IEEE Transformers Committee October 2008 Status of CIGRE JWG A2/B4-28 HVDC Converter Transformers HVDC Converter Transformers Ugo Piovan 1
IEEE Transformers Committee October 2008 Guidelines for conducting design reviews for HVDC converter Transformers The design review guide is now complete and is in • process of being published end of 08 / beginning of 09 process of being published end of 08 / beginning of 09 One of the main novelty regards the extensive use of • numerical simulations to address dielectric and loss/thermal aspects which cannot be replicated during tests. 2
IEEE Transformers Committee October 2008 HVDC Transformers reliability survey The survey is almost complete. • Latest data show an improvement in HVDC transformer • reliability FAILURE RATE BY YEAR COMMISSIONED 0.18 0.16 ACTUAL PREVENT TOTAL 0.14 E T 0.12 A R 0.10 E R U 0.08 IL A 0.06 F 0.04 0.02 0.00 1970 -1980 1981-1985 1986-1990 1991-1995 1996-2000 2001-2006 YEAR COMMISSIONED 3
IEEE Transformers Committee October 2008 PR Test Present PR test can be inadequate (less stress during test than in service) because of insufficient polarization time. This problem can be addressed by : • increasing the PR test level by 20% U 1 , 5 (( N 0 , 5 ) U 0 , 35 U ) = − + pr dm vm • lengthening the PR test by 45 min, i.e. 90-120-60 min However the JWG has decided that it is more urgent to address oil conductivity issues and to leave recommendations about PR test to IEC / IEEE 4
IEEE Transformers Committee October 2008 Issues related to uncertainty about oil conductivity HVDC transformer Manufacturers and End Users, pay great attention to some properties (particle content for example) of oil used in their HVDC transformers. However from the information gathered by the group, we learned that: • oil electrical conductivity is not measured on a regular basis (not even by oil producers) • there is no test method employed consistently 5
IEEE Transformers Committee October 2008 Issues related to uncertainty about oil conductivity (continued) • existing standard test methods measure oil conductivity at dielectric stress below 0.5 kV/mm while during a polarity reversal the oil can be subject to dielectric stresses in the range of 5-10 kV/mm Considering that it is very common that the HVDC transformer in service will be filled with an oil different from the one used during tests, it follows that we have no information about the effectiveness of the DC and PR tests performed until now. 6
IEEE Transformers Committee October 2008 Goals of a new CIGRE A2/D1 JWG • Understanding of the effect of insulation material characteristic on reliability of HVDC converter transformers • To come up with a methodology for measuring and To come up with a methodology for measuring and monitoring these values • Guidance for evaluation and interpretation for these values • Recommendation for new standards 7
IEEE Transformers Committee October 2008 Goals of a new CIGRE A2/D1 JWG (continued) • Advise on a simple test of oil quality to be used by supplier, OEM and End Users. This is to be used for tests and during service • Understanding of aging under repetitive overvoltages Understanding of aging under repetitive overvoltages • Effects of E on conductivity of oil/board systems • Understanding of conductivity variation with aging • Effects of polarization and charge migration on apparent conductivity 8
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