Experimental Investigations on Mineral and Ester Oils for Power Transformers

This paper presents results of experimental investigations on mineral oils, natural esters (vegetable oils) issued from different origins and synthetic esters. Different characteristics such as the dielectric strength, ageing stability, gassing tendency and heat transfer efficiency are analyzed. It is shown that before ageing, natural and synthetic esters present a breakdown voltage (BDV) close to that of mineral oil while after ageing, natural esters generally keep a good BDV and are even sometimes better than mineral oils. Natural esters appear to be gas absorbing oils whereas synthetic ester is gas evolving. Natural esters have inherently low oxidation stability that can be improved by adding anti-oxidant additives and thus compete with non inhibited mineral oils. Their heat transfer (by convection) seems to be less efficient than with mineral oils even if some ester oils can have a viscosity closer to that of mineral oil.