FLASHOVER PROCESSES ON THE SURFACE OF SOLID INSULATORS UNDER POSITIVE IMPULSE VOLTAGE IN THE ATMOSPHERE

The surface discharge phenomena on acrylic plates, 1 to 10 mm thick, were investigated under positive impulse voltage in air at atmospheric pressure. The corona inception voltages and the flashover voltages were measured and the discharge phenomena were observed with an image converter camera. As the result of this study, various stages in the flashover processes have been investigated. When 50% flashover voltages are applied and the creeping distance of the surface discharge is in the range from 2.5 to 10 cm, the positive leaders proceed towards the cathode generating positive streamers intermittently from their tips until the flashover occurs, as in air gaps. If the creeping distance is in the range from 0.4 to 1.0 cm, the flashover occurs without any positive leader. Therefore, the relations between the 50% flashover voltages and the creeping distance in these two regions are different. In the range of the creeping distance from 1.0 to 2.5 cm the flashover may occur through both processes, so that the fluctuation in the 50% flashover voltage is quite large. These flashover processes are illustrated with typical photographs, the conceptual figures constructed on the basis of the photographic observations, and the flashover characteristics are explained with the flashover processes.