When measuring the large current of alternating current, in order to facilitate the measurement of the secondary instrument, it needs to be converted into a relatively uniform current (China stipulates that the secondary rating of current transformers is 5A or 1A). In addition, the voltage on the line is relatively high, such as direct measurement. It is very dangerous. The current transformer plays the role of current conversion and electrical isolation. It is a sensor used by secondary equipment such as measuring instruments and relay protection in the power system to obtain electrical primary circuit current information. The current transformer converts high current into low current proportionally. The primary side of the current transformer is connected to the primary system and the secondary side Connect to measuring instruments, relay protection, etc.