Lightning impulse polarity effect in ester oils and mineral oil for transformer applications

Ester oil is becoming increasingly popular as a potential alternative to mineral oil due to eco-friendly, fire safety and higher continuous over loading capability in comparison to conventionally used mineral oil. The high interest on ester oil has led to several studies aimed at prediction of their dielectric breakdown voltage for transformer applications. In this paper, to evaluate the effectiveness of ester oil as a dielectric for transformers the withstand voltages of ester oil under both positive and negative polarities of lightning impulse waveshape in comparison with mineral oil are evaluated for various oil gaps using statistical analysis. In addition, the variation of streamer velocity with various gap distances between electrodes in mineral oil, natural ester oil and synthetic ester oil under different polarity of impulse are extracted to analyze the streamer behavior in ester oil. Finally, the ratios of breakdown voltage, streamer velocity of ester oil with respect to mineral oil are calculated from the experimental results under both positive and negative polarities which serves as a point of reference for insulation design of transformer. Sphere-needle electrode configuration is used to create nonuniform field under lightning impulse stress. The breakdown voltage is determined by multiple level test method and best estimate of 3-parameter Weibull distribution is used for 1% breakdown probability due to breakdown voltage.