Three Winding Converter Transformer Gassing and Failures Les

Three Winding Converter. Transformer Gassing and Failures Les Recksiedler

Background – Transformers with a Wye and Delta secondary winding - 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

Case A– Three Phase 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– Gassing Results - Duval’s Triangle – about 700 o. C -No Furans one test only Hydrogen H 2 37 Normal 77 Normal 247 Low Methane CH 4 385 Low 443 Medium 3139 High Acetylene C 2 H 2 5 Normal 20 Normal Ethylene C 2 H 4 350 High 579 High 3955 High Ethane C 2 H 6 293 High 172 High 1245 High CO 187 Normal 1154 Medium 140 Normal CO 2 5248 Medium 5300 Medium 5724 Medium TDCG ppm 1257 Low 2427 Medium 8746 High CO 2/CO 28. 6 Abnormal 4. 59 Abnormal 40. 89 Abnormal Years 27 16 4

Case A – Field Load Run Tests – Two Days Hydrogen 117 ---- 133 Methane 273 -----259 Ethane 95 ------99 Ethylene 305 ----311 Acetylene 3 ----3 CO 438 ---- 419 CO 2 1671 ----1689 TDCG 1231 ----1224

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 800 500 Medium �Ethylene 780 650 High �Acetylene < 5 Normal �Extreme gassing above 70% loading �Gassing 100 to 1000 times the daily gas volume in IEC 60599 table A 3 at high loading

Case C – Gassing and Failures linked to Harmonics IEEE transactions Volume 19, No 1, pp 208 -213 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