Exposure of vegetable oil impregnated transformer windings to air

A large quantity of research into the use of vegetable oils as transformer dielectrics has been performed over the years. Together with over ten years of practical experience, vegetable oil based dielectrics have been demonstrated to be effective for use within a transformer. One of the practical issues of maintaining transformers filled with these fluids is that a Utility is likely to extensively service the unit twenty or so years into its operating life. During this service the fluid impregnated cellulosic structure is exposed to the air. We have therefore been conducting investigations into the effect of air on the windings. Specifically, whether the rate of water adsorption and the formation of residues on the surface of the paper insulation is different when a vegetable oil is used instead of mineral oil. We are firstly investigating residue formation in the laboratory setting, by studying the interactions between paper, vegetable oil and air. Secondly we will apply the findings of the laboratory studies to small transformers, by exposing their windings to the atmosphere under controlled conditions then searching for gelling. In this paper we present some preliminary findings of the project.