Comparison of partial discharge inception voltages for ester and mineral oils under divergent fields

Ester oils are making their way into the transformer market, but still some doubts exist about their ability to be successful fluids for high and extra-high voltage transformers. In this paper, we compare the PDIV of two commercially available fluids: a natural ester and mineral oil. Two different setups were considered: Point to plane in oil and point to plane on a pressboard sheet immersed in oil. Fluids and pressboard sheets were fully conditioned prior testing. Results suggest that ester oils perform slightly better with simple point-to-plane electrodes, independently of the gap length. When testing oil/plane electrode configurations with pressboard, ester oils perform much better at short gaps. This behavior has been explained considering the hydrophilicity of ester oils, which helps in further drying the pressboard prior testing. Therefore, ester oils could prove superior to mineral oils in suppressing creeping discharges.