Space charge dynamics of oil-impregnated paper insulation with high water content

Oil and oil-impregnated cellulose paper and pressboard are widely used as insulation materials in converter transformers. The existence of DC component in electric stress imposed on converter transformer insulation unfavorably causes space charge accumulation inside the oil impregnated paper insulation. It has been reported that the water content of oil impregnated paper has an impact on space charge behaviors [1][2]. This paper focuses on space charge dynamics in oil-impregnated paper insulation with high water contents. Pulsed electro-acoustic (PEA) technique was employed for measuring space charge in 400 μm thick single-layer paper samples impregnated in mineral oil. The samples were stressed with different field strengths of 7.5 kV/mm and 20 kV/mm, different temperatures of 30T and 70T and different polarities. The results show that, with high water content, negative hetero charges are firstly formed in the vicinity of the anode and then shifted to the cathode at a decreasing velocity. This process is accelerated by a higher temperature or a higher field strength.