Partial Discharges With Fast Switched Voltage on Impregnated Pressboard in a Mineral Oil and a Synthetic Ester

Partial discharges (PDs) on liquid-impregnated pressboard insulation are investigated for sinusoidal and switched voltages for two different liquids—one mineral oil and one synthetic ester. The test arrangements used are a rounded electrode resting on a pressboard plate on a plane electrode representing semi-uniform field and a sharp-edged conical electrode representing divergent field. The main result shows that the PD inception voltage (PDIV) is considerably lower at switched than at sinusoidal voltage. Moreover, the mineral oil shows a lower inception voltage than the synthetic ester. Finally, a lower PDIV for the sharp electrode compared to the spherical electrode is seen. The differences can be explained by the 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.

I. Introduction

Transformer insulation design meets new challenges when exposed both to new stresses and new materials. Sharp rising voltage flanks produced from power converters stress the insulation differently from sinusoidal voltages. Additionally, new insulation liquids behave differently from the classical mineral oils used in impregnated insulation systems [1]. The new ester liquids are introduced as alternatives to mineral oils due to reduced fire risk and improved biodegradability. Synthetic esters have been a preferred choice for transformer insulating liquid for offshore oil exploitation and for windfarms, often in connection with converter operated variable speed drives, basically because of their good biodegradability. While many of the properties of the new liquids are well documented, there are also many questions remaining concerning their functional properties [2].