Influence of fiber impurities on DC partial discharge process in oil-pressboard insulation

To study the generation mechanism of fiber impurities in oil-pressboard insulation and their influences on partial discharge process under DC voltage, a HVDC experimental platform was set up, based on which characteristics including impurity shape, partial discharge inception voltage (PDIV), partial discharge repetition rate (n) and partial discharge apparent charges were all recorded and analyzed. The experimental results show that under the influence of electric field, fiber chains at the surface of pressboards will be damaged and detach into oil. According to stress conditions, impurities exhibit different shapes. Due to the influences of fiber impurities: PDIV of oil-pressboard insulation decreases comparing with no-impurity condition. The degree of this inclination reaches the maximum at an oil distance between 12.5mm and 15mm, and among three kinds of impurities, pile-like bridge introduces the most severe influence; PD repetition rate of oil-pressboard insulation increases, and partial discharge process exhibit "expansion phenomena" on time; PD charges of oil-pressboard insulation decrease. Strong PD happens at the end of intensive PD period, after which PD amplitude drops among the sparse PD period. Further analysis indicated that fiber impurities distort the distribution and recovering process of electric field in oil, which consequently cause the above-mentioned phenomenon.