Research on forming mechanism of surface discharge paths for solid dielectric with different shapes under micro-second impulse in liquid nitrogen

As the increasing of voltage grades for high temperature superconducting (HTS) devices, the research on characteristics of surface flashover and methods of improving the surface flashover voltages in liquid nitrogen are necessary. Designing different surface shapes of solid dielectric is a commonly used method to improve surface flashover voltage. But in liquid nitrogen, the related characteristics of surface flashover must be studied firstly. In this paper, several surface shapes of solid dielectric suitable for liquid nitrogen environment are designed, their surface discharge phenomenon is studied by experiments and the forming mechanisms of surface discharge paths for solid dielectric with different surface shapes are analyzed. It is assumed that because of the lack of oxygen and the low electron energy in the forming discharge, the surface discharge paths in liquid nitrogen under micro-second impulse easily deviate the surfaces of solid dielectric. Surface shapes of solid dielectric changes the tractive efforts to the head of electron avalanche for the discharge streamer, together with the characteristics of liquid nitrogen that easily gasified, so the discharge arcs choose different paths under different surface shapes of solid dielectric. The results further reveal the mechanism of surface discharge phenomenon in liquid nitrogen and provide method guidance for the shell design of HTS power devices.