Study on AC Breakdown Voltage Characteristics of Dissolved Water in Insulating Oils by JIS C 2101

We investigated the breakdown voltage of ester-based insulating oils, which are currently attracting attention as a substitute for mineral oils in oil-filled transformers. In this study, dielectric breakdown voltage tests were performed in accordance with the Japanese Industrial Standard (JIS C 2101) using four types of ester-based insulating oils, with approximately 100 measurements, for each oil type. The results showed that the dielectric breakdown voltage decreased slightly when the relative water saturation of the insulating oil exceeded approximately 20 %. Statistical analysis based on the Weibull probability distribution showed that as the water content in the insulating oil increased, the shape parameter decreased while the dispersion between the measured data increased. As a practical consideration, it was confirmed that the relative water saturation of the insulating oil should be maintained below 20 % of the saturated water content in the oil.