In order to ensure the safe and economical operation of the power system, it is necessary to monitor and measure the operation of power equipment. However, general measurement and protection devices cannot be directly connected to primary high-voltage equipment. Instead, it is necessary to proportionally convert the high current of the primary system into a small current for use by measurement instruments and protection devices.

When measuring high currents of alternating current, it is necessary to convert them into a relatively uniform current for the convenience of secondary instrument measurement (China stipulates that the secondary rating of current transformers is 5A or 1A). In addition, the voltage on the line is relatively high, and direct measurement is very dangerous. The current transformer plays a role in current conversion and electrical isolation. It is a sensor for measuring instruments, relay protection and other secondary equipment 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 is connected to measuring instruments, relay protection, etc.

So current transformers are divided into measuring current transformers and protective current transformers; The function of measuring current transformers is to measure (bill) and measure the current of operating equipment; The protective current transformer is mainly used in conjunction with the relay device to provide a signal to the relay device to cut off the faulty circuit in case of short circuit overload or other faults in the line, in order to protect the safety of the power supply system.