《HighVoltageEquipmentTesting》GroundingResistanceMeasurementofGroundGrid主講人：何發(fā)武（副教授）OperatingWorksheet：OperatingWorksheet：TableofContents01Followme02Workshop03Training01FollowmeⅠ.OverviewofGroundingResistanceGroundingresistancereferstotheresistancevaluebetweenthegroundingelectrode,equipmentenclosure,orbuildinggroundingelectrodeandtheearth.Thesmallerthegroundingresistance,thebetteritcandivertleakagecurrentsorlightningsignalsintotheground,preventingpersonalandequipmentdamage.Groundingresistanceincludescontactresistancebetweenelectricalequipmentandgroundingwires,resistanceofgroundingwiresorgroundingbodiesthemselves,contactresistancebetweengroundingbodiesandtheearth,andresistanceoftheearth.Ⅰ.OverviewofGroundingResistanceAtpresent,thegroundingresistanceofthegroundinggridismainlymeasuredbythepowerfrequencylargecurrentthree-polemethodinthepowersystem.Inordertopreventthepowerfrequencyinterferencegeneratedduringtheoperationofthepowergridandimprovetheaccuracyofthemeasurementresults,theinsulationpreventivetestprocedurestipulatesthatthetestcurrentofthepowerfrequencylargecurrentmethodshallnotbelessthan30

A.Ⅰ.OverviewofGroundingResistanceThe50

Hzdataobtainedfrommeasurementsinstronginterferenceenvironmentsofsubstationsareparticularlysuitableforenvironmentswithlargefluctuationsinrailwaysubstationreturncurrent.Theanti-interferencegroundinggridgroundingresistancemeterisaspecializedinstrumentusedtomeasurethegroundingresistanceandconductivityofagroundinggridbetweengroundingpoints.Itiscapableofmeasuringbothgroundingimpedanceandresistancesimultaneously,providinganaccuratereflectionofthegroundinggrid’sactualcharacteristics.Ⅱ.PrinciplesofGroundingResistanceMeasurementGroundingresistancereferstotheresistancedisplayedwhenreleasingcurrentthroughagroundingdevice,numericallyequaltotheratioofthevoltagedropacrossthegroundingdevicetothecurrentflowingintotheearth.Intheequation,theUrepresentsthevoltagetothegroundofthegroundingdevice,whichisthepotentialdifferencebetweenthegroundingelectrodeandthereferencepointatzeropotentialwithrespecttotheearth.TheIrepresentsthecurrentflowingintotheearththroughthegroundingdevice.Therefore,theprincipleofmeasuringgroundingresistanceisbasedonequation(3-1):applyingacurrentItothegroundingelectrodeorgroundinggrid,measuringthevoltageUonthegroundingelectrode(grid),anddividingthevoltagebythecurrenttoobtainthegroundingresistance.Ⅲ.FiveCommonConnectionMethodsforGroundingResistanceTestersThegroundingresistancetesterisspecificallyusedtotesttheelectricalintegrityofgroundingdevices.Similartoaninsulatorfaultdetector,itispartofelectricaltestingandaregularinspectionitem.Themainobjectsofmeasurementincludethegroundingresistanceofpowerlinetowers,transformers,etc.Thetestercommonlyutilizesthefollowingfiveconnectionmethods:(Ⅰ)ClampMeasurementMethodThismethodmeasuresthegroundingresistanceateachgroundingpointinamulti-pointgroundingsystemwithoutdisconnectingthegroundingconnectionstopreventpotentialhazards.Itissuitableformulti-pointgroundingsystemswheretheconnectionscannotbedisconnectedandtheresistanceofeachgroundingpointneedstobemeasured.Ⅲ.FiveCommonConnectionMethodsforGroundingResistanceTesters(Ⅱ)Double-ClampMethodThismethodmeasuresthegroundingresistanceofasinglegroundingpointinamulti-pointgroundingsystemwithoutusinganauxiliarygroundingrod.Whenconnecting,thecurrentclampisconnectedtothecorrespondingsocket,andbothclampsareclampedonthegroundingsemiconductor.Thedistancebetweenthetwoclampsshouldbegreaterthan0.25

m.Ⅲ.FiveCommonConnectionMethodsforGroundingResistanceTesters(Ⅲ)Two-PoleMethodTheremustbeaknownwell-groundedpoint,andthemeasuredresultisthesumoftheresistancesbetweenthetestedgroundandtheknownground.Iftheresistanceoftheknowngroundismuchsmallerthanthatofthetestedground,themeasurementresultcanbeusedastheresultofthetestedground.Itissuitableforareaswithdensebuildingsorclosedareassuchascementfloorswhereitisimpossibletohitagroundingrod.Ⅲ.FiveCommonConnectionMethodsforGroundingResistanceTesters(Ⅳ)Three-PoleMethodTheremustbetwogroundingrods,oneauxiliaryground,andonedetectionelectrode,andthedistancebetweeneachgroundingelectrodemustbenolessthan20

m.Itismainlyusedforfoundationgrounding,constructionsitegrounding,andlightningprotectionballlightningrodgrounding.Themeasurementconnectionmethodsofthegroundingresistancetesterforgroundinggridscanbedividedintothethree-polemethodandthefour-polemethod.TheschematicdiagramsareshowninFigures3-96and3-97.Ⅲ.FiveCommonConnectionMethodsforGroundingResistanceTestersFigure3-96

Three-PolemethodwiringdiagramforgroundingresistancemeasurementFigure3-97

Four-polemethodwiringdiagramfor

groundingresistancemeasurementⅢ.FiveCommonConnectionMethodsforGroundingResistanceTestersDuringthethree-polemethodconnection:MeasurementofcurrentlineD:Thewirediameteris≥1.5mm2,andthelengthis3to5timesthediagonallengthofthegroundgrid.Measurementofvoltageline1:Thewirediameteris≥1.0mm2,andthelengthis0.618D.Measurementofvoltageline2:Connecttothegroundgridbeingtested.Measurementwire:Connecttothegroundgridbeingtested.Ⅲ.FiveCommonConnectionMethodsforGroundingResistanceTesters(Ⅴ)Four-PoleMethodSimilartothethree-polemethod,itreplacesthethree-polemethodinlowgroundingresistancemeasurementandeliminatestheinfluenceofmeasurementcableresistanceonthemeasurementresults.Thismethodisamoreaccurategroundingresistancemeasurementmethod,asshowninFigure3-97.Ⅲ.FiveCommonConnectionMethodsforGroundingResistanceTestersDuringthefour-polemethodconnection,twoconnectingwiresaredrawnfromthegroundingrodofthegroundinggridandconnectedtothevoltageterminalP2andthegroundinggridterminalC2oftheinstrument.Followingthemeasurementoperationsteps,theinstrumentwillautomaticallyeliminatewiringerrorsduringthefour-polemethodmeasurement.Inthethree-polestraight-linemethodandthetriangularmethodformeasuringgroundingresistance,itisnecessarytotemporarilycreateanauxiliarycurrentelectrodeinthedistancetoprovideareturnpathforthecurrent.02WorkshopⅠ.InstrumentoperationinterfacePaneldiagramisshowninFigure3-98.1–Currentpole(C1);2–Voltagepole(P1);

3–Voltagepole(P2);

4–Groundinggrid(C2);

5–232serialport;

6–Groundingpost;

7–LCDscreen;

8–Printer;9–Keyboard;

10–Powerswitch;

11–220

Vpoweroutlet.Figure3-98

PanelDiagram“”

Increase/storekey-modifythemenucontent,usingcircularscrolling.“”Decrease

key-modifythemenucontent,usingcircularscrolling.“”Function/printkey-Selectthemenuitem,andtheselecteditemisdisplayedinreversewhitefont.“”

Start/StopKey-Pressthiskeyonthe[Test]optiontoentertheteststate.Ⅱ.OperationSteps1.Usethethree-polemethodforwiring(1)Useamultimetertocheckifthereisanyopencircuitinthetestcurrentline,voltageline,andgroundingwire.Checkiftherustonthegroundingrodhasbeencleanedoffandifthedepthofitsburialisappropriate(>0.5meters).Also,checkiftheconnectionbetweenthetestwiresandthegroundingrodisconducting.Ifitisnotconducting,pleaseresolvetheissueandreconnect.(2)Thelengthratiobetweenthetestcurrentlineandthetestvoltagelineshouldbe1∶0.618.Thelengthofthetestcurrentlineshouldbe3to5timesthediagonallengthofthegroundinggrid.(3)Connectoneendofthetestcurrentlineandthetestvoltagelinetotheinstrumentasrequired,andthenlaythemoutinparallel.Connecttheotherendofeachlinetotwogroundingrodsrespectively.Ⅱ.OperationSteps(4)Double-checkthelaid-outtestlines.Connectoneendofthemultimetertothetestcurrentlineorthetestvoltageline,andconnecttheotherendtothegroundingwire.Ifthereisnoresistancevaluedisplayed,itindicatesanopencircuit.Confirmthateverythingisingoodorderbeforeproceedingwiththetest.(5)Afterconfirmingthattheconnectionsarecorrect,connecttheinstrumenttoanAC220

V/50

Hzpowersourceandpoweriton.Ⅱ.OperationSteps(6)SelectGroundingResistanceTest.Onthestartupinterface,whenmovingthecursortotheGroundingResistanceTest,usetheincreaseanddecreasekeystomodifythetestwirelength.Ingeneral,choose20metersasthetestwirelength,asshowninFigure3-99.Figure3-99

MovingtheCursortoGroundingResistanceTestⅡ.OperationSteps(7)Movethecursortothedesiredtestitem,andpresstheStart/Stopkeytoenterthetestforthatitem.PresstheMeasurekeytostartthemeasurement.(8)Aftertheinstrumentdisplaystheendofthetest,recordthetestdata(multiplemeasurementscanberepeated),asshowninFigure3-100.Figure3-100

Testdata①

Themeaningsofthemeasurementresultsareasfollows:Z=1.491W:Groundingimpedancevalue.U=1.45

V:Testvoltagevalue.I=0.97

A:Testcurrentvalue.F=47.5/52.5

Hz:Representsthetestfrequencyof(50±2.50)Hz.Ⅱ.OperationSteps(9)Afterturningofftheinstrument’spower,removetheconnections,andthetestprocessends.Figure3-101

Testdata②

Themeaningsofthemeasurementresultsareasfollows:Z=1.491W:Groundingresistancevalue.ρ=187.4W·m:Soilresistivity.U=1.45

V:Earthvoltagevalue.I=0.97

A:Testcurrentvalue.F=47.5/52.5

Hz:Representsthetestfrequencyof(50±2.50)Hz.Ⅱ.OperationSteps2.Double-clampmethodformeasuringgroundingresistancevalueThedouble-clampmethodissuitableformeasuringthegroundingresistancevalueofanindependentmulti-pointgroundingsystem.AsshownintheFigure3-102,thereisnoneedtohitagroundingstaketomeasurethegroundingresistancevalueinamulti-pointgroundingsystem.BothcurrentclampsAandBareclampedtothetestgroundingleadwiresimultaneously.Notethatthedirectionofthetwocurrentclampsshouldbethesame,andthedistancebetweenthemshouldbegreaterthan30

cm.Thetwocurrentclampsmustnotbeinterchanged,otherwise,errorsmayoccur.Ⅱ.OperationStepsPressthered“TEST”buttonontheinstrumenttostartthetest.Afterthetestiscompleted,stabledatawillbedisplayed,whichrepresentsthegroundingresistancevalueofthetestedgroundingelectrodeR=Re+R1//R2//R3,whereR1//R2//R3.Figure3-102

WiringDiagramforDouble-clampMethodofGroundingResistanceMeasurementⅢ.PrecautionsWhenthetestcurrentis0.0

A,itmay