Influence of Defects on the Electric Field Distribution of ±800kV Butt-type DC Wall Bushing

UHVDC wall bushing is one of the key equipment of UHV DC power transmission. In recent years, a number of bushing failures have occurred at home and abroad due to the defects in the connection of the conductive rods. It is necessary to study the mechanism and preventive measures of these faults. The inner insulation of the epoxy resin impregnated butt-type dry wall bushing in this paper is formed by the butt joint of two converter transformer dry bushings' capacitor cores. And SF6 gas is filled between the composite insulated jacket and capacitor cores. This paper analyzes the influence of the defects and impurities at different positions on the electric field distribution of the bushing under rated voltage by finite element simulation. The results show that when there are metal particles on the surface of the grading cover, the end of the core, and the end of the plate, the field strength increases to about 1.2 times, 1.25 times and 3.8 times respectively. Besides, when the air gap occurs inside the core, the distortion of the casing electric field strength increases from 5kV/mm to 37kV/mm, which increases to about 7.4 times. What's more, the type of the powder doesn't have typical influence on the electric field distribution. When the 100% solid powder decomposition products were found on the surface of the epoxy resin core near the butt joint structure of the bushing, the distortion of the electric field of the bushing is the greatest. Under the rated voltage, the electric field increases from 9.93kV/mm to 89.1kV/mm. The maximum electric field strength and the ratios of powder are approximately proportional. These defects may result in partial discharge of the grading cover or epoxy resin core near the butt joint part. The internal electric field distortion will seriously threaten the safe and stable operation of the bushing.