Investigation of 500-kV current transformer external flashovers

Several external flashovers across the porcelain shell occurred on the 500-kV current transformers (CTs) located on one of the Victorian stations approximately 10 km from the sea coast. A thorough investigation was conducted in order to explain the flashovers since no system born or lightning overvoltages were registered in times of CT flashover. The investigations identified three factors which could have contributed to the external flashovers of the CTs: contamination of the porcelain, uneven voltage gradient distribution along the shell, and reduced air clearance due to grading rings. In the investigation, analytical and experimental data were gathered, compared and analyzed. Methods used included finite element analysis, and numerical integration and differentiation. A novel method of measurement of porcelain surface potential distribution was developed and successfully tested on different CTs. The method can be used on any porcelain or synthetic solid insulation, including insulator strings, shells, and bushings where even axial distribution of voltage gradient is of paramount importance