Deterioration characteristics due to partial discharges in insulating structure constituting gas-filled power transformers

The soundness of a transformer under operating voltage is evaluated in a partial discharge (PD) test of a long-duration ac withstand voltage test. Presently, 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 may therefore have different strengths against PDs to oil-filled transformers. With this in mind, it is necessary to establish PD evaluation criteria for gas-filled transformers. A preceding paper discussed the raw materials of solid insulating ones used in gas-filled transformers in terms of the generated PD magnitudes and the degrees of insulation damage and evaluated the harmful PD levels of the insulating materials themselves. In this paper, the harmful PD levels of actual equipment were studied using the turn-to-turn and section-to-section insulation models, insulating structures that constitute gasfilled transformers, in order to examine harmful PD levels that lowered the residual impulse withstand voltages. Consequently, it was confirmed that, on the turn-to-turn insulation model at normal test voltage levels, the generated PDs were too small to damage the insulator, even with a particle present. To damage the insulator, a voltage significantly exceeding the test voltage had to be applied. Therefore there is little necessity to consider the harmful PD level on the turn-to-turn insulation. On the other hand, on the section-to-section insulation model, PDs were found to be generated if a particle completely bridged separate sections. As a result, the degradation statuses related to the magnitude and duration of PDs were identified and the harmful PD levels were estimated at 10000 pC according to the residual impulse withstand voltages.