Partial discharge-induced degradation characteristics of insulating materials of gas-filled power transformers

The soundness of a transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. At present, the same criteria for evaluating oil-filled transformers are adopted as those for evaluating gas-filled transformers in this PD test. However, gas-filled transformers are made of different insulating materials than those for oil-filled transformers, and therefore they may have different strengths against PDs from oil-filled transformers. Therefore, it is necessary to establish PD evaluation criteria for gas-filled transformers. This paper, as the first step toward this objective, discusses the study of solid insulating materials used in gas-filled transformers in terms of the generated PD magnitudes and the degrees of damage of insulation and examines the harmful PD levels that lower the residual impulse withstand voltage. As a result, it identified the PD-induced degradation patterns of some insulating materials used in gas-filled transformers, i.e., polyethylene terephthalate (PET) films and Nomex^® boards and evaluated the harmful PD levels based on the residual impulse withstand voltages. It was also verified that the insulations of pressboards are not degraded even by relatively large PDs.