N S Sodha, Former Executive Director, POWERGRID …

1 N S Sodha, Former Executive Director, POWERGRID Corpn, GurgaonPOWERTRANSFORMERSF actory conditionPOWERTRANSFORMERSSite conditionPOWER TRANSFORMER FAULTSF aults in a FaultsTransformer failureratesDEVELOPMENT AND RESULTS OF A WORLDWIDE TRANSFORMER RELIABILITY SURVEYS. Tenbohlen, J. Jagers, G. Bastos, B. Desai, B. Diggin, J. Fuhr, J. Gebauer, M. Kruger, J. Lapworth, P. Manski, A. Mikulecky, P. Muller, C. Rajotte, T. Sakai, Y. Shirasaka, F. VahidiOn behalf of CIGRE WG at CIGRE SC A2 COLLOQUIUM 2015 during Sep 20-25, 2015 at Shanghai, China 6 IntroductionWG was formed with an objective of Review of all existing surveys and study different practice Conduct new International survey Compiling and analysis of collected data Interpreting the results Calculation of failure rates Classification of failure locations Failure causes and modes7 Failure definition Only transformers/reactors > 60 kV considered Major failure Situation requires removal from service > 7 days Involves major repair factory or repair bay Opening of transformer.

2 Tap changer or exchange of bushing < 7 days, but with extensive oil processing8 Failure Rates9 Failure locations > 100 kVSubstation Transformers (536 Failures)10 Failure Locations > 100kVGSUs (127 Failures)11 Failure Locations > 100kVManufactured before 1980 (333 Failures)12 Failure Mode all Voltage class964 Failures (SS and GSU)13 Failure Modes all Voltage Class799 Failures (SS transformers)14 Failure Mode analysis according to Voltage Class15 Failure Cause 964 Failures16 External effect Analysis % of failures did not result into any external effects % failures resulted in Fire % failures are related with explosions or blasts17 External Failure effects964 Failures18 Failures with Fire or explosions(126 Failures)

3 19 External effects of Bushing failures115 Failures20 Action taken analysis964 Failures21 Failure location analysisof 242 scrapped transformers22 Failure location analysis of 465 repaired transformers23 CIGRE WG-Conclusions Overall failure rates remained < 1 % GSU failure rate marginally higher than SS X merfailure rate -difference is < GSU (300-500kV)failure rate exceeded 1% Winding, tap changer, bushings followed by lead exits major contributors24 CIGRE WG Conclusions GSU contribute higher windingand lead failures SS transformers contributed more towards tap changer failure Both type had similar contribution in bushing failures Bushing and Lead exit failurestend to have increasing trend with higher voltage upto700 kV25 CIGRE WG Conclusions Dielectric failures highest contributors Design and manufacturing, ageing and external short circuits -major contributors Bushing failures most often result in fire and explosions Large contributors being unknown.

4 Results should be treated with caution26 POWERGRID , IndiaPower Transformer/Shunt Reactor Asset Performance & Preventive ActionsAsset Growth in POWERGRID , IndiaGrowth of 765kV Transformers in POWERGRID , India050100150200250300 Till 2011201220132014As on dateQuantityPeriodGrowth of 765kV Reactors in POWERGRID , India0100200300400500600 Till 2011201220132014As on dateQuantityPeriodFailures of AC Transformers2011-122012-132013-142014-15 2015-16 Population10100135206247 Failure-1-12% of Shunt Reactors Transformer and Reactor Failures First 765kV Transformer commissioned in the year 2007 and first 765kV Reactor commissioned in 2007. First failure of Transformer was encountered in 2013 and first Reactor was encountered in the year 2015 So far, failure of 4 Nos.

5 Transformer and 3 nos. Reactor were encountered. All of the failures were attributed due to failure of winding unlike 400kV Transformers and Reactors where most of the failures were due to bushingPOWERGRID s Preventive Measures/Philosophy Eachbankof765kVTransformerandReactorsare havingadedicatedhotstandbyspare ChargingofspareTransformerandReactoronro tationalbasisinevery4to6months Each765kVTransformersandReactorsareequip pedwithonlineDGAmonitoringSystem,Onlined ryingSystemFOtemperaturesensors&DigitalR TCC. CommissioningofControlledSwitchingDevice s,OnlineDGA/DryoutSystemalongwithTransfo rmersandReactorsandValidationandSCADAI ntegrationofFiberOpticTemperatureSensors SixMonthlyReviewofDGAandOilTestresultsby CommitteeofExpert.

6 PeriodicreviewofAMPtestresultsbyCommitte eofExpert. MonitoringofBushingTandeltabeforeexpiryo fwarranteeperiodPOWERGRID s Preventive Measures/Philosophy Monitoring of oil: Norms for DGA Testing of oil : Just after charging, After 24 Hrs. of charging, After one week of charging 15 days of charging One month of charging. On monthly basis till warrantee Period Thereafter 2 monthly basis Monitoring of all oil parameter tests (except oxidation stability) within six month of charging and thereafter yearly basis Particle counts measurement before and within three months of Improvement: InclusionofUHFPD monitoringSystemandSelfDehydratingBreath ersinTS.

7 SCADA integrationofonlineDGAmonitoringSystem,O nlinedryingSystem,FOtemperaturesensorsar eunderprogressforremotemonitoring. StandardisationofTransformerandReactorde signforeaseofmaintenance InclusionofRIPbushingswithpolymerhousing toavoidcatastrophicfailure BushingandTransientvoltagemonitoringsyst emPOWERGRID s Preventive Measures/PhilosophyWater theworst enemyof A transformer with low moisture content is like a person in goodcondition A transformer can be used at high load without riskfor catastrophicfailure. A person can work hard without risk for heartattack A wet transformer is like an overweight person in bad condition The transformer owner has to limit load to avoid bubbling(may lead to catastrophicfailure) Moisture in insulation increases the rate ofaging The person can not run Water/moisture and (high) temperature will sooner or later kill thetransformerHow Water affects the transformerperformance?

8 Loadingcapability Limits the loading capability due to decreasedbubbling inceptiontemperature Dielectricstrength Decreases the dielectric strength of the oil and decreasesPD inceptionvoltage Aging High temperature and moisture will dramaticallyaccelerate aging that lowers the mechanical strength of the cellulose insulationLoadingcapability is limited by high moisture Moisture determines the maximumloading/hot-spot temperature for bubble inception Knowing moisture content and oil quality allows for correct decision on maximumloading Leaveas-is Dry-out/re-generateoil Replace/Relocate ScrapG. K. Frimpong et al, Estimation of Moisture in Cellulose and Oil Quality of Transformer Insulation using Dielectric Response Measurements , Doble ClientConference,Paper 8M, limits from of a transformer Moisture andaging During manufacturing, the cellulose insulation in the transformer is carefully dried out before it is impregnated withoil The moisture content in the solid insulation of a new transformer is typically targeted to be < byweight As the transformer gets older, the moisture content willincrease Open-breathing transformers, typically Sealed conservator transformers.

9 Typically In an old and/or severely deteriorated transformer, the moisture content can be >4% The aging process of the insulation is directly related to moisture content504540353090100110120130140 Winding Hot Spot CrK25rof20rota15cFn10gig A5080aftPaper(IEEE)IEEEK raft acceleratesagingLeft: Lars E. Lundgaard, Walter Hansen, Dag Linhjell, TerenceJ. Painter, Ageing of oil-impregnated paper in power transformers , IEEE PWRD,2003~2 years @3%~12 years @1%Where does the water comefrom? Leaking gaskets and faulty water traps may exposethe inside of the transformer to moisture humidair Exposure to humid air during site installation/commissioning Exposure to humidair duringmaintenance Normal aging ofcellulose produceswater Insufficient dryingat manufacturing Typical moisture content inpaper/pressboard: New transformer: <1% Aged transformer: 2 -4% Normal increase of water content is of moisture content by various standards andpractices < 1% - Asnew 1-2% - Dry 2-3% - Moderatelywet - Wet > - Excessivelywet 1)OriginaldatainIEC60422,annexA,areprese ntedas relativesaturationinpercentage.

10 Dry , Moderatelywet , Wet and Extremely wet are recalculated to percent moisture )OriginaldatainIEC60422fornewtransformer sispresentedaspercentmoistureincellulose )OriginaldatainCIGRE349isbycategories. Good, Fair , Probablywet and Wet arerelabeledto Good/New , Dry and Wet .Transformer drying Methods/Examples Two major techniques areused: Drying the insulation by drying the oil Field Drying the insulation with heat and vacuum Field andfactory Drying theoil Molecularsieves Cellulosefilters Coldtraps Combined oil regeneration anddegassing Drying theinsulation Vacuum andheat Pulsation drying through oilcirculation Hot oil spraydrying Low frequencyheating Vapour phasedryingA.