Analysis of converter transformer failures and application of periodic on-line partial discharge measurements

Of a group of eight 3-phase core form HVDC converter transformers, six have now failed in service. The final failure mechanism has consisted of a turn-to-turn fault, usually in the valve winding. All the windings were produced using continuously transposed conductors. Thermal aging of the insulating materials was evident. This paper summarizes the results of the various tests and inspections that were made in an attempt to better understand the circumstances that may have contributed to these failures. On-line partial discharge measurements using an electrical detection method are being made on a periodic basis on a number of large power transformers, primarily those used in HVDC conversion. This paper describes the measuring system and methods of noise deletion. An example of the test results has been provided describing a significant failure hazard that was detected.