On-line insulation diagnostics for current transformers

High voltage current transformers (CTs) perform a secondary function in a power utility. In the Transmission group in Eskom, more than 30% of over 10000 CTs have been in service for more than 30 years. A real need exists for a method by which on-line partial discharge (pd) measurements can be recorded which translate into a qualitative interpretation of the degree to which both bulk insulation deterioration and voids have progressed with operational conditions. This paper will present the concepts of high frequency elements in the insulation leakage current which uniquely describe different types of failure modes in high voltage insulation. The paper will present results of forcibly created voids and insulation deterioration and the corresponding signatures for these conditions.