Aging Performance and Moisture Solubility of Vegetable Oils for Power Transformers

This paper discusses the suitability of vegetable oil as an insulating medium in power transformers. A high flash point and very good environmental compatibility compared to conventional mineral oil are good reasons to use vegetable oil in power transformers. In this paper, the physical and electrical performance and the aging behaviour of three natural vegetable oils are compared to one synthetic and one traditional mineral oil. The breakdown voltage of new, dry, natural vegetable oil is more than 80 kV, which is above the breakdown voltage of mineral oil. Regarding the aging of paper, natural esters are more friendly toward paper than mineral oil. The water solubility clearly exceeds that of mineral oil but the same dependence of dielectric parameters on relative moisture saturation was observed for all liquids. Aging of vegetable oils under the presence of air leads to oxidation and, thus, to a strong increase of viscosity. Therefore, transformers filled with these liquids need to exclude oxygen from air. The conducted experiments show vegetable oil to be applicable for use in power transformers.