Gas Convection Affecting Surface Charge and Electric Field Distribution around Tri-post Insulators in ±800 kV GIL

A multi-physics simulation model of the tri-post insulator in +/- 800 kV GIL laid horizontally is established to study the effects of gas convection on surface charges and electric field distribution. Results show that gas density together with the dielectric strength around the upper post is reduced over 11% under gas convection generated by the load current. The larger temperature gradient (29 K) along the lower post accelerates more surface charge injection through insulator buck conduction and distorts the electric field around the bottom of the lower post, which is 19.4% higher than that around the bottom of the upper post. The insulation performance of SF6/N2 filled GIL has the most striking spatial distribution characteristics compared with the insulation performance of SF6 and C4F7N/CO2. Though the flashover voltage is improved with high gas pressure, the flashover is most likely triggered near the lower post of the insulator according to the stream theory. It is hoped that this study can contribute to a useful guide for the design and manufacture of DC-GIL.