RESIDUAL LIFE OF TRANSFORMERS - Central Board …

1 RESIDUAL life ASSESSMENT OF GENERATOR TRANSFORMERS IN OLD HYDRO POWER PLANTSA uthored by:Sanjay Srivastava, Chief Engineer (HE&RM), RakeshKumar, Director (HE&RM), Jayaswal, Dy. Director (HE&RM) & AmitRoy Singal, Asst. Director (HE&RM).Presented by:RakeshKumar, Director (HE&RM), CEAINTRODUCTION Renovation,Modernization,UpratingandLife Extension(RMU&LE)hasbeenrecognizedworldw ideasawellprovencosteffectivetechniquefo rimprovingtheperformance,efficiencyandre liabilityofoldHydroPowerPlants(HPPs). GeneratorTransformerisoneofthemostimport antandcostlycomponentofpowergenerationne xtonlytothegenerator. OF GENERATOR TRANSFORMERS AgeingofsolidinsulationmaterialslikePape r,PressBoard,Woodbasedlaminatesetc.

2 AgeinganddegradationofInsulatingOil. WearandtearofmovingpartslikeTapChangingM echanism,PressureReliefDevice,Monitoring equipmentsetc. ,acceleratedageingtakingplaceinthetransf ormeronaccountofthefollowingabnormaloper atingconditions: OverloadingofTransformers. Operationunderovervoltageconditions. Impactoflightning/switchingsurgesfromthe system. ON GENERATOR TRANSFORMERS Dissolved Gas Analysis (DGA) of transformer oil. Monitoring of water content in transformer oil. Monitoring of Capacitance and Tan-Delta values of Condenser Bushing. Partial Discharge (PD) measurement. Winding Resistance Measurement Percentage Impedance Measurement Insulation Resistance Test Furan Analysis Magnetizing Current Test Sweep Frequency Response Analysis (SFRA) DIRANA Test Harmonics MeasurementDISSOLVED GAS ANALYSIS (DGA) 5.

3 Acetylene2. Methane 6. Carbon Dioxide3. Ethane 7. Carbon Monoxide4. Ethylene 8. OxygenMECHANISM OF GAS GENERATION IN TRANSFORMERSP artial Discharge in OilLarge amount of H2 and little CH4 Arcing in Oil (Power Discharge)Large amount of H2and C2H2 Oil OverheatingCH4,C2H6,C2H4areformedinincre asingorderofseverityTraces of C2H2in case of very high temperaturesCellulose Overheating by Electric FaultLarge quantities of CO & CO2 DISSOLVED GAS ANALYSIS (DGA)Method recommended by CIGRE Task forceRatioValueIndicationC2H2/C2H6H2/ CH4CO2/ COC2H2/ H2>1>10>10<3>2 (with C2H2>30)DischargePartial dischargeCellulose overheatingCellulose degradation by electric faultFault gases diffusing into main tank from OLTCGas Concentration RatiosDISSOLVED GAS ANALYSIS (DGA)Method recommended by IEC 60599 (1999)CaseCharacteristicfaultC2H2/C2H4CH 4/H2C2H4/C2H6CO2/COC2H2/H2O2/N2 PDPartialdischarges(*)< < > > >2---T1 Thermalfault(<300oC)(*)(*)<1---T2 Thermalfault(300,700oC)< >11,4---T3 Thermalfault(>700oC)< >1>4---Cellulose(paper)degradationbyelect ricfault---<3--FaultgasesdiffusingintomaintankfromOLTC---->2to3-Excessiveconsumptionofoxygenasaresu ltofoiloxidationand/orpaperageing-----< (*) Not significantNote.

4 In some countries, the ratio CH4/H2is used instead of C2H4 ratio between gasesWATER CONTENT MEASUREMENT Hydrogen generated during ageing combines with the free oxygen present in the oil and forms water. A part of this is absorbed by the cellulose insulation and the balance remains as moisture in oil. As paper is highly hygroscopic in nature, when transformer is filled with oil, it absorbs the moisture from oil, which affects its insulation properties as well as reduces its lifeSystem Voltage of Transformer kVWater content ppm, to and DELTA AND CAPACITANCE ON WINDINGS AND BUSHINGS Thistestisusedtoassesstheconditionoftheo ilandcelluloseintermsofmoisture,insulati on,etc.

5 Theadvantageofthismethodisthatitidentifi esthewinding(HVorLV)thathasapossibleprob lem. RESISTANCE MEASUREMENT Transformerwindingresistancemeasurementi scarriedouttocalculatetheI2 Rlossesandtocalculatewindingtemperaturea ttheendofatemperaturerisetest. Windingresistanceresultshouldnotvaryby5% IMPEDANCE MEASUREMENT Thepercentageimpedanceofatransformeristh evoltdroponfullloadduetothewindingresist anceandleakagereactanceexpressedasaperce ntageoftheratedvoltage. RESISTANCE TEST Insulationresistancetestsaremadetodeterm ineinsulationresistancebetweenindividual windingstogroundorbetweenindividualwindi ngs. It shows the condition of INDEX TEST ItisaratiooftheInsulationResistanceMeasu redfor10minutestotheinsulationresistance valuemeasuredafter1minute.

6 PIisagoodindicationofwindingcontaminatio n,moistureingress(leakagecurrents),and/o rbulkinsulationdamage(conductioncurrents ). , OF POLYMERISATION (DP) Remnantlifeofatransformerinservicedepend smainlyonthedegreeofdegradationoftheinsu latingmaterials DPistheaveragemoleculesizeandcloselyrela testothemechanicalpropertiesofpaper Earlyandprecisediagnosisofdegradationnot possiblethroughDGA. ,oxygenfree,andwith110degChotspotfor135, 000hoursuntil25%ofretainedtensilestrengt h(75%loss),or150,000hoursuntilaDPof200is reached(80%lossoftensilestrength). life ASSESSMENT BY DPMETHODDEGREE OF POLYMERISATION (DP) Testisinvasiveinnature. Samplecanbetakenonlyaftertakingtheequipm entoutofservice.

7 Takingpapersampleisatediousjob. ANALYSIS Theconcentrationoffuransinthemineraloilo fapowertransformerisagoodindicatorofthec onditionofthecellulosematerialssuchaspap er,pressboardandcottontapesusedforinsula tion. Notanoilqualitytest. HistoricallyusedasamajorinputforRLAoftra nsformers. Furananalysisisusedasasupplementalongwit hDGA&DPtests LimitingvalueoftotalfuraniccontentisGood Health:<3mg/KgSatisfactoryhealth:3-5mg/KgDeterioratedhealth:>5mg/KgMAGNETIZING CURRENT TEST MagnetizingcurrenttestofTransformerisper formedtolocatedefectsinthemagneticcorest ructure,shiftingofwindings,failureinturn -to-turninsulationorproblemintapchangers . Usefulindistinguishingbetweensevereandle ssseverecases.

8 Anagreementtowithin25%ofthemeasuredexcit ingcurrentwiththeprevioustestisusuallyco nsideredsatisfactory. Ifthemeasuredexcitingcurrentvalueis50%hi gherthanthevaluemeasuredduringpre-commis sioningchecks, FREQUENCY RESPONSE ANALYSIS (SFRA) Thistestpassesarangeoffrequencies(betwee n10 Hzto2 MHz) ,themechanicalconditioncanbeassessed. Reliableandsensitivemethodtoevaluatethem echanicalandelectricalintegrityoftransfo rmersactivepart. FRAisalsocarriedoutperiodicallyfordetect ionofdeformationofsmallermagnitude,whent ransformerexperiencesseveralshortcircuit forces. RESPONSE ANALYSER (DIRANA) TEST DIRANA determinesthewatercontentinpaperwithoutt heneedofoilsampling. ItcombinesFrequencyDomainSpectroscopy(FD S)andPolarizationDepolarizationCurrent(P DC+) Thepowerdissipationfactoroftheinsulation ismeasuredoverawidefrequencyrangingfromt he Theverylowfrequenciescontaininformationo nmoistureinthesolidinsulation,whilethepo sitionoftheslopeinthemid-rangefrequencie sindicatestheconductivityoftheliquidinsu lation.

9 , MEASUREMENT Theharmfuleffectsofharmonicvoltagesandcu rrentsontransformerperformanceoftengounn oticeduntilanactualfailureoccurs. Applicationofnon-sinusoidalexcitationvol tagestotransformersincreasetheironlosses inthemagneticcoreofthetransformer. Amoreseriouseffectofharmonicloadsservedb ytransformersisduetoanincreaseineddycurr entlosses. CASE STUDY InsulationsectionInsulationResistance60s ec (G )PolarizationIndexTan (%)Moisture content (%) (Ungrounded) within permissible limitsDGAHigh concentration of , ON TEST RESULTS LimitofTanDeltaforagedtransformeris1%. Moisturecontentshowsthatthetransformeris inwetcondition. Theoiltestresultsarenormalandtheoilparam etersarewithinpermissiblelimitsasperIS18 66:2000.

10 TheDGAresultsexhibitshighconcentrationof ethylenegasindicatingpossibleoverheating orthermalfaultinthetransformer. Furananalysisshowspresenceofhighquantity ( ) You