Corrosive sulphur in transformers.

Abstract

Failures in power transformers, in the majority of cases, have been linked to the formation of copper sulphide (Cu₂S) and corrosive sulphur. Cu₂S is conductive and affects the dielectric properties of the insulation system in the transformer. The formation of Cu₂S and corrosive sulphur in transformers is of worldwide concern to power utility companies, like Eskom, and large industrial manufacturing plants that maintain their own transformers. This research deals with determining and understanding the formation mechanism of Cu₂S as well as investigating factors that influence the acceleration of the corrosive sulphur formation. Data from oil test results was obtained from an experimental set-up belonging to eThekwini Electricity. The set-up consists of two 100 kVA transformers, one containing corrosive oil and the other containing clean oil. While varying the load and temperature of the transformer, oil samples were taken at various intervals and tested for corrosive sulphur by monitoring the concentration of dibenzyl disulphide (DBDS) and the dielectric strength of the oil. This data was used to investigate the reaction rates, activation energies and various thermodynamic parameters of the corrosive sulphur and Cu₂S, and to establish the factors affecting their formation. At high temperatures, the DBDS concentration was found to reach equilibrium. The activation energy for the DBDS reacting with copper and further formation of DBDS was found to be 47.4 KJmol⁻¹ and 35.2 KJmol⁻¹ respectively. This research also determined that a significant amount of the initial concentration of DBDS was needed in order to react with copper. This experimental study also showed that DBDS and Cu₂S reactions do not play a major part in influencing the physical properties of the transformer and transformer oil. Surface physics concepts were employed to discuss the interaction dynamics of Cu₂S on copper surfaces. The main focus of the surface physics investigation compared the results from this experiment with results from other surface physics investigations in published literature.