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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
9.3 Conclusionofthechapter.....................................................................................................42
10 Acasestudy...............................................................................................................................43
10.1 KristiansandArendal.............................................................................................................43
10.2 Findingsofthecasestudy.....................................................................................................47
10.3 Savingpotential.....................................................................................................................47
10.4 Conclusionofthechapter.....................................................................................................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
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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2
Aflowchartismadetogivethereaderabetteroverviewofthedifferentissuesthatareinvestigated
andhowtheinvestigationisperformed.
Figure1Theprojectpresentedbyaflowchart
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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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6
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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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
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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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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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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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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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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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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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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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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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Figure8Verticalandhorizontalspanlengths.Visthelengthbetweenthelowerpointsofthelinewithintwospanswhile
Histhelengthbetweenthemiddleoftwospans.
ForcalculationofswinganglesoftheinsulatorstringsaprogramnamedPLSCADDwhichisa
computerprogramwithgraphicaluserinterfaceusedfordesigningofoverheadpowerlines.Figure9
showstheanglesoftheinsulatorstringsinatoweratnowindconditions,3yearswindconditions
and50yearswindconditions.
Figure9CalculationsofswinganglesoftheinsulatorstringsinPLSCADData)nowind,b)3yearwindandc)50year
wind.
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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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Figure11ExtensionofaVstringinsulator.
7.2 Alternativemeasuresfornarrowtowers
Fortowerscoveredbycategory1nomeasuresareneededotherthanextensionoftheinsulator
strings.
Towerscoveredbycategory2requireacloserinvestigationofthetowertoidentifythemostcritical
airgap.Whenthisisidentifiedonecanstartevaluatingpossiblemeasures.Insuchcasesitmightbe
enoughtojustdosomeminorchanges,suchasreplacingexistingfittingequipmentlocatedbetween
phaseandinsulatorwithmorecompactequipment,asillustratedinfig.12willgainalimiteddistance
intheairgap.
Figure12Fittingequipmentbetweenphaseandinsulator.
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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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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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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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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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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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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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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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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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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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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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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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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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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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However,asfig.21showsadecreaseofthegapfactorbetween13metres,theEFIreportclaims
thatthegapfactorincreaseswithincreasedstrikelengthinthesamearea.
TheEFIresearchsuggeststhatforairgapsof1m
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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
Istringinsulator15segments Dry 0.18 2.38 2.56 4.00 1.09 1.079
"
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
Istringinsulator18segments Dry 1.20 2.80 4.00 4.50 1.11 1.072
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
Vstringinsulator15segments Dry 0.18 2.37 2.31 4.50 1.07 1.049
" " 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
Istringinsulator9segments Dry 0.60 1.35 1.95 3.06 1.12 1.051
" Wet 0.60 1.35 1.95 3.06 1.11 1.051
Istringinsulator15segments Dry 0.18 2.38 2.56 4.50 1.07 1.048
" Wet 0.18 2.38 2.56 4.50 1.07 1.048
ForlightningimpulsesthegapfactorsfromtheEFItestareverysimilartothegapfactorsobtained
fromthestandardforbothdryandwetconditions.Rainseemtohavenoinfluenceonthewithstand
strengthofIstringsorVstringsexposedtolightningimpulses.
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9.2.1.2 Switchingimpulse
Outerphase
Table10Gapfactorsforouterphaseatswitchingimpulse.
Testobject Dry/
wet
Geometrym Kg
EFItest
Kg
Standards z y x
Istringinsulator9segments Dry 0.18 1.35 1.53 2.40 1.19 1.374
" " 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
Istringinsulator15segments Dry 0.18 2.38 2.56 4.00 1.31 1.298
" 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
Vstringinsulator9segments Dry 0.18 1.34 1.26 2.40 1.04 1.220
" " 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
Vstringinsulator15segments Dry 0.60 2.37 2.63 4.50 1.15 1.182
" 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
Istringinsulator15segments Dry 0.18 2.38 2.56 4.50 1.22 1.182
" 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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andtheENstandardseemtofolloweachotherrelativelyindryconditions.ThegapfactorsoftheEN
standardaresomewhathigherthantheEFItestresults.
9.2.1.3 50Hzpowerfrequency
Outerphase
Table12Gapfactorsforouterphaseatpowerfrequencyvoltage.
Testobject Dry/
wet
Geometrym Kg
EFItest
Kg
Standards z y x
Istringinsulator9segments Dry 0.18 1.35 1.53 2.40 1.15 1.061
" " 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
Vstringinsulator9segments Dry 0.18 1.34 1.26 2.40 0.98 1.037
" " 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
Istringinsulator9segments Dry 0.60 1.35 1.95 3.06 1.19 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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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:
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