Partial Discharge Evolution Toward Surface Flashover in a Resin-Impregnated Paper Bushing Core Under Combined AC–DC Voltage

Partial-discharge (PD) evolution toward surface flashover in resin-impregnated paper (RIP) bushing cores was investigated through experiments and an idealized electroquasistatic calculation. Two independent 40.5 kV cores with 100 mm surface paths were tested under alternating-current (AC) and combined AC and direct-current (DC) voltages, respectively. All 5342 intermittent 20 ms records were analysed using phase-resolved PD (PRPD) maps, five amplitude thresholds, and conditional block-bootstrap intervals. The illustrative model examined tangential-field asymmetry under prescribed voltages and material parameters. In the core tested under AC voltage, dense opposite-sign PRPD lobes developed. In the separate core tested under AC–DC voltage, a broad positive-output band and a concentrated high-amplitude branch developed. With AC held at 10 kV rms, the detected-event rate above 200 mV-equivalent rose from 6.1 to 282.0 s−1 during the positive-DC ramp. Four sequential acquisition sets at 50 kV DC showed overall high-amplitude activity enhancement, with the highest rate in S4 and overlapping S2–S3 conditional intervals. Bridging surface flashover was observed during this 50 kV exposure. Separately, the idealized calculation showed a DC-induced half-cycle difference in mean tangential-field magnitude. The experimental observations support joint evaluation of PRPD morphology and amplitude-stratified event rates, while the calculation illustrates the applied-field asymmetry. These complementary results inform further investigation of discharge development toward surface flashover.