Electric Field Simulation and Optimization of Environmentally Friendly ±400 kV DC Wall Bushing

SF6 is still widely used as insulation medium for DC wall bushing, although it possesses strong greenhouse effect. C4F7N/CO2 mixture gas has attracted attention as a potential SF6 alternative gas. In this paper, the dielectric strength of the 4% ~ 6% C4F7N/CO2 mixture gas is calculated. When the gas pressure is 0.6 MPa, the breakdown field strength is about 18.03 kV/mm under power frequency voltage and about 21.56 kV/mm under lightning impulse. Secondly, a 3-D model of the DC wall bushing is established, and the internal electric field distribution is simulated and calculated. The maximum field strength is 25.24 kV/mm at the end of the shield. Finally, the shield structure is optimized by BP neural network, the maximum value of the field strength is reduced by 7.9 kV/mm and the electric field nonuniformity coefficient is reduced to 3.2 after optimization. Based on the breakdown field strength, liquefaction temperature and other factors, the 4% ~ 6% C4F7N/CO2 gas mixture at 0.6 MPa has the potential to be applied to ± 400 kV DC wall bushing.