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Case Study - Can Programming in DCS can also lead to Tripping of HT Motors

Today I am going to present a Case of GRR (Grid Rotor Resistance) which is used in Slip ring induction motor, which might be rare in Industries. Hence want to share the experience, so that the problem we faced must not be faced by others or can be avoided<br>

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Case Study - Can Programming in DCS can also lead to Tripping of HT Motors

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  1. ISO 9001:2015 ISO 45001:2015 (OH & S) Certified Company Case Study Can Programming in DCS can also lead to Tripping of HT Motors www.systemprotection.in +91-265-2225137

  2. HelloFriends / Sirs, TeamdecidedtoremovePLCandtakeall theDI, DOandfeedbacktoDCSandmake aprogramjustlikePLCintheirDCS, soasto removelocalPLCandreducebreakdown andmalfunctioning. TodayIamgoingtopresentaCaseofGRR (GridRotorResistance) whichisusedinSlip ringinductionmotor, whichmightberare inIndustries. Hencewanttosharethe experience, sothattheproblemwefaced mustnotbefacedbyothersorcanbe avoided Projectwastakenandwasdoneduring shutdown, everyinputandoutputwas checkedandconfiguredandJustlikePLCa programmewasmadeinDCSwhich removedLocalPLC. Inacementplant, therewasaHTMotorof 6.6kV750RPMwhichwasusedforFan. A modificationwasplannedtoremovesome breakdownwhichhashappeneddueto malfunctioningofPLC. Butduring modification, theyoverlookonecondition, whichcanbehappenedwithanyone, asno oneeverthoughtofsuchcondition. Nowteamdecidedtotaketrialoffan duringshutdown, toensurethateverything isright. Trialtakeninofflinemode, GRRwas workingfineeverystepaswascorrect. Now itwasdecidedtotakeonlinetrialand Motorstartedsuccessfully. Currentwas normal, everythingwasnormal. Nowitwas decidedtotakemotortofullRPMto confirmeverything. Stepwasincreasedand currentwaschecked, butsuddenlyafter onestepMotorwastrippedinovercurrent. 1st Question What is GRR? Ans:ItisGridRotorResistance, where3 phaseresistanceischangedonbasisof combinationofpowercontactors. 2nd Question Where it is used? Whathappened? DidmotorGoneordid theirmodificationfailed. Teamwaslooking ateachother. Theydecidedtocheck meggerandhealthinessofmotorandstart again. Motorstartedagainnormallybut afterthatsamestep, Motorwastrippedin overcurrent. Ans:ItisusedinSlipringinductionmotor, wheremotorspeedneedstobecontrolled (MostlyinFans, Fanspeeddependson ProcessrequirementandAirflowrequired inasystem) 3rd Question Where it is C1 to C6 ? Atleasttheygotthatsomethingiswrong after8thstepofGRR, astill8thstepmotor runsfineassoonasGRRgoesto9thstep, motorgetstripped. Ans:C1, C2, C3, C4aremainpower contactors, throughcombinationofwhich resistancechanges, C5isStarContactorand C6isDeltaContactor. IfC5isonitmeans GRRisinStarandifC6isonitmeansGRR isinDelta. BothC5andC6willneverbeon atsametime Nowtheinvestigationstarted. GRR Resistancereadingofeverystepandevery phasewastakenthroughmicro-ohmmeter. Buttheresistancewasbalancedforeach stepandeveryphase. GRRStepisgiven below. InGRRthereisLocalPLC, whichcontrols thestepofGRR, whichworksonthe feedbackofpowerandAuxiliaryContactors andCommandfromDCSandLocalto increaseanddecreaseresistancetoRotor, therebycontrollingspeedoffan. ActuallythisFanPLCwascreatingsome problemandwasmalfunctioning, dueto whichtherewasprobleminincreaseand decreasingofspeedand2timescaused trippingofplantalso, ultimatelycreating problemontheprocessside. www.systemprotection.in +91-265-2225137

  3. Thisproblemwasnotsolvedtill2days. Both daytrialtaken2timesandcompleteGRR andmotorwaschecked. In8thStepC1, C2, C3andC5Contactors wereoni.eGRRwasinStar. Nowasthe commandcomestoGRRtogoto9thStep, InsteadofC3contactordroppingfirstand thenC4ContactorpickingupitwasPicking upC4Contactorfirstandthenitwas droppingC3Contactor, duetowhichallthe resistancewereshortingmomentarilyand GRRwasbypassed, whichledtoincreasein Statorcurrentandconsequentlytrippingof Motor. Till8thstepofGRR, everythingisfineand assoonasitgoeston9thstepMotortrips. Theyaskedinsomeotherplants, onetold them “increasethetimedelaybetween changeofsteps”. On3rdDaydelaywasgivenbetween changesofstepofGRR. Andtoeveryone surprise, itworked. Nowthequestionwas whattimedelayhasdonetoGRR? SothequestionwasduringStepchange ContactorshoulddropfirstorPick-upfirst. Nowweknewtheproblemwasindelay. I lookedagainitintoGRR8thand9thstep andthenrealizewhattimedelayhasdone. Itwasgreatlearning, asimplePLClogicwas trippingourHTmotor. www.systemprotection.in +91-265-2225137

  4. CONTACT INFORMATION 301/2/306, SilverlineComplex,OppositeBBC Tower,Sayajigunj,Vadodara - 390005, Gujarat, India. bdm@systemprotection.in www.systemprotection.in +91-265-2225137

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