Partial discharges (PDs) on liquid impregnated pressboard insulation have been investigated for sinusoidal and switched voltages for two different liquids-one mineral oil and one synthetic ester. The insulation models used were a rounded electrode resting on a pressboard plate on a plane electrode representing semi-uniform field, and a sharp-edged electrode representing divergent field. The main result shows that the PD inception voltage (PDIV) is considerably lower at switched than at sinusoidal voltage. Similarly, the mineral oil showed a lower inception voltage than the synthetic ester. Finally, a lower PDIV for the sharp conical electrode compared to the spherical electrode was seen. The differences can be explained by formation and transport of space charges. The space charges will shield the high-field regions when having the same polarity as the voltage and then enhance the electric field right after a voltage polarity reversal. There is minimal time available for charge drift at polarity reversal of switched voltage. Contrary, for sinusoidal slowly varying voltage there is plenty of time for the charges to drift before the opposite voltage peak occurs. Thus, less field enhancement from space charges under sinusoidal voltage. The field enhancement due to space charges of opposite polarity is enhanced for liquids with higher injection current, such as mineral oil.
