Influence of thermal stress on the dielectric properties of oil paper insulation

The oil impregnated insulation paper is one of the oldest insulation systems used in the electrical power equipment. The mineral oils have been used for decades as transformer fluids because of their excellent dielectric properties and availability. However the performance of the mineral oil starts to be limited due to the environmental consideration. The natural or synthetic ester (as transformers liquids) is one of the alternatives that can replace the mineral oils in transformers. The aim of this paper is a study of the thermal stress on the insulation system oil-paper. There were used three types of oil; the inhibited mineral oil ITOX, the synthetic ester Midel 7131 and the natural food rapeseed ester. They were combined with a transformer paper. The samples of a combination paper and…