50givenintheENstandard.Thecalculationsare,asfaraspossible,carriedoutwiththesame

conditionsasthetestconditions.Gapfactorsarecorrectedforinsulatorsandrain.Thefollowing

tablesshowacomparisonbetweentestresultsandcalculations.

9.1.1

Lightningimpulse

Thelightningtestwasperformedonaninsulatorstringof15insulatorscorrespondingtoaflashover

lengthof1.96munderdryconditions.Theup anddownmethodwasusedtodetermineU50.

Table5U50,50%flashoverprobabilityforlightningimpulse(LIdry).

Test Numberof

insulators

U50corrected

test

U50calculated

conductor

window

kdry=1.041

Deviationbetween

testandcalculation

referredto

calculatedvalues

LIdry 15 1216kV 1081kV 12.4%

Forthelightningimpulsethedeviationis12.4%.Thecalculationsinthepreviouschapterindicated

thattheminimumairclearancesforthelightningimpulsewerethemostcriticalonesandrequired

minimumairclearancecouldnotbeobtainedforconfiguration1and3.


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27

Inthestandardsforinsulationcoordination,therequiredwithstandvoltage,Urwforlightning and

switchingimpulsesaresettobethevoltagethatgivesa10%probabilityforflashovertooccurinthe

airgapi.e.Urw=U10.Thisvalueisobtainedbymoving1.3standarddeviationstotheleftonthe

probabilitycurve.Forlightningimpulseonestandarddeviationis3%ofU50.Onewillthenendupat

10%probabilityforflashoverontheprobabilitycurve.

Urwisdeterminedbytheformula

10 50 1.3rwU U U Z kV (9.1)

whereZisthestandarddeviation

Figure17showstheprobabilityforaflashovertooccurasafunctionofthemagnitudeofthe

lightningimpulseforaninsulatorstringof1.96m,correspondingto15insulators.

Figure17

Cumulative

normal

distribution

probability

curves

for

flashover

for

lightning

impulse

(LI

dry)

and

15

insulators

(1.96m).

ThedifferencebetweenU50andUrwis1.3standarddeviations,whilethedifferencebetweenthetest

resultandcalculationcorrespondstoapproximatelyfourstandarddeviations.IfthevalueofU50

givenbythebluecurvethatrepresentthestandardisinsertedintotheformulaforUrw,weobtain

therequiredwithstandvoltage:

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

950 1050 1150 1250 1350

Prob

ability

Voltage[kV]

Test

ENstandard


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28

10 50 1.3 1081 1.3 0.06 1081 1039rwU U U Z kV (9.2)

wherethestandarddeviation,Z=0.03U50.

U10of1039

kV

on

the

blue

curve

gives

aprobability

of10

%for

flash

over,

while

itgives

aprobability

of3.17*105forflashoverontheredcurvethatrepresentsthelaboratorytestresult.

Conclusion:TheUrwvaluedeterminedfromthestandards,whichissupposedtohaveaprobabilityof

10%foraflashovertooccur,hasamuchlowerprobabilityforoccurrenceaccordingtothelightning

impulsedisruptivedischargetestwith15insulators.

9.1.2 Switchingimpulse

Theswitchingtestwasperformedunderwetconditionsonaninsulatorstringof14,15and16

insulatorscorrespondingtoaflashoverlengthof1.8,1.96and2.1mrespectively.Theup anddownmethodwasusedtodetermineU50.

Table6U50,50%flashoverprobabilityforswitchingimpulse(SIwet).

Test Numberof

insulators

U50corrected

test

U50calculated

conductor

window

kwet=1.155

Deviationbetween

testandcalculation

referredto

calculatedvalues

SIwet 14 786kV 753kV 4.4%

SIwet 15 851kV 802kV 6.1%

SIwet 16 919kV 843kV 9.0%

TheresultsfromtheswitchingimpulsedisruptivedischargetestandthecalculatedvaluesofU50have

adeviationthatvariesbetween4.49%,wherethedeviationincreasingwiththelengthoftheairgap.

Figure18showstheprobabilityforaflashovertooccurasafunctionofthemagnitudeofthe

switchingimpulseforinsulatorstringof1.96m,correspondingto15insulators.


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29

Figure18Cumulativenormaldistributionprobabilitycurvesforflashoverforswitchingimpulse(SIwet)and15insulators

(1.96m).

ThedifferencebetweenU50andUrwis1.3standarddeviations,asforlightningimpulses,butfor

switchingimpulses,onestandarddeviationcorrespondsto6%ofU50.Thedifferencebetweenthe

valueofU50basedonthestandardandU50testedisapproximatelyequaltoonestandarddeviation

whenconsideringaflashoverlengthof15insulatorsor1.96m.IfthevalueofU50givenbytheblue

curvethatrepresentthestandardisinsertedintotheformulaforUrw,weobtaintherequired

withstandvoltage:

10 50 1.3 802 1.3 0.06 802 739rwU U U Z kV (9.3)

wherethestandarddeviation,Z=0.06U50.

U10of739kVonthebluecurvegivesaprobabilityof10%forflashover,whileitgivesaprobability

of0.014forflashoverontheredcurvethatrepresentsthelaboratorytestresult.

Table7belowshowstheresultsoftheU10switchingimpulsedisruptivedischargetestoninsulator

stringsof14and15insulatorscorrespondingtoaflashoverlengthof1.8and1.96mrespectively.

0

0,1

0,2

0,3

0,4

0,50,6

0,7

0,8

0,9

1

650 750 850 950

Probabilit

y

Voltage[kV]

Test

ENstandard


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30

Table7U10,10%flashoverprobabilityforswitchingimpulse(SIwet).

Test No.of

insulators

No.of

impulses

U10correctedtest

No.offlash

over

U10calculatedconductor

window

kwet=1.155

Deviation

betweentest

andcalculation

referredto

calculatedvalues

SIwet 14 15 732kV 1 695 5.3%

SIwet 15 15 804kV 3 739 8.8%

Again,whenaninsulatorstringof15insulatorsisconsidered,thetestresultshowsthatthevalueof

U10=804kVwhichis8.8%higherthanU10calculatedaccordingtostandards.

IfthetestresultsoftheU50 testareinsertedintotheformulaforUrw,ideallyweshouldendupwith

aresultsimilartotheU10 testofequallynumberofinsulators.WheninsertingthetestresultofU50

at15insulatorsobtain:

10 50 1.3 851 1.3 0.06 851 785rwU U U Z kV (9.4)

whichissomewhatbetweentheactualtestresultof804kVandthevalueobtainedfromthe

standardof739kV.Thisindicatesthattheremaybesafetymarginsaddedinboththeformulafor

requiredwithstandvoltage,UrwandthevalueofthestatisticalwithstandvoltageU50.Thismayin

sumgiveanunnecessarilyhighsafetymargin.

Conclusion:TheUrwvaluedeterminedfromthestandards,whichissupposedtohaveaprobabilityof

10%foraflashovertooccur,hasa1.4%probabilityforoccurrenceaccordingtotheswitching

impulsedisruptivedischargetestwith15insulators.

9.2

Researh

report

2:

EFI

TheresearchworktitledLuftisolasjon.Underskelseavelektriskholdfasthetforlinjeisolasjon[7]

conductedbyformerSINTEF,EFIin1971discussesdimensioningofinsulationintowersand

probabilityforflashover,i.e.probabilisticinsulationdimensioning.Thetestobject,aninsulator

stringof9to18cap andpininsulators,wasexposedtolightningimpulses,switchingimpulsesand

50Hzoperatingvoltage.Theresearchtakesaimtoestablisharelationshipbetweenflashover

voltageforarodplanegapandrelevantinsulationconfigurationsofequalstrikedistancei.e.thegap

factor,Kg.

Otherissuesthatwereinvestigatedweretheimpactsoftheinsulatorswingangleandraintothe

electricalwithstandvoltage.ThereportpresentsthevalueofU50forthethreedifferenttypesof


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31

electricalstressatfivedifferentinsulatorconfigurationsformid andouterphaseandatfour

differentswinganglesoftheinsulatorstringoftheouterphase.

Thetest

object

used

toperform

the

tests

was

simulating

outer

phase

and

tower

window

asshown

in

fig.19a),b)andc).

Figure19Testobjectusedtosimulatethetowerarrangement.Figurea)andb)representstheouterphase.Swingangle

issimulatedaccordingtofigureb).Figurec)representsthemidphase[7].

Thetestobjectconsistedof:

Istringinsulatorsof9,15and18cap andpininsulatorsandVstringinsulatorsof9and15

cap andpininsulatorstypeNTP33019,21ton.

Towerarrangementaccordingtofig19.

Thetestprocedurewasasfollows:

Towercrossarmwassimulatedaccordingtofig19a)andb)forIstringinsulatorsandtower

windowweresimulatedaccordingtofig19c)forbothVstringandIstringinsulators.

Insulatorswingingwassimulatedfortheouterphaseaccordingtofig19b)for10,20and

30.


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32

Theexperimentswereperformedusingdifferentvoltageimpulseshapessimulatinglighting

impulsesandswitchingimpulses.Tosimulatelightingstrikeanimpulseshapewithtimeto

peakT1=1.2sandtimetohalfvalueT2=50swasusedaccordingtofig20.The1.2/50s

impulseisdefinedasthestandardlightningimpulse.

TheswitchingimpulsewassimulatedbyapplyingavoltageshapewithtimetopeakT1=200

sandtimetohalfvalueT2=3000stothetestobjectaccordingtofig20.

Figure20Voltageimpulse.Xaxis:T1=timetothepeakvalueoftheimpulseisobtained,T2=timetohalfofthepeak

valueoftheimpulseremains.Td=timewheretheimpulsehasavoltagelevelbetween0.9and1.0PU.Yaxis:Valueof

thevoltageoftheimpulseinPU.

9.2.1 Gapfactors

Theresearchestablishedarelationshipbetweenflashovervoltageforarodplanegapandrelevant

insulationconfigurationsofequalstrikedistanceaccordingtofig.19a),b)andc).Therelation

betweentheflashovervoltageforacertaininsulationconfigurationandarodplanegapisdescribed

bythegapfactor.Severaldisruptivedischargetestsareperformedforseveralvariantsofthe

geometry(s,z,y,andx)forlightning /switchingimpulsesand50Hzpowerfrequencyandforwet

anddryconditions.Thetestresultsarecomparedtodisruptivedischargetestsofarodplanegapof

equallengthasareferencegap.

Thegapfactoristherelation:

50

50

_ _

_ _ . _ .g

U test objectK

U rod plane pos pol

(9.5)

ThegapfactorsobtainedfromtheEFItestarecomparedtogapfactorsobtainedaccordingtothe

formulas3.3,3.4,3.5and3.6inchapter4Gapfactors.


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33

ParisandCortinasuggestedseveralgapfactorsdependingonthegapconfiguration,from1forarod

planegapto1.9foraconductorrodgap.EN50341reproducestypicalvaluesforgapfactorsthatare

basedontypicaldimensionsfordifferentinsulatingconfigurations.Thesearefixedvaluesgivenfor

onespecificconfigurationwhichdimensionsarewithinanareaofapplication.Ifamoreaccurate

valueofthegapfactorisdesired,itcanbecalculatedaccordingtotheformulas3.5and3.6fromthe

Cigr72technicalbulletin[2]whichdescribeshowtocalculatethegapfactorsforthedifferent

geometricconfigurations.Bydoingthisoneobtainagapfactorthatshoulddescribetheelectric

propertiesoftheairgapmoreaccurately,sincethestrikedistanceisalsotakenintoaccount.

Whenputtingindifferentstrikedistancesfrom210metresintheformula3.6fortowerwindow,

andplottingtheresults,oneobtainsthefollowingcurvewiththegapfactorasafunctionofthe

strikedistancethatis,thestrikedistanceinthexaxisandthegapfactorintheyaxis.

Figure21Gapfactorforpositivepolarityswitchingimpulsetowerwindow(midphase)asafunctionofthestrike

distance.

Fromthecurveitiseasytovisualizethattheinfluenceofthestrikedistancetothegapfactorissmall

andthattheoutcomefromcalculatingthegapfactorforaspecificstrikedistancedoesnotdiffer

significantlyfromthefixedvaluesgiveninthestandard,whichinthiscase,thetowerwindow,would

be1.25.ThisobservationsupportsthestatementfromtheCigr72technicalbulletinwhichstate:

Somethingthatshouldbenoticedisthatthegapfactorispracticallyunaffectedbythelengthofthe

airgap[2].

TheresearchworkconductedbyEFIin1971[7]concludesthatforswitchingimpulsethegapfactoris

dependentonthestrikedistanceandthatthegapfactorsvaryapproximatelylinearlywhenthestrike

distanceisintherangeof13metres.Thisconclusionseemsreasonablecomparedtothecurveinfig.

21intherangeof13metres.

1,21

1,22

1,23

1,24

1,25

1,26

1,27

1,28

2 3 4 5 6 7 8 9 10

Gap

factor

Strikedistance(m)


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34

However,asfig.21showsadecreaseofthegapfactorbetween13metres,theEFIreportclaims

thatthegapfactorincreaseswithincreasedstrikelengthinthesamearea.

TheEFIresearchsuggeststhatforairgapsof1m


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35

9.2.1.1 Lightningimpulse

Outerphase

Table8Gapfactorsforouterphaseatlightningimpulse.

Testobject Dry/

wet

Geometrym Kg

EFItest

Kg

Standards z y x

Istringinsulator9segments Dry 0.18 1.35 1.53 2.40 1.13 1.099

" " 0.60 1.35 1.95 2.40 1.15 1.041

" " 0.60 1.35 1.95 3.06 1.16 1.041

" " 1.12 1.35 2.47 4.00 1.15 1.080

" Wet 0.18 1.35 1.53 2.40 1.10 1.099

" " 0.60 1.35 1.95 2.40 1.10 1.041

" " 0.60 1.35 1.95 3.06 1.13 1.041

" " 1.12 1.35 2.47 4.00 1.14 1.080


"

Wet

0.18

2.38

2.56

4.00

1.09

1.079

" " 0.60 2.38 2.98 4.00 1.12 1.075


Midphase

Table9Gapfactorsformidphaseatlightningimpulse.

Testobject Dry/

wet

Geometrym Kg

EFItest

Kg

Standards z y x

Vstringinsulator9segments Dry 0.18 1.34 1.26 2.40 1.07 1.058

" " 0.60 1.34 1.54 3.06 1.08 1.054

" " 1.12 1.34 1.90 4.00 1.13 1.051

" Wet 0.18 1.34 1.26 2.40 1.08 1.058

" " 0.60 1.34 1.54 3.06 1.09 1.054

" " 1.12 1.34 1.90 4.00 1.07 1.051


" " 0.60 2.37 2.63 4.50 1.06 1.048

" Wet 0.18 2.37 2.31 4.50 1.05 1.049

" " 0.60 2.37 2.63 4.50 1.06 1.048


" Wet 0.60 1.35 1.95 3.06 1.11 1.051


" Wet 0.18 2.38 2.56 4.50 1.07 1.048

ForlightningimpulsesthegapfactorsfromtheEFItestareverysimilartothegapfactorsobtained

fromthestandardforbothdryandwetconditions.Rainseemtohavenoinfluenceonthewithstand

strengthofIstringsorVstringsexposedtolightningimpulses.


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36

9.2.1.2 Switchingimpulse

Outerphase

Table10Gapfactorsforouterphaseatswitchingimpulse.

Testobject Dry/

wet

Geometrym Kg

EFItest

Kg

Standards z y x


" " 0.60 1.35 1.95 3.06 1.22 1.332

" " 1.12 1.35 2.47 4.00 1.26 1.302

" Wet 0.18 1.35 1.53 2.40 0.84 1.323

" " 0.60 1.35 1.95 3.06 0.79 1.297

" " 1.12 1.35 2.47 4.00 0.78 1.276


" Wet 0.18 2.38 2.56 4.00 1.05 1.274

Midphase

Table11Gapfactorsformidphaseatswitchingimpulse.

Testobject Dry/

wet

Geometrym Kg

EFItest

Kg

Standards z y x


" " 0.60 1.34 1.54 3.06 1.09 1.206

" " 1.12 1.34 1.90 4.00 1.14 1.194

" Wet 0.18 1.34 1.26 2.40 1.00 1.213

" " 0.60 1.34 1.54 3.06 0.98 1.201

" " 1.12 1.34 1.90 4.00 0.86 1.190


" Wet 0.60 2.37 2.63 4.50 1.08 1.179

Istringinsulator9segments Dry 0.60 1.34 1.90 3.06 1.20 1.194" Wet 0.60 1.34 1.90 3.06 0.94 1.190


" Wet 0.18 2.38 2.56 4.50 1.08 1.180

Forswitchingimpulses,theEFItestshowsahigherdecreaseofthegapfactorasaconsequenceof

rainthanthoseobtainedfromthestandard.TheEFItestshowsadecreaseofkgandthusthe

withstandstrengthintheorderof613%forVstringinsulatorsand2034%forIstringinsulators.

Theinfluenceofraintothegapfactorisdecreasingwithincreasedinsulatorlength.Thegapfactors

obtainedfromthestandardshowadecreaseofkgintherangeof04%.However,theEFItestresults


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37

andtheENstandardseemtofolloweachotherrelativelyindryconditions.ThegapfactorsoftheEN

standardaresomewhathigherthantheEFItestresults.

9.2.1.3 50Hzpowerfrequency

Outerphase

Table12Gapfactorsforouterphaseatpowerfrequencyvoltage.

Testobject Dry/

wet

Geometrym Kg

EFItest

Kg

Standards z y x


" " 0.60 1.35 1.95 2.40 1.17 1.054

" " 0.60 1.35 1.95 3.06 1.17 1.026

" " 1.12 1.35 2.47 4.00 1.20 1.050

" Wet 0.18 1.35 1.53 2.40 0.75 1.061

" " 0.60 1.35 1.95 2.40 0.69 1.054

" " 0.60 1.35 1.95 3.06 0.69 1.026

" " 1.12 1.35 2.47 4.00 0.69 1.050

Rodplane Dry 1.35 1.03

Midphase

Table13Gapfactorsformidphaseatpowerfrequencyvoltage.

Testobject Dry/

wet

Geometrym Kg

EFItest

Kg

Standards z y x


" " 0.60 1.34 1.54 3.06 1.00 1.034

" " 1.12 1.34 1.90 4.00 1.00 1.032

" Wet 0.18 1.34 1.26 2.40 0.71 1.037

" " 0.60 1.34 1.54 3.06 0.79 1.034" " 1.12 1.34 1.90 4.00 0.74 1.032


" Wet 0.60 1.35 1.95 3.06 0.80 1.032

Asfortheswitchingimpulses,the50Hzpowerfrequencytestshowsthatthedifferenceinkgindry

andwetconditionsseemstobelargerfortheEFItestthanproposedbythestandard.TheEFItest

showsadecreaseofthegapfactorasaconsequenceofrainintheorderof25%forVstring

insulatorsand3340%forIstringinsulators.Thereisnodifferencefordryandwetconditionsforthe

gapfactorsobtainedfromthestandard.However,thetestresultsandthecalculationsseemto


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38

followeachotherrelativelyindryconditions.Oppositetoswitchingimpulses,thegapfactors

obtainedbytheENstandardaresomewhatlowerthanthosefromtheEFItestresults.

ThetestsperformedwithswitchingimpulseandpowerfrequencyvoltageshowthattheIstring

suffersagreaterlossofelectricwithstandstrengththantheVstringasaconsequenceofrain.This

canbeexplainedbythenaturallydraineffectcausedbythe45angleoftheVstringinsulators.ThesefindingsareconsistentwithwhatwasfoundinCigrreport72[2](seechapter4.4).Asfor

lightningimpulsesitdidnotseemtobeanydifferencesondryandwetconditions.Thismayindicate

thatrainhasalesserimpactonshortdurationimpulsewaves.

9.2.2 U50disruptivedischargetest

9.2.2.1 ElectricalstressfromLightning

OuterphaseEFItest:

Table14Conductorcrossarmexposedtoa1.2/50spositivelightningimpulse,(LI)underdryconditions.

Testobject Geometry

D1/dxO.Pref.fig.7

Swingangle

U50EFI

Kg=1.31at0

%reductionat

swingangle

Istringinsulator 2.56/4m 0 1380 0

" " 10 1354 1.9

" " 20 1289 6.6

" " 30 1204 12.8

Midphasecalculated:

voltage upgrading of overhead lines.pdf

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