Effect of gas pressure on partial discharge in voids in epoxy

The pressure inside an enclosed void in a solid dielectric is generally determined by calculation based on equating the measured partial discharge (PD) onset voltage and the breakdown voltage from the Paschen curve for a particular gas. Here it is assumed that the gas is known or that physical properties remain approximately constant. In the present experiment, two different large voids were placed in epoxy adjacent to the ground electrode containing a small pump out port. This arrangement allowed for direct measurement and setting of the pressure for different gases filling the void. PD onset and extinction voltages were determined as a function of pressure over the range from 13.3 Pa (0.1 torr) to 101 kPa (760 torr) for nitrogen and SF₆ at room temperature. The onset voltages generally followed the Paschen curve dependence on pressure but were higher in voltage at pressures around the minimum, while the extinction voltage followed the Paschen curve more closely in both pressure dependence and voltage. Agreement with the Paschen curve is better at the higher pressures. One possible explanation is the dominant role that real surfaces play at lower pressures.