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Case Study - How HT Motor Protection should be designed when running in parallel with Capacitor Bank

Protection is a vast field and itu2019s designing and implementation is a subject of involved Technicality and Engineering Experience to larger extent. For Switchgears, Protective Relays are designed to provide protection for electrical equipment and hence Engineering toward Designing of Protection Scheme and Protection Relaying must be done keeping all the aspects in mind.<br>

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Case Study - How HT Motor Protection should be designed when running in parallel with Capacitor Bank

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  1. ISO 9001:2015 ISO 45001:2015 (OH & S) Certified Company Case Study How HT Motor Protection should be designed when running in parallel with Capacitor Bank www.systemprotection.in +91-265-2225137

  2. Weshare, sothattheproblemoccurredin oneplantcanbeavoidedinothers, reducingtheirdowntimeandincreasing productivityandprofitmargins. InoneoftheProcessPlantwhere switchboardwasdesignedbysome consultantwhoshowedignoranceowingto lesserexperienceandknowledgeinDesign & EngineeringofProtectionSchemes. This hascostsfrequentfailureof6.6kVHT Motorswithoutanywarningsand abnormalityreflected. Protectionisavastfieldandit’sdesigning andimplementationisasubjectofinvolved TechnicalityandEngineeringExperienceto largerextent. ForSwitchgears, Protective Relaysaredesignedtoprovideprotection forelectricalequipmentandhence EngineeringtowardDesigningofProtection SchemeandProtectionRelayingmustbe donekeepingalltheaspectsinmind. Wewerecontactedandcalleduponto identifyandrectifytherootcause, Few MotorshavebeenburntduetoOverheating / Overloadinginjust9 -10months. UsuallyweneversuspecttheEngineeringof ProtectionSchemes, hencestarted analyzingthedataviz. MotorRatings, CT Sizing, TransformerRatings, RelaySettings, Loadpatterns, connectedloadsetc. DuringalltheseyearsinEngineering Services, wehavemasteredtheartof DesignandSelectionofProtectionScheme. Wehavealsohelpedmanyofourvalued customerstorectifytheissuesrelatedto wronglyDesignedandImplemented ProtectionSchemesandPhilosophies. Examiningfurtherwhennocheckpoints left, wefinallycheckedtheProtection SchemeandSLDthatleadustoaninstant conclusionthatitwaswrongProtection SchemeforMotorscausingfrequentfailure andlossofrevenuewithsignificant repairingcostanddowntime. Presentingyouoneofsuchcasewhichwe haveIdentifiedandRectifiedforwrong DesignofProtectionSchemewhichinturn reducesSelectivityandincreasesdown timeorremainInoperativeincaseofsome genuinefaultconditions. Below is the SLD Representation of existing / old scheme and corrected Protection Scheme for HT Motors with Capacitor Bank. www.systemprotection.in +91-265-2225137

  3. Now if motor keep running at 115% normally, it will be overheated and shall surely results into breakdown. ItwascommonCTandRelaysforBoth MotorsandCapacitorbankswhichwas causingthefrequentfailureofmotors. Here underarethereasonswhyCommon Protectionshouldn’tbeusedforHTMotor withCapacitorBanks. 2. AlsowheneverRelaytripsduetofault, it takeslongertimeforEngineerstoidentify thefault / locationsincecommon protectionforMotorandCapacitorBank hasbeenused, andhenceincreasesthe downtimeofbreakdownasEngineershave tocheckMotors, Associatedrotor equipment's (Ifany) inthefieldand CapacitorBankintheSubstations. 1. Protectiverelaywillnotsensethefault properly: InacommoninstalledProtectionScheme forParallelconnectedMotorsand Capacitors, currentsensedbyCTwillbeless thanwhatactuallyisbeingconsumedby Motor. SystemProtection, providedthemcompete EngineeringsupportwithSelectionofCT, Relay, Design, SupplyandInstallationof newCTandRelays, Completescheme Implementationandtesting. Suppose3600kW, 6.6kV, 384AmpFLC (0.82P.F) SlipRingInductionMotoris runningatfullloadwithoutcapacitorbank thanMotorwillbetaking384Amp normally. Duetoabovedescribedfactors, weadvise thePlantauthoritiestomodifyprotection scheme, byimplementingseparate protectionformotorsandcapacitorbank. WealsoadvisetomanipulatetheMotor ProtectionRelaysettingsbyreducingitto 88% approximatelytillthemodificationin protectionschemeismade. Butwhenconnectedinparallelwith capacitorbankof1350KVAR, loading remainsthesamei.e. 3600kWbutaspower factorisimprovedto0.95, netcurrent reducesto335- 340Ampere. NormallyMotorProtectionRelaysettings shallbedoneasper384AmpasFLC + permissiveoverloadforatimebeing. Where asinexistingprotectionschemeRelaywill take340Amponly. Soinordertoreach Thresholdof384Amp, motorhastorunat approx4250kWwhichisinactual115% of ratedcapacity. Below are few examples of some wrong and correct design www.systemprotection.in +91-265-2225137

  4. Howeverihavepersonallyseensuchcases duetosuchdesignsinvariousplants. And tomysurpriseManypaneldesignerstilluse thisCase -1schemeandmodifyonlyifsome engineerspinpointsuchlapses. Normally duringprojecttime, thisaresmallthings andhencesometimestheydon'tseesuch thingindetails. Butithasamoreimpact duringoperationandmaintenance. StitchinTime, SavesNine! Don’tletyourPlantsufferthesameasin describedcasestudy, docheckyour ProtectionSchemeandifyouhavethe same, thenfeelfreetoCall / Writetous. Weareyouronestop ElectricalSolutionProvider. Regards SystemProtection www.systemprotection.in +91-265-2225137

  5. 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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