The surface flashover process under positive lightning impulse voltage: initial stage and evolution

Surface flashover is a crucial issue in the field of electrical insulation, and it involves many complex physical processes. In this paper, the development process of the surface flashover was studied from different methods. The initial process of positive surface streamers in different gas environments (air, N-2, CO2) was studied by photoelectric observation. The evolution of positive surface streamers in the air was described based on a 2D fluid model. The influence of the surface trap energy level on flashover development and the relationship between gas adsorption and surface trap energy level was discussed by density functional theory calculation preliminarily. The results showed that the initial ionization process of surface flashover is considered as the collision ionization between the initial electrons and gas molecules and photoionization of high-energy photons. Some of the high-energy photons can not only ionize some gas molecules but also cause the surface of the insulator to emit electrons (photoemission process), which could promote the development of the streamer. Both the ionization of the gas molecules and the photoelectric emission of the insulator surface may determine the initial development of surface flashover. During the process of the flashover, the electron density of the surface streamer (similar to 10(21) m(-3)) is high and the main streamer tends to develop towards the insulator surface. The attraction of surface streamers changes with the position of the initial electron, and the positive surface charge brings the stronger ionization process, and the negative surface charge has the opposite effect. The band gap of the insulator surface is affected by the adsorption of gas molecules, which is considered as introducing shallow traps on the surface. For impulse voltage, the charge accumulation and internal charge migration may be not evident, the initial photoionization process and initial surface charge distribution affect the flashover process primarily.