Abstract
This paper summarizes experiments that were carried out on insulating ceramic substrates and a needle-plane arrangement with the purpose to characterize the inception of partial discharges (PD) in the insulation of high-voltage semiconductors. PD measurement results obtained on substrates are correlated with the high-field pre-inception currents measured in a needle-plane electrode arrangement in liquids and gel. The tested substrates were embedded in either silicone liquid, silicone gel, or mineral oil. The PD inception was measured at sinusoidal and switched voltage of different polarities and rise times. For all test objects, the PD inception voltage (PDIV) is significantly lower under switched voltage compared to the sinusoidal voltage. Silicone gel and silicone liquid as insulating media surrounding the substrate are similar in terms of the PDIV. The PDIV is lower for substrates placed in mineral oil, particularly under switched voltage. Silicone gel and liquid are characterized by very low pre-inception currents measured at a high electric field. In contrast, a higher pre-inception current is observed in mineral oil. These results are consistent with the hypothesis that the high pre-discharge current in the more conductive mineral oil leads to space charge build-up in the high-field region that, by deforming the electric field, affects the PD inception probability.