The partial discharge phenomena on the surface of oil impregnated paper with parallel electric field

In power transformers, the interface between the oil and the paper is always the weak point of the electrical insulation. Along on the surface of the paper and the pressboard, Partial discharges (PD) may often appear. They could destroy the paper and lead to electric fault. In this paper, the authors used a needle - plate electrode construction to form parallel electric field, and study the partial discharge phenomena under this situation. High voltage of 30kV or 35kV was applied on the oil impregnated pressboard specimens for 30 h to study the PD phenomena, especially the development of the carbon trace on the surface of the pressboard and the pattern of the PD. The white mark, gas bubbles, flashover, carbon trace, and PD phenomena will be described. Within high voltage experiments it was found that the carbon trace can reduce the strength of the electric field near the needle and the partial discharge activities will be rapidly reduced at the beginning of high voltage test. PD with small magnitude will not change the carbon trace in some ten hours and the pattern of the PD is not related to the carbon trace.