Partial Discharge and Degradation Characteristics of Cast Resin Transformers Due to Bipolar Impulse Application

The objective of our work is to investigate partial discharge (PD) behaviors and degradation characteristics in winding insulation of an actual 22 kV cast resin transformer (CRTr) when subjected to bipolar impulse voltages. A differential high-frequency current transformer (HFCT) was employed on the high-voltage coils to measure PD pulses induced by impulse voltages. Additionally, PD measurement at ac voltage was carried out to characterize the aging process resulting from the application of bipolar impulses. Experimental findings indicated that higher-level bipolar impulses could cause large-amplitude PDs in the windings, occurring at the peak value of the applied impulse voltages and increasing with the number of applied impulses. PD inception voltage (PDIV) at ac voltage decreased to the operating voltage, and the discharge amplitude and phasewidth in phase-resolved PD (PRPD) patterns increased due to impulse PD occurrences. Considering these results, it is evident that the bipolar lightning impulses could cause large-amplitude PDs, intensify impulse PD activity, and ultimately lead to dielectric breakdown in the CRTr windings. Space charge accumulation and residual charge deposition resulting from impulse PD play a significant role in the generation and the process of development for impulse PDs in the winding insulation, contributing to an acceleration effect in the degradation process and eventual insulation failure.