A type of ±800 k V wall bushing at the Jinhua Station in Xizhe UHVDC project had a flashover during the summer operation. It is very important to find out the cause of flashover of UHVDC wall bushing supporting insulators to improve the reliability. This article dissects the bushing and analyzes the cause of the failure. A practical experimental system under SF6 environment has been developed to verify the sufficient insulation margin of bushing insulators without pollutants or partial discharge. Anatomical analysis indicates that little metal particles are generated due to the relative movement of the spring finger structure components in the casing

which is the main cause of the failure. It was found that when the internal partial discharge is suppressed and there is no internal contamination

the insulation of the supporting insulator is large. The internal design of the casing has been studied and improved

a supporting insulator has been cancelled

and the design of the connecting device has been optimized. The improved ±800 kV project was put into operation and it has stable performance for more than two years. The lessons learned from this accident were all adopted by our own self-developed ±1 100 kV UHV DC bushings and achieved good results. The ±1 100 kV busing passed the type test.