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Very Fast Transients (VFT) in Gas Insulated Substations (GIS)

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Les Recksiedler
Three WindingConverterTransformerGassing and Failures
Similar transformers not gassing or failingIncreased harmonic currentsNo gassing during factory load runNo increase in gassing during 2 day Field test load runOne supplier known for “cookie cutter” approachFailure of three winding transformers in USA analyzed to be due to harmonics.All transformer ineach HVDC system either gassing or not.Data is sparse
Background – Transformers with a Wye and Delta secondary winding
HVDC System one year earlier all transformers not gassingAll Transformer one year later gassing badlyOne transformer failed after 5 years3 transformer failures so far in 26 yearsReports not available for transformer repairsNo gassing during factory load run assumed to be 12 hoursNo appreciable gas increase during 2 day Field load run.Appreciable gassing within two months of in-service
Case A– Three Phase Transformers with a Wye and Delta secondary winding
Duval’s Triangle – about 700oCNo Furans one test only
Case A– Gassing Results
Case A – Field Load Run Tests – Two Days
Hydrogen 117 ---- 133Methane 273-----259Ethane 95------99Ethylene 305----311Acetylene 3--------3CO 438---- 419CO2 1671----1689TDCG 1231----1224
Case B – Single Phase Transformers with a Wye and Delta Secondary winding
All transformers in one HVDC system gassingOne unit has acetylene and is being repairedSimilar HVDC scheme all transformers not gassingSame supplier for both.Non gassing transformers produced 11 years earlierRemaining units are acceptable ????
Case B – Gassing Results – Two worst units excluding unit with Acetylene
Unit 1 Unit 2 Condition/RiskMethane 800 500 MediumEthylene 780 650 HighAcetylene < 5<5 NormalExtreme gassing above 70% loadingGassing 100 to 1000 times the daily gas volume in IEC 60599 table A3 at high loading
Case C – Gassing and Failures linked to Harmonics
IEEE transactions Volume 19, No 1, pp208-213 dated January 2004, “Thermal problems caused by Harmonic Frequency Leakage Fluxes in 3 Phase 3 winding Converter Transformers”Magnetic fluxes must be managedUse CTC cableLeads arranged to cancel fluxesIncreased distancesNon-magnetic structural framework
Factory Testing cannot ensure a reliable converter transformer?Design Review a possible solution?What are critical factors to consider? Winding configuration, lead routing, flux cancellation?Should warranty consider gassing results? If so at what level?






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Very Fast Transients (VFT) in Gas Insulated Substations (GIS)