A main insulation breakdown fault of two 500 kV SF6 current transformers

which occurred within 65 s in a thunderstorm

is analyzed by building a model of transformer substation with transmission line with the simulation software ATP. The amplitude of the lightning overvoltage on the current transformer is calculated

the impact of the lighting overvoltage on the faulty process of SF6 current transformer is analyzed

the disassembly inspection of the faulty current transformers is conducted

and the faulty process of the current transformer is deduced to find out the major causes of the fault. The results show that:1)The lightning overvoltage didn’t break down the main insulation of the current transformers directly

but made the main insulation of the current transformers with defects deteriorated more quickly; 2)The underlying cause of the fault is the insulation defect of fourth insulation support at L2side;and 3)Insufficient insulation margin of the fiberglass insulation cylinder in the current transformer may result in deterioration and ablation of cylinder wall when the internal SF6 gas is contaminated

thus induce discharge between wall and capacitance screen to ablate the capacitance screen surface. In addition

quality control measures and fault prevention methods are offered for this type of current transformers.