The principle of current transformer is based on the principle of electromagnetic induction. The current transformer is composed of a closed iron core and windings. The number of primary winding turns is very small, and the string is in the line of the current that needs to be measured. Therefore, it often has a full current of the line. The secondary winding has a relatively large number of turns. It is connected in series in the measurement appearance and maintenance circuit. When the current transformer is in operation, its secondary circuit is always closed, so the impedance of the series coil of the appearance and maintenance circuit is very small, and the operating state of the current transformer is close to short circuit. The current transformer is used to convert the large current on the primary side to a small current on the secondary side, and the secondary side cannot be opened.