The transformer, as an important measuring instrument, is used in the power system, which makes the high voltage and current visible through the transformer. The transformer can measure the voltage, current and electric energy of the primary system in combination with measuring instruments and metering devices. Together with the relay protection and automatic device, it can constitute the electrical protection and automatic control of various faults in the power grid. The performance of transformer directly affects the accuracy of power system measurement and measurement and the reliability of relay protection device.