Effects of thermal aging on creepage discharge in oil- impregnated pressboard under combined AC-DC voltage

In this study, we investigate the characteristics of creepage discharge on oil-impregnated pressboard under combined AC-DC voltage conditions using a sphere-plane electrode configuration. In our experiments, we use four oil-impregnated pressboard samples subjected to different thermal aging time. Experimental results show that the creepage discharge inception voltage (CDIV) and creepage discharge flashover voltage (CDFV) are lowest under pure AC voltage and gradually increase with higher DC voltage components. Longer aging time may contribute to higher CDIV with low DC voltage components; however, with higher DC voltage components, the effect of aging time is less pronounced. The CDFV is not significantly influenced by aging time. The length of carbonized tracks and the development velocity of the creepage discharge increase and the endurance time decreases with longer aging time and higher DC voltage components. We divide the development of creepage discharge in aged pressboard into four stages according to changes in the discharge characteristic parameters. With longer aging time, characteristic parameters exhibit less fluctuation and development velocity exhibits greater sensitivity to the DC voltage component. Our experimental results demonstrate that the characteristics of creepage discharge are significantly influenced by the age of the pressboard and by DC voltage components.