A new approach to study the effects of copper sulphide on electric stress distribution in paper oil insulation of transformers

Many cases of failures of power and converter transformers due to copper sulphide deposition on the insulating paper have been reported and the presence of corrosive sulphur in mineral oil has been identified as the cause of some of these failures. Though extensive research work has been carried out to understand the mechanism of formation and migration of copper sulphide, contribution towards understanding of electric stress distribution under copper corrosion is very limited. Hence electric stress distribution in paper-oil insulation of transformer under copper corrosion is studied using Finite Element Method (FEM) and the results are presented in this paper. This study attempts to correlate the deposition of copper sulphide on the individual paper layers to the variation in overall capacitance and conductivity of copper sulphide which gives better understanding of electric stress distribution in paper-oil insulation under conditions of copper corrosion. The results of this study will be useful in assessing the insulation damage caused by electric stress enhancement in transformer insulation due to deposition of copper sulphide.