Techniques for diagnosing deterioration of oil-impregnated paper-film power capacitors

Power capacitors are widely used for regulating voltage and improving the power factor of power systems, and the type in most frequent use today are capacitors made of oil-impregnated paper film, a growing number of which will soon have been in operation in the field for 20 years. This has prompted the authors to study techniques for diagnosing deterioration in oil-impregnated paper film capacitors. It was found that thermal stress causes deterioration of the insulating paper, which increases the amount of dissolved CO and CO₂ in the oil and lowers the partial discharge inception voltage, determining the useful lifetime of the capacitor. Based on this idea, techniques have been proposed for diagnosing deterioration, consisting of caution level and danger level, from actual stress in the field. Further, a trial calculation was made for the number of years and months it takes for a capacitor to reach this caution level, based on an investigation of capacitors in service for many years