In order to transmit electrical energy, the power system often uses AC voltage and large current loops to send power to users, which cannot be directly measured with meters. The function of the transformer is to proportionally reduce the AC voltage and large current to a value that can be directly measured by the meter, which is convenient for the meter to directly measure, and at the same time provides power for relay protection and automatic devices. Transformer for power system is a special transformer that transmits the information of high voltage and high current of power grid to the metering, measuring instrument, relay protection and automatic device of low voltage and low current secondary side. It is a primary system and a secondary system. The primary winding is connected to the power grid, and the secondary winding is connected with measuring instruments and protection devices. The mutual inductor can measure the voltage, current and electric energy of the primary system in conjunction with measuring instruments and metering devices; in conjunction with relay protection and automatic devices, it can constitute electrical protection and automatic control of various faults in the power grid. The performance of the transformer directly affects the accuracy of power system measurement and metering and the reliability of relay protection devices.