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

65

8

DISTRIBUTIONTAPS

65A

15A

25A

40A

15A

9

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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65

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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