Abstract: The function of Current transformer (CT) is to convert the primary Current of larger value into the secondary Current of smaller value through a certain ratio, which can be used for protection, measurement and other purposes, in series in a circuit of current to be measured. When the internal overheat or discharge fault occurs, its internal insulation function of the insulating oil will produce gas, the operation of current transformer can be evaluated and analyzed by detecting the content of these characteristic gases, and the latent fault and the severity of the fault can be judged.