The transformers after long-term use should be demagnetized. The transformer should also be demagnetized before inspection. Demagnetization is the alternating magnetic field that is passed through the primary or secondary windings to alternate the excitation current. Gradually increase the alternating magnetic field (excitation current) from 0 to bring the iron core to saturation, and then slowly reduce the excitation current to zero to eliminate residual magnetism.
Demagnetization of the split core current sensor, the primary winding is open, and the secondary winding is connected to the power frequency current, gradually increasing from zero to a certain current value (this current value is related to the design measurement upper limit of the transformer, generally 20- of the rated current) About 50%. It can be judged that if the current suddenly becomes sharply large, it means that the iron core enters the magnetic saturation phase). Then slowly reduce the current to zero, so repeat 2-3 times. Before disconnecting the power supply, the primary winding should be shorted before disconnecting the power supply.
The core of the split core current sensor is demagnetized, and for some devices, the number of turns of the secondary winding is relatively large during operation. If an open demagnetization method is used, the open winding may generate a high voltage. Therefore, a large resistance (10-20 times the rated impedance) can be connected to the secondary winding. The primary winding is energized, ramping from zero to the maximum allowable current of the primary winding of the transformer, and then ramping to zero, thus repeating 2-3 times. It may not be completely demagnetized due to the load of the core. Since the maximum current of the primary winding is limited, if it is too large, the primary winding may be burned out. If the secondary winding connected to the load produces a voltage that is not too high, the load resistance of the secondary winding can be increased. This can improve the demagnetization effect.