Mechanism of Increase of Electrostatic Charging Tendency in Insulating Oil for Oil-immersed Transformer

Both of the mechanism and the causing compound to increase the electrostatic charging tendency (ECT) of insulating oil was investigated by accelerated deterioration tests with the addition of various compounds. Although the ECT of the insulating oil was almost constant when only sulfoxide compounds were added, remarkable increase was observed when either of hydrochloric acid or moisture, which was considered to be generated by the aging of insulating oil, was further added to the oil containing sulfoxide compounds. It is presumed that sulfonium ion, which is generated by the reaction between sulfoxide compound and hydrogen ion, is the compound to directly contribute the increase in ECT. Hydrogen ion can be supplied from the organic acids, which is generated by the oxidation of hydrocarbon in aging insulating oil. It is considered that the increase in ECT of insulating oil is caused by the generation of sulfoxide by the oxidation of sulfide, which is contained in new oil (origin compound), and the generation of sulfonium ion from the reaction between sulfoxide and hydrogen ion, which is generated due to aging.