The Purpose and Method of Current Transformer Demagnetization

According to the national metrology standards, metrological grade transformers must be demagnetized before measuring errors. The error is not accurate if it is not demagnetized. The main reason is that the iron core has residual magnetism.

When the current suddenly decreases, the transformer core is likely to cause magnetic losses. For example, under high current conditions, the current transformer suddenly cuts off the power transformer, and the secondary winding resistance suddenly leads. The current transformer coil of this product has magnetic losses, which reduces the magnetic permeability of the transformer coil and affects the characteristics of the current transformer. Current transformers used for a long time should be demagnetized. Demagnetization should be carried out before inspection. Demagnetization is to alternately change the current of the excitation regulator according to the winding resistance of the primary coil or secondary coil, so that the transformer core generates an alternating magnetic field.

The current transformer is demagnetized. The primary and intermediate winding resistances guide the direction, and the secondary winding resistance produces the DC current. Starting from zero, the winding resistance gradually increases to a certain current value (this value is related to the precise measurement limit of the current transformer scheme design, generally around 20-50% of the rated current). It can be said that if the current suddenly increases sharply, the transformer core enters the magnetic saturation stage. Then reduce the current to zero and repeat this two or three times.

Before disconnecting the power transformer, please wind it once and then disconnect the power supply. Demagnetize the transformer core. This method is called directional demagnetization. With some current transformers, many turns are created due to the resistance of the secondary winding. If the guide rail demagnetization method is selected, the winding resistance in the guide rail direction may easily cause the operating voltage to be too high. Then connect a large resistor (10-20 times the characteristic impedance of the rated current) with a good tight connection to the second winding resistor.

The color gradually changes from zero to the maximum current allowed by the initial winding resistance of the current transformer, and then gradually changes to zero, and so on 2-3 times. Due to the presence of coils in the load transformer, complete demagnetization may not be possible. Because there is a very large current limit in the primary winding, if it is too large, there is a risk of damaging the primary winding. When the standard operating voltage is not too high, the secondary winding of the input resistor can be amplified. Therefore, the specific demagnetization effect is improved.

      Demagnetization method of current transformer:

      After the polarity of the current transformer is checked by the DC method, or when the power supply is cut off and the secondary winding is accidentally open-circuited under AC current, the iron core may produce residual magnetism, which affects the error characteristics of the transformer. Therefore, before the error test, the current Each iron core of the transformer is demagnetized. The iron core can be demagnetized by any method of open circuit demagnetization or closed circuit demagnetization.

      1. Open circuit demagnetization method

      When the secondary (or primary) winding of the current transformer is open-circuited, pass the power frequency current to the primary winding (or primary winding) of the demagnetized core to increase the current from zero to 10% of the rated current, and then slowly and evenly The current drops to zero. Repeat this process 2 to 3 times, and the applied current is gradually reduced to magnetize the current transformer. If the voltage peak induced at both ends of the open-circuit winding exceeds 75% of the value specified in the inter-turn insulation test at 10% of the rated current, demagnetization should be performed at a smaller current value. Generally, it is advisable to carry current through the primary winding of the current transformer. For current transformers with multiple secondary windings, the secondary winding that is not demagnetized should be open-circuited.

      2. Closed circuit demagnetization method

      Connect a resistor equivalent to 10 to 20 times the rated load to the secondary winding, and pass industrial frequency current through the primary winding. The current increases from zero to 1.2 times the rated current, and then decreases evenly and slowly to zero to demagnetize the current transformer. . When demagnetizing a current transformer with multiple secondary windings, when one of the secondary windings is connected to a large load, the remaining secondary windings should be open-circuited.

It is advisable to use the closed-circuit demagnetization method for demagnetizing current transformers above level 0.2.

HZCT-100C Transformer Tester