Evaluation of Dielectric Withstand Levels of Ester Oils (Natural & Synthetic) and Oil Impregnated Pressboard under Impulse Stress Conditions for Transformer Application

Abstract

Ester oil is becoming increasingly popular as a potential alternative to mineral oil. Main reason behind this is environmental benefit and high continuous over loading capability in comparison to the conventionally used mineral oil. Major attractive features of esters which has been already evaluated are: good biodegradability, non-toxicity, high moisture absorption ability and high flash and fire points (usually above 300°C) which lowers the risks for humans as well as environment. Due to high moisture saturation levels of esters it protects the paper insulation by absorbing the water from cellulosic materials. The major technical challenge that usage of esters is facing in recent years is to extend its use from distribution transformers (low voltage) to power transformers (high voltage). The high interest on ester oil has led to several studies aimed to understand complete dielectric behaviour of esters and their impact on transformer design for manufacturing. Therefore, this work focuses on evaluation of their dielectric breakdown performance for transformer application. Main objective is to evaluate impulse withstand voltage of ester oils and their impregnated pressboard under standard positive and negative impulse for uniform (nondivergent) and non-uniform (divergent) electrode configurations. Sphere-sphere and Sphereneedle electrode configurations are considered to create a uniform and non-uniform field respectively. The study compares different ester oils with conventional mineral oil, which serves as point of reference. Electrodes and test procedures followed are according to standard ASTM–D3300. The best estimate of 3-parameter Weibull distribution is calculated for 1% breakdown probability voltage. From experimental investigations, conclusions have been made considering geometry effect and polarity effect. Furthermore, electric stress experienced by all insulating liquids at 1% failure probability voltage is determined with the help of ElecNet software. Similarly, for impregnated pressboards, electric stress is determined at average breakdown voltage. Last but not the least, an effort has been made to study effect of aromatic content and polar contaminant on dielectric breakdown performance of mineral oil. This section discusses the effect of aromatic content by comparing paraffinic inhibited and naphthenic inhibited mineral oil. Further to analyse effect of polar contamination in mineral oil, natural ester insulating oil (polar contaminant) is mixed to mineral oil, which is highly non-polar in nature. The results indicated that these effects are prominent under negative polarity of impulse.