A mechanism for the formation of copper sulphide in oil filled electrical equipment

The formation of copper sulphide within oil filled high voltage electrical equipment can lead to the failure of the equipment. Incidents involving large transformers have been attributed to such contamination. Sealed equipment, working under high loads and containing mineral oil with high sulphur content, appear to be most at risk. The failure is caused by copper sulphide deposits which produce a low resistance path across and through the cellulose insulation and can lead to internal discharge and flash-over. However, the mechanism for how and why the copper sulphide forms on copper is largely an open question. A simple heating test, using strip copper, is available which indicates the likelihood of copper sulphide formation when using a mineral oil type. This paper outlines a series of experiments using variations to this heating test. The results suggest a mechanism for how copper sulphide forms on the copper itself.