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About the current transformer core demagnetization check

2018-12-07 17:13:09

Current transformer core is a general term for current transformers and voltage transformers. It is used to convert high voltage and high current into low voltage, low current devices for measuring or protecting systems. Next, I will introduce how to solve the core remanence problem of the transformer.
How to solve the core remanence problem of the transformer:

In the case of a sudden drop in current of the current transformer, the transformer core may generate residual magnetism. For example, the current transformer suddenly cuts off the power supply under a large current condition, and the secondary winding suddenly opens. The transformer core has residual magnetism, which reduces the magnetic permeability of the core and affects the performance of the transformer. 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 applied to the core through alternating primary or secondary windings. 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.

 

 

For current transformer demagnetization, the primary winding is open, the secondary winding is connected to the power frequency current, and gradually increases from zero to a certain current value (this current value is related to the design measurement upper limit of the transformer, generally 20-50 of the rated current) 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 is demagnetized. This method is called open circuit demagnetization. For some current transformers, the number of turns in the secondary winding is relatively large. 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 secondary 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 a 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.

 

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