Gaseous Ionization Dependence of Surface Charge Pattern on Epoxy Insulator With Complex Surface Profile Under DC Voltage

Surface charge accumulation on epoxy insulator installed in gas-insulated transmission line (GIL) and gas-insulated switchgear (GIS) has been recognized as a crucial factor for electric field distortion that results in the reduction of surface flashover voltage. This article reports on the gaseous ionization dependence of the charge accumulation pattern on a real-size epoxy insulator with multiarc surface profile by using various gas atmospheres. A coaxial electrode system was designed to investigate the charging behavior under dc voltages of -10, -20, and -30 kV. The insulator and electrodes were placed in a metal-enclosed test chamber, which was filled with 0.1 MPa air or 0.4 MPa SF6/N-2 mixture to estimate the influence of the gas atmosphere on the charge accumulation behavior by a Kelvin-type probe. The accumulation mechanism and the electric field distortion induced by the charges were demonstrated based on numerical simulation. The results indicated that positive charges were accumulated in the shape of speckle on insulator surface in air, whereas both positive and negative charges were accumulated in SF6/N-2 mixture and positive charge speckles appeared in SF6/N-2 mixture only when the voltage raised up to -30 kV. It is considered that the charge accumulation behavior is determined by the competition result among the bulk, the gas and the surface conductions. As compared with the case in air, gaseous ionization in SF6/N-2 mixture is more difficult to occur, which leads to the obvious difference in the charge accumulation pattern.