Les Recksiedler

- Similar transformers not gassing or failing -Increased harmonic currents -No gassing during factory load run -No increase in gassing during 2 day Field test load run -One supplier known for cookie cutter approach -Failure of three winding transformers in USA analyzed to be due to harmonics. -All transformer in each 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 gassing -All Transformer one year later gassing badly -One transformer failed after 5 years -3 transformer failures so far in 26 years -Reports not available for transformer repairs -No gassing during factory load run assumed to be 12 hours -No 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

- Duvals Triangle – about 700 oC -No Furans one test only Case A– Gassing Results HydrogenMethaneAcetyleneEthyleneEthane Years H2H2 CH 4 C2H2C2H4C2H6COCO 2 TDCG ppmCO 2/ CO NormalLowNormalHigh NormalMediumLowAbnormal NormalMediumNormalHigh Medium Abnormal LowHighNormalHigh NormalMediumHighAbnormal

Case A – Field Load Run Tests – Two Days Hydrogen Methane Ethane Ethylene Acetylene CO CO TDCG

Case B – Single Phase Transformers with a Wye and Delta Secondary winding All transformers in one HVDC system gassing One unit has acetylene and is being repaired Similar HVDC scheme all transformers not gassing Same supplier for both. Non gassing transformers produced 11 years earlier Remaining units are acceptable ????

Case B – Gassing Results – Two worst units excluding unit with Acetylene Unit 1 Unit 2 Condition/Risk Methane Medium Ethylene High Acetylene < 5 < 5 Normal Extreme gassing above 70% loading Gassing 100 to 1000 times the daily gas volume in IEC table A3 at high loading

Case C – Gassing and Failures linked to Harmonics IEEE transactions Volume 19, No 1, pp dated January 2004, Thermal problems caused by Harmonic Frequency Leakage Fluxes in 3 Phase 3 winding Converter Transformers Magnetic fluxes must be managed Use CTC cable Leads arranged to cancel fluxes Increased distances Non-magnetic structural framework

Conclusions 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?

QUESTIONS & ANSWERS