Designing epoxy insulators in SF6-filled DC-GIL with simulations of ionic conduction and surface charging

The problem of surface charge accumulation on gas-insulator interface is one of the critical factors for the development of DC Gas Insulated transmission Lines (GIL). In order to solve this problem, researchers have proposed a variety of mathematical models in which the volume conductivity of SF₆ was assumed to be constant for calculating the distribution of surface charge. However, the conductivity of SF₆ was inconstant, affected by electric field strength. In this paper, a gas model is developed, taking into account the generation, recombination and motion of charge carriers in SF₆ of DC-GIL. The surface charge density and the electric field distribution on the insulator surface under DC voltage were simulated. Afterwards, the electric conduction through the volume of the insulator was proved to be the dominating accumulation mechanism. In order to improve the insulation performance, the influence of practical insulator shape on the electric field distribution in DC-GIL was studied. Three different shapes of insulator which were disc insulator, conical insulator and obtuse conical insulator were introduced and compared. The simulation results indicate the obtuse conical insulator was the most suitable configuration for DC-GIL.