DiscriminationCurvesImageSource:AmtechPower

ByStevenMcFadyenonNovember3rd,2014

Carryingoutaprotectionsystemdiscriminationstudyiscriticaltoensurethecorrectfunctioningoftheelectricalsystemintheeventoffaults.TheIECdefinesdiscriminationas:

overcurrentdiscrimination

coordinationoftheoperatingcharacteristicsoftwoormoreovercurrentprotectivedevicessuchthat,ontheincidenceofovercurrentswithinstatedlimits,thedeviceintendedtooperatewithintheselimitsdoesso,whiletheother(s)does(do)not

NoteDistinctionismadebetweenseriesdiscrimination,involvingdifferentovercurrentprotectivedevicespassingsubstantiallythesameovercurrent,andnetworkdiscriminationinvolvingidenticalprotectivedevicespassingdifferentproportionsoftheovercurrent.

Electricalengineersarecalledupontoproduceadiscriminationstudyatvariousstageswithinaproject.Ifyouarelookingtohaveaprotectiondiscriminationstudycarriedoutorhavebeentaskedwithdoingone,hereareafewtipstohelpensureyouhaveabetterthanaveragestudy:

1. DoITEarly2. Theearlierintheprojectastudyiscarriedout,the

better.Ideallytheinitialstudyshouldbecarriedoutinthedesignphaseandthenrefinedduringconstructionandasequipmentselectionbecomesfinalized.Leavingthestudytilltheendoftheinstallation,willmakeanynecessarychangesmorecostlytoimplement.Carryingoutastudyearlier,willnotonlyleadtoabetterdesignedsystem,butcaninfluenceequipmentselectionanddrivethecostofthesystemdown.

3. ShortCircuitandEarthFaultsEnsurethestudycoversbothshortcircuitandearthfaults.Inpractice,boththeshortcircuitandearthfaultcurrentscanbedifferentandcouldaffecttheoverallfunctioningoftheprotectionsystems.Inaddition,relayandprotectivedevicesoftenhavedifferentsettingsforshortcircuitandearthfaults.

4. MoreThanCurves[CurrentLimitingBreakers]Oftendiscriminationstudiesfinishoncethecurveshavebeendrawn.IEC60947isclearthatforcurrentlimitingbreakersandhighlevelsofshortcircuit,discriminationcanonlybeverifiedbytests.Allmanufacturershavecarriedoutthesetests,andreviewingtheirliteraturetoensurediscriminationinthisregioninadditiontoproducingdiscriminationcurvesforlowerfaultlevelsmakesforacompletestudy.

5. NoSourceandLoadEndDiscrimination

Incomplexsystems,achievingfulldiscriminationbecomesalmostimpossible.Avoidtryingtodiscriminatebetweenthesourceandloadendofcables(ifbothhaveprotectivedevices).Bothdevicesisolatethesamesectionoftheelectricalsystem,makingnodifferenceifeitheronetrips.Anypotentialbenefitsoftryingtonarrowthefaultlocationtowithinthecableisgoingtobelostinmoretimerequiredforupstreamdevicestotripandlongerfaultdurations.

6. NotEverythingNeedsToDiscriminateThereisnorulethateverythingneedstodiscriminateandoftenitisimpossibleorcostlytotryandachievethis.Byunderstandingthesystem,it'soperationandtheeffectsonthebusinessofpowerloss,amoreengineereddiscriminationschemecanbeachieved.

7. UseSoftwareInallbutthesimplestinstances,youshouldusesoftwaretocarryoutboththefaultcalculationsandsettingoftheprotectivedevices(discriminationcurves).Tryingtodothingsbyhand,willleadtomistakesandcompromiseinsimplifyingtheamountofwork.Softwarealsoallowsthetestingofwhatiftypescenariostoimprovesystemdesign,installation,andoverallcost.

8. ABetterReportTheresultofanystudyisusuallytheprotectiondiscriminationreport.Spendtimeandeffortonthis.Considerthereaderandfocusthereportonthem.Keepitsimpleandavoidthetemptationtoexpanditwithwaffle.Identitywhythestudyisbeingcarriedout,anymajorissuesandtheoveralloutcome.Keepssettings,curves,supportingdata,etc.inappendices.

9. FullyDocumentTheStudyFullydocumentthestudyinareport.Includeonlyappropriateinformationinthemainbodyandusetheappendicesforfullydocumentingthestudy.Includeallnecessaryinformationsupplyauthoritytechnicalinformation,shortcircuitandearthfaultcalculations,discriminationcurvesandcurrentlimitingchecks,recommendsettingsand/orsettingchanges,problemsandrequiredsystemmodifications/changesandanyothertechnicalinformationwhichisrequiredaspartofthestudyscope.

Hopefully,theabovetipsshouldhelpinachievingabetterprotectiondiscriminationstudy.Thetipsaregenericandapplytonearlyalldiscriminationstudies.However,bearinmindthateachstudyisdifferent,protectionsystemscanbecomplex,focusedaroundaparticulartechnologyorwithaspecificenduserrequirementinmind.Itisuptotheelectricalengineertobringeverythingtogetherandtodelivertotheclientaprotectiondiscriminationstudywhichdeliversresults.