Monitoring of total and Mercaptan sulphur under combined thermal electrical ageing of paper-oil insulation system

Corrosion of copper conductors due to Sulphur in transformer oil is a serious problem affecting both Power and converter transformers. The reactive and corrosive sulphur components are of very low order in fresh transformer oils but during service, some sulphur components are converted to reactive and corrosive forms which react with copper forming copper sulphide. Copper sulphide migrates from conductor surface to outer layers of paper and from there it is carried to other parts of the transformer. Since copper sulphide is conductive, it affects the voltage distribution leading to surface discharges. The paper insulation is damaged due to these discharges and leads to breakdown of insulation. In this study, samples of paper insulated conductors are subjected to combined thermal/electrical ageing and Total sulphur and Mercaptan sulphur levels of oil are regularly monitored. Mercaptan is also added to oil and then subjected to thermal ageing and measurements are repeated to determine the rate of Mercaptan sulphur consumption in oil. The discharge characteristics of the insulation were also studied after thermal/electrical ageing for establishing correlation between copper corrosion and PD characteristics.