Adopt front-end digital technology, optical fiber transmission, good electromagnetic compatibility. The amplitude-frequency characteristics and phase-frequency characteristics are good, and high measurement accuracy can be obtained in a wide range of amplitude, frequency and phase. It is a digital sensor, the secondary instrument will not introduce errors, and the sensor error is the system error. The secondary rated capacity of the current transformer should be greater than the actual secondary load, and the actual secondary load should be 25-100% of the secondary rated capacity. The capacity determines the secondary side load impedance, and the load impedance affects the measurement or control accuracy. The load impedance is mainly affected by the resistance of measuring instruments and relay coils, reactance and wiring contact resistance, and the resistance of secondary connecting wires.