Surface Charging and Flashover Characteristics of DC-GIS Epoxy Insulators Under AC–DC Composite Voltage

In this article, the surface charge distribution and flashover characteristics of epoxy basin insulators under ac–dc composite voltage with specific proportions in a down-sized dc-gas-insulated switchgear (GIS) are investigated by experiments. Results show that more heteropolar charges are observed on the insulator surface and tend to diffuse toward the central high-voltage electrode with increasing the composite voltage amplitude. Higher prestressing would result in a decline of the ac component but lifting of the absolute amplitude of the flashover voltage. With direct ramp voltage stressing, the insulators present higher flashover voltage than that with prestressing. Besides, the higher the component of ac voltage, the easier the surface flashover. The surface modification of the insulators is performed by direct fluorination. The surface charge distribution and flashover voltage of the fluorinated insulator are improved due to the increase in surface conductivity. It is confirmed that ac/dc superimposed voltage would have significant effects on the insulation performance of dc-GIS and direct fluorination is an effective method to improve the flashover voltage.