Partial Discharge Characteristics in Oil Gap between Polypropylene Films

Film power capacitors are made out of polypropylene (PP) films co-wound with Al foil electrodes, impregnated with insulating oil. In a void, if any, or around the electrode edge, partial discharges (PDs) occur, leading to PP film deterioration and eventually to dielectric breakdown. In this work, PD characteristics were investigated using the oil gap electrode system in which an oil gap with thickness of 12 μm or 50 μm was formed between PP films. PD inception voltage (PDIV), discharge amplitude-PD number-PD phase (q-n-φ) pattern and PD light emission were observed under AC voltage application. The q-n-φ pattern and light emission of PDs generated in the oil gap containing oil-decomposed-gas voids differed from those without gas voids. The sequential change of PD characteristics showed that the oil-decomposed-gas voids were generated by PD in the oil gap, and then the PD occurred in the gas voids. The relation between the PP film deterioration and the difference in sequential change of PD characteristics were discussed.