Observation of residual surface charge distribution inside an artificial air-filled void due to partial discharge activities at room and liquid nitrogen temperatures

Partial discharge (PD) characteristics within an artificial air-filled void contained in a solid insulator were experimentally investigated under 60 Hz ac voltages at liquid nitrogen temperature (77 K) and room temperature of 298 K. The results show that the first PD inception voltage at 77 K is about twice higher than that at 298 K while successive values at both temperatures are almost the same. The results also show that PD charge magnitude noticeably decreases and PD pulse number remarkably increases with the decrease in the temperature from 298 K to 77 K. The latter results are further discussed based on the observed remarkable decrease in the charged area by a single PD on an inner void surface with the decrease in the temperature.