Aiming at the issue of abnormal voice during the dielectric test period of the electromagneticunit of 500 k V CVT and of excessive standard of acetylene of oil chromatographic analysis and withcombination of both the operating principle and disassembly of the CVT

it is found that the reason for theabnormality is the damage of voltage withstand capability of the electromagnetic unit after the abnormalcondition due to unreliable earthing of the LV terminal of capacitive divider of the CVT. During thesubsequent operation period the arc discharge is occurred by the electromagnetic unit under the action ofovervoltage

causing the transformer oil to decompose and generate acetylene. The preventive measuresare proposed so to provide reference for the analysis treatment and prevention of the similar fault.