Partial Discharges at Boundaries of Oil-Pressboard Immersed in Mixed Insulating Oils

Liquid and solid insulation work alongside in the transformer to efficiently insulate and cool it. Coadoption and compatibility of both insulations are responsible for the transformer's reliable operation and long life. In recent years, ester oils have become the choice for liquid insulation due to excellent biodegradability, high moisture saturation capability, and good dielectric strength. However, the cost of ester oils is relatively high compared to mineral oil, which lead researchers to come up with mixed insulating oils (MIOs) to balance the cost and performance. It is crucial to study the performance of such insulation as a compatible coexistent with solid insulation. Partial discharge (PD) diagnosis provides an inference into the compatible performance under the presence of the electric field. Therefore, this study focuses on studying the PD at the MIO, pressboard and conductor interface. The experiments are carried out in accordance with IEC 60270 on various mixtures of fresh and oxidative aged MIO soaked pressboards. The blended oil soaked pressboards have higher PD inception voltage than those soaked in base oils. The inception voltage is found to reduce with the aging of the oil. The discharges are observed to a large extent in the negative half cycle.