Long-Term AC Partial Discharge Characteristics Till Dielectric Breakdown in an Aged 22 kV Cast Resin Transformer

This article presents a trend analysis of long-term ac partial discharge (PD) characteristics till dielectric breakdown by accelerated degradation test in an aged 22 kV cast resin transformer (CRTr). The long-term ac PD characteristics till dielectric breakdown showed that the maximum PD charge ${Q}_{\max }$ and the average PD charge ${Q}_{\text {mean}}$ increased sharply during the cumulative charging time ${t}_{\text {c}}$ , which was between 100 and 200 h before dielectric breakdown. These results suggest that defects could expand and propagate due to PD degradation and penetrate the insulating paper, resulting in large PD as a precursor to dielectric breakdown. With a voltage acceleration factor ${\text {AF}}_{\mathrm {\textit {v}}}$ and a frequency acceleration factor ${\text {AF}}_{\mathrm {\textit {f}}}$ in the accelerated degradation test considered, the time at which this sign was observed was well before dielectric breakdown. Therefore, this sign can be expected as one of the indicators of equipment replacement. The time sequence of ac PD characteristics just before dielectric breakdown was successfully measured. The result indicates the possibility that a fatal dielectric breakdown can be prevented owing to a fast interruption utilizing a circuit breaker. Furthermore, a breakdown site in the winding of the aged 22 kV CRTr could be located by the thermography, i.e., high electric field sections between the top block layers of the high-voltage side winding.