Partial discharge progress and breakdown characteristics of aluminum foil/oil-impregnated paper insulation system

Static capacitors (SC) and capacitor voltage transformers (CVT) operated over 30 years in substations are composed of aluminum foil/oil-impregnated paper insulation system. For the condition monitoring and diagnosis of aged SCs and CVTs, partial discharge (PD) measurement is expected. However, PD characteristics from PD inception and progress to breakdown (BD) are not yet clarified. Thus, in this paper, we carried out electrical and optical investigation of PD progress and BD characteristics of a fundamental insulation model composed of aluminum foil/oil-impregnated paper insulation system. As a result, we clarified that PD progress can be classified into 3 periods. In the Period 1, PD signals less than 500 pC were intermittently generated around the peak phase of applied AC voltage and dot-like PD light emission was observed at the edge of aluminum foil. In the Period 2, PD signals larger than 500 pC were continuously generated around zero crossing (void discharge) to peak phase and PD light emission was observed on the peripheries of the aluminum foil. In the Period 3, PD signals larger than 10,000 pC were generated with the extended PD light emission and bubbles, leading to BD through the oil-impregnated papers.