Soil Resistivity Testing Services - Resistivity...Soil Resistivity Testing Services ... Accurate soil resistivity for power system earthing ... equations from IEEE80 give large errors

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  • www.libertyconsulting.com.auCopyrightLCS2016

    SoilResistivityTestingServicesLibertyConsultingServices(LCS)hasconductedextensiveelectricalsoilresistivitytestingformanyyears,insomeofthemostdifficultsoilconditionsAustraliahas tooffer.During this time,LCShasdevelopedreliable techniques toprovideaccuratetestdatasothata lowmarginoferror isabletobeobtainedforearthsystemdesignsusingtheprovided testmeasurements. Theuseof adequatelypowered test equipment to suit the conditions is crucial toperformingmeaningfulelectricalsoilresistivitytesting.

    Whysoilresistivitytestingissoimportant

    Soilresistivity isthebasisuponwhichearthgridsaredesignedsothatelectricalparametersaredeterminedandmodelled(e.g.gridresistance,andmeshvoltagesduetoinjectedfaultcurrents).Thisisacrucialstepindesigningandassessinganyearthingsystem.

    Soilisrarelyhomogeneous,andiscommonlymadeupof horizontal layers which need to be identified sothatthecorrectplacementofearthingconductorscanbedeterminedduring thedesignprocess.Theuseofuniform or averaged tested resistivity values canleadtosignificanterrorsinearthingdesigns.

    Qualified LCS engineers perform testing so thatdifficult conditions encountered in the field can behandledonsite,andretestingsuspectdatapoints isdone as required. This ensures the provision ofaccuratetestdatatothedesigningengineer(s).

    TheseStandardsdetailsoilresistivitytestmethods:

    SoilResistivityTestingStandards ENAEG0PowerSystemEarthingGuide

    (Section7especially7.2.3) ENAEG1SubstationEarthingGuide

    (Section5.2especially5.2.2) AS/NZS1768Lightningprotection

    (AppendixC.10especiallyC10.1) IEEEStd80IEEEGuideforSafetyinACSubstation

    Grounding(Sections13.3&13.4) IEEEStd81IEEEGuideforMeasuringEarth

    Resistivity,GroundImpedance,andEarthSurfacePotentialsofaGroundingSystem(Section7)

    Soilelectricalcharacteristics

    The key determinant of error in the earthing designprocess is the inaccuracy or variation of the soilresistivityinputtothedesign.

    Therefore it is important to understand electriccurrent flow innonuniform,multilayered soils, andhow to accurately test soil resistivity in a diverserangeofconditions.

    www.libertyconsulting.com.auTel:0434005393

  • www.libertyconsulting.com.auCopyrightLCS2016

    There are four basic parameters that influence soilresistivityvalues:

    1. Chemicalcontent(soiltype,constituents);2. Moisturecontent;3. Temperature;and4. Particlesizeanddistribution.

    The use of geotechnical data from the proposedtesting area will often provide good backgroundinformationandsanitycheckoftestresults.

    Accuratesoilresistivityforpowersystemearthing

    TheWennerorfourpinequalspacingmethodisthepreferred test regime used by LCS. Other methodsinclude the Schlumberger or Driven Rod, and thesehaveadvantagesincertainspecificconditions.

    IntheWennermethod, it is importanttounderstandhow far apart test probes need to be in successiveteststoprovideanaccuratesoilprofileasaninputtothedesignsoftware.

    The largest spacing (see a below) needs to be atleastequaltothezoneof influenceoftheearthingsystem being designed. A sample test instruction isprovidedwithrecommendedspacings.

    The meter measures apparent resistivity, not theactualresistivity.Thevalueobtained isaweightedaverageof the actual resistivitydown to the depth(spacing) being tested. This raw data must beinterpretedbysoftware(e.g.CDEGSRESAPmodule)inordertodeterminetheactualsoilresistivity.

    Howmanytestpointsarerequiredinatraverseset?

    Experiencehasshownthatthegreaternumberoftestpoints in aWenner traverse, a higher accuracy soilmodel(multilayer)willbedevelopedbysoftware.Werecommendatleast10datapointsineachtraverse.

    Softwarecomputationsaremoreaccurate ifthedatarecordeddoesnothavetoomanygapsbetweenthespacings.Thepreferredratiobetweenspacingsis1.5.A reading taken at 5 metres, would need to befollowedupbya7.5metrespacing,andprecededbyatleast3.5metres,inordertoadheretothis1.5rule.

    Soilresistivitytestresultsandanalysis

    Theuseof sophisticatedmodelling software such asCDEGS RESAP module is able to provide accuratemultilayer soil profiles. A % error is provided witheachsoftwaremodel.

    Multilayer software analysis is critical to providingaccurate inputs to the design process. High/Lowversus Low/High soil models give quite differentresultsforaninjectedearthfaultcurrent.

    Large variations in soil resistivity near the surfaceinfluence step and touch voltage performance.Whereas large variation deeper below the groundsurface affect the transfer of earth potential rise toadjacentmetalworkandservices.

    Usingequivalentuniformsoilresistivityandempiricalequations from IEEE80 give large errors. Thereforesimply averaging the raw test data results is notadvisable.

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