Study on Development Process of White Marks on Pressboard Surface layers in Needle-Plate Model Under Different Curvatures

The phenomena of white marks on the pressboard could reduce the performance of oil-paper composite insulation, which is accessed on the scrapped transformers. Whereas it is much difficult to find white marks during the overhaul of the power ac transformer because the white mark channel could generally disappear when connected to the air. So it is much important to find a method to detect the phenomena of the white mark on pressboard. At first, a throughout understanding of the development process of the white mark should carry out under the laboratory condition. A typical needle-plate electrodes model with pressboard was selected to initiate ac divergent stress on pressboard surface. The research found the development process of the white mark could be divided into four stages named the generation, fast extension, slow extension, and flashover stage respectively, the development process of the partial discharge could be divided into five stages. The one-to-one relationship was observed between the four stages of the white mark and the second to fifth stages of the partial discharge. The parabola relationship between the flashover voltage and needle curvature could be due to the concentrated degree of spark discharge on pressboard surface.