LW13-550 tank circuit breakers are used widely in 500 kV power system.Their insulation faults were caused by discharge between contact shield and breaker shell.Disassembly inspection and test results indicate that the metallic particulates on the wall of tank under the shield caused the discharge

or even insulation breakdown.To quantitatively analyze the relationship between the discharge and the size of metallic particulates

3D electric field in the tank circuit breaker needs to be calculated.We proposed a 3D electric field calculation model of the tank circuit breaker with metallic particulates in it

and solved the model with FEM.The calculation results show that: ① a metallic hemisphere-head column with the height of 5 mm and the diameter of 1mm can induce a maximum surface electric field intensity far higher than the breakdown field intensity of 0.5 MPa SF6 gas to lead to insulation breakdown in the break;② a metallic column with the height less than 5 mm will induce electric field deformation around it to result in local discharge or even insulation breakdown.