The Transition of Surface Charge Accumulation Dominating Way in HVDC GIS

The presence of SF 6 -insulator interface in gas insulated switchgear (GIS) leads to a large number of charges accumulating on the insulator surface under high voltage direct current (HVDC) voltage, which may result in the decrease of surface insulation performance, and hence restrict the development and application of HVDC GIS. Therefore, it is necessary to investigate the surface charge accumulation mechanism. In this paper, a simulation model with regard to surface charge accumulation of disc-type insulator based on the drift and diffusion equations of positive and negative ions were constructed. In terms of it, the changes of surface charge accumulation characters with ion pair generation rate were studied. The results indicated that, as the ion pair generation rate became larger, the surface charge density of insulator gradually decreased. When the ion pair generation rate further increased, the polarity of the surface charges on the insulator was reversed. By analyzing the distribution of the electric field, it is inferred that the dominant accumulation way of the surface charges varied from the electric conduction through the insulator volume into the gas conduction during this process. Moreover, a critical value of ion pair generation rate was defined to describe the transition of surface charge accumulation dominating way, at which net charges on a surface of insulator equaled to zero. However, due to the differences of electric field distribution around the upper and lower surfaces, there were different critical values for each surface. And it is found that the higher the voltage, the greater the critical value. Our research is helpful to understand surface charge accumulation mechanism and provide suggestions aiming to inhibit surface charge accumulation.