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

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

Post on 05-May-2018

219 views

Category:

Documents

6 download

TRANSCRIPT

<ul><li><p>www.libertyconsulting.com.auCopyrightLCS2016</p><p>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.</p><p>Whysoilresistivitytestingissoimportant</p><p>Soilresistivity isthebasisuponwhichearthgridsaredesignedsothatelectricalparametersaredeterminedandmodelled(e.g.gridresistance,andmeshvoltagesduetoinjectedfaultcurrents).Thisisacrucialstepindesigningandassessinganyearthingsystem.</p><p>Soilisrarelyhomogeneous,andiscommonlymadeupof horizontal layers which need to be identified sothatthecorrectplacementofearthingconductorscanbedeterminedduring thedesignprocess.Theuseofuniform or averaged tested resistivity values canleadtosignificanterrorsinearthingdesigns.</p><p>Qualified LCS engineers perform testing so thatdifficult conditions encountered in the field can behandledonsite,andretestingsuspectdatapoints isdone as required. This ensures the provision ofaccuratetestdatatothedesigningengineer(s).</p><p>TheseStandardsdetailsoilresistivitytestmethods:</p><p>SoilResistivityTestingStandards ENAEG0PowerSystemEarthingGuide</p><p>(Section7especially7.2.3) ENAEG1SubstationEarthingGuide</p><p>(Section5.2especially5.2.2) AS/NZS1768Lightningprotection</p><p>(AppendixC.10especiallyC10.1) IEEEStd80IEEEGuideforSafetyinACSubstation</p><p>Grounding(Sections13.3&amp;13.4) IEEEStd81IEEEGuideforMeasuringEarth</p><p>Resistivity,GroundImpedance,andEarthSurfacePotentialsofaGroundingSystem(Section7)</p><p>Soilelectricalcharacteristics</p><p>The key determinant of error in the earthing designprocess is the inaccuracy or variation of the soilresistivityinputtothedesign.</p><p>Therefore it is important to understand electriccurrent flow innonuniform,multilayered soils, andhow to accurately test soil resistivity in a diverserangeofconditions.</p><p>www.libertyconsulting.com.auTel:0434005393</p></li><li><p>www.libertyconsulting.com.auCopyrightLCS2016</p><p>There are four basic parameters that influence soilresistivityvalues:</p><p>1. Chemicalcontent(soiltype,constituents);2. Moisturecontent;3. Temperature;and4. Particlesizeanddistribution.</p><p>The use of geotechnical data from the proposedtesting area will often provide good backgroundinformationandsanitycheckoftestresults.</p><p>Accuratesoilresistivityforpowersystemearthing</p><p>TheWennerorfourpinequalspacingmethodisthepreferred test regime used by LCS. Other methodsinclude the Schlumberger or Driven Rod, and thesehaveadvantagesincertainspecificconditions.</p><p>IntheWennermethod, it is importanttounderstandhow far apart test probes need to be in successiveteststoprovideanaccuratesoilprofileasaninputtothedesignsoftware.</p><p>The largest spacing (see a below) needs to be atleastequaltothezoneof influenceoftheearthingsystem being designed. A sample test instruction isprovidedwithrecommendedspacings.</p><p>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.</p><p>Howmanytestpointsarerequiredinatraverseset?</p><p>Experiencehasshownthatthegreaternumberoftestpoints in aWenner traverse, a higher accuracy soilmodel(multilayer)willbedevelopedbysoftware.Werecommendatleast10datapointsineachtraverse.</p><p>Softwarecomputationsaremoreaccurate ifthedatarecordeddoesnothavetoomanygapsbetweenthespacings.Thepreferredratiobetweenspacingsis1.5.A reading taken at 5 metres, would need to befollowedupbya7.5metrespacing,andprecededbyatleast3.5metres,inordertoadheretothis1.5rule.</p><p>Soilresistivitytestresultsandanalysis</p><p>Theuseof sophisticatedmodelling software such asCDEGS RESAP module is able to provide accuratemultilayer soil profiles. A % error is provided witheachsoftwaremodel.</p><p>Multilayer software analysis is critical to providingaccurate inputs to the design process. High/Lowversus Low/High soil models give quite differentresultsforaninjectedearthfaultcurrent.</p><p>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.</p><p>Usingequivalentuniformsoilresistivityandempiricalequations from IEEE80 give large errors. Thereforesimply averaging the raw test data results is notadvisable.</p></li></ul>

Recommended

View more >