Influence of Diverse Compounds on Electrostatic Charging Tendency of Mineral Insulating Oil used for Power Transformer Insulation

Some cases reported recently indicate flow electrification that becomes obvious due to aging deterioration of insulating oil and cellulose insulation in aged oil-immersed transformers. It is supposed that the inclusion of minor components influences the aging deterioration of mineral insulating oil, but details including the aging mechanism have not been clarified. This paper examines the influence of various compounds on the electrostatic charging tendency (ECT) of insulating oil. Mineral insulating oil mainly consists of hydrocarbon compounds, but it also contains extremely small amounts of sulfur compounds, nitrogen compounds and oxygen compounds. Sample oil was prepared by adding various compounds to synthetic oil suffering from small aging changes of the ECT. After heating the sample oil, the ECT was measured, and the influence of various compounds was examined. Aging changes of the ECTs were small in pure hydrocarbon compounds. On the other hand, increases by aging were detected in the ECTs of the sample oils to which impurities such as sulfur compounds and nitrogen compounds were added. In particular, outstanding increases were detected in the ECTs of sample oil to which sulfide compounds and sulfoxide compounds classified as sulfur compounds were added. It has been estimated that sulfur compounds influence the increase by aging of the ECT of mineral insulating oil. It has been also estimated that the increase by aging of the ECT of mineral insulating oil starts from sulfide compounds contained in new oil and that the ECT increase process triggered by sulfide compounds progresses by the production of sulfoxide compounds and compounds of high ECT.