Pre-compressed pressboard was commonly used in power transformers along with mineral oil. Pressboard in combination with mineral oil is suitable composite insulation for oil filled transformers over several decades. Synthetic ester is a suitable alternative for mineral oil due to its biodegradable and fire resistant property. It is also compatible for transformer insulations as per recent research studies. Intent of this paper is to find out the suitability of synthetic ester as transformer insulation, with respect to partial discharge studies. Understanding partial discharge characteristics of synthetic ester with an influence of moisture, conducting particles and bubbles, the experimental study was carried out. Measurements of PDIV, PDEV, maximum apparent charges, and PRPD patterns were recorded in order to analyse the effect of various impurities over partial discharge activity. Oil moisture has more impact over board moisture. © 2016 IEEE.