Thermal ageing and corrosive sulphur effect on partial discharge and electrical conductivity of oil impregnated paper

Corrosive sulphur has been identified as the cause of numerous failures of relatively new transformers. In this paper, models of transformer turn insulation were put into artificially-contaminated (315 ppm of DBDS) mineral oil. The ageing procedure consisted of thermal cycles with maximum temperature of 140°C. Between cycles, partial discharges were measured and processed in order to extract diagnostic markers able to highlight copper sulphide contamination. In particular, attention was paid to identify specific PD patterns associated with this type of degradation. Other additional tests, such as electrical conductivity measurements, were also performed.