40 ohm ground fault resistance – still applicable? · nov. 8 – 10, 2016 tom guttormson, pe...
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40OHMGROUNDFAULT RESISTANCE–STILLAPPLICABLE?
MNPowerSystems ConferenceNov.8– 10,2016
TomGuttormson,PEPrincipleTechnologyEngineer
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130Kmembercustomers1,000squaremilesterritory44substations(48transformers)12.5kV- 9,121milesofline92%residentialbycustomerSystempeakdemand550MWWholesale PowerSupplier – GreatRiverEnergy
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OUTLINE
q Definition– groundfaultresistanceq Basisforresearchq Historyofgroundfaultresistancevalueq Practicesamongutilitiesq Recenttestingandpublicationsq Conclusion
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GROUNDFAULTRESISTANCE
Resistancethroughreturnpathbetweenfaultedconductorandtransformersource
A
C
B
N
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SIMPLEDIAGRAM
ZL
ZN
ZLoadZFaultZT
IFault
ZFault IFaultBoltedFault:0Ω 3470ampsFaultimpedance:40Ω 173amps
Assume:15MVAtransformerwithZ=5.75%Line impedance ZL=1.4+j1.7Ω
1mile4/0ACSR3miles1/0ACSR
7200volts/40Ω =180amps
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BASISFORSTUDY:
2011– 2015Project–• ReplacedKSlinefusewithTfuses• Auditsystem:fix&optimizecoordination
Ongoingchallenge:• Protectingfor“minimumfault”• “Standard”hasbeen40ohms• Ifwecouldsqueezeinonemorelayeroffusesizes…
Question:dowehave touse40?Howabout30???
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40
140AGnd Trip 140AGnd Trip
400APhase Trip400APhase Trip
164min fault 164min fault
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DISTRIBUTIONTAPS
65A
15A
25A
40A
15A
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140Gnd Trip
400Phase Trip
164min fault
15 4025
Limitedto40ampfuse
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DISTRIBUTION– LOOPEDTAP
ILoad ILoad25amps 25amps
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Question:Isthe“40ohmrule”stillapplicable?
Goal:• Investigateopportunity toapplylowervaluetoimprovesectionalizing
coordinationConsiderations:• Continueconformancewithindustrypractices• Safetyisnotcompromised
https://www.youtube.com/watch?v=WdekH0UnWXk
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GROUNDFAULTIMPEDANCEHISTORY
• Studyandengineering report– EdisonElectric Institute(EEI)&Bell Telephonein1937
• Groundfaultimpedance testingwithstagedfaults– measured impedance inrangeof5to20ohms
• Someutilitycompaniesadopted20ohmsasstandard– oneadopted40ohms
REApublishedBulletin61-2in1953;recommended:• 40ohmsforsubstationtransformers5000kVAandless• 30ohmsofOHlines&10-20ohmsforUGlinesfortransformers>5000kVA
Manycoopsadopted40ohmsasstandard
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MORERECENTRESEARCH/PUBLICATIONS
REABulletin61-2rescindedin1992,replaceby1742E-102“GuideforSectionalizingDistributionLines”(10/23/2012)• OHline30-40ohms>5000kVAtransformer,40ohms5000kVAandless• UGline10-20ohms• “40ohmrule”was intendedtobeguild line,notcast-in-stonerule
CRNStudy1997– GroundFaultImpedanceValuesforSystemProtection• SameconclusionsasRUS bulletin(didnotconductactualtesting)• CRN&otherdocumentsstate“othervaluesandmethodsmaybeapplied”
- uptotheCoop’sengineer
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RECENTTESTING…
• IEEEandABBresearch– testingbydroppingenergizelineonvarioussurfaces
• HighimpedancefaultsR>>40ohms– cannotbedetectedbytraditionalmeans
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GROUNDCONTACTRESISTANCE
Burke, James. Hard to Find Information About Distribution Systems. 5th Edition. ABB Inc. Consulting. Raleigh, NC. August 2002
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FAULTCURRENTAMPSBASEDONRESISTANCE
Surface Current(Amps)fromFigure3
Correspondingimpedance (Ω)
Dryasphalt,concreteordrysand <5 >1400
Wetsand <20 >360
Drysod 20 360
Wetsod 40 180
Drygrass 25 288
Wetgrass 45 160
Reinforced concrete 70 103
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VALUES/METHODSUSEDACROSSUTILITIES
Ø 40ohms(acrossanycoops,basedonREA/RUSBulletins)
Ø Calculatefaultcurrentusingmultiplierofloadcurrent
Ø Applypercentageofboltedfaultcurrent
Ø Nofaultimpedance(assumeboltedfault)
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MEASUREDFAULTCURRENTS- CONNEXUS
Beganrecordingfaultcurrentvaluesin2011Findingsfromfallenlines:
• Higherfaultcurrentvalues(“low”impedance) <<30ohms
• Lowerfaultcurrentvalues(“high”impedance) >>40ohms
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CONCLUSIONS
Ø ZfaultvaluesaregenerallyNOTinthe30to40ohmrange- basedontestingbyothersand internalmeasurements
Ø “GroundFaultImpedanceisadesignnumberforpurposeofmodelingandprotection/coordinationdesign”– not intended torepresentactual impedances andcurrentvalues
Changingfrom40to30ohms:ü enablesbettersectionalizing >enhancedreliabilityü will notmeasurablycompromise publicsafetyü Stillcomplieswithacceptable industrypractices
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560APhase Trip560APhase Trip
200AGnd Trip 200AGnd Trip
15 25 40 65
212min fault 212min fault
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TEAM
Contributions by:DaveAndersonJeanneAdventMikeFelberJohnGasalEricJacksonWaltLepistoNickLoehleinJimPaulsonJonWahlgrenMattYsethDeniseZurn
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