What checks need to be done before the current transformer error experiment?

1. Accurately compare the measured transformer with the standard transformer, the secondary current difference of the two transformers is 1. Accuracy check: the transformer error test generally uses the measured transformer to compare with the standard transformer, two transformers The secondary current difference is the error of the measured transformer. This test method is called the comparative method. The standard transformer requires two levels higher than the measured transformer, and the error of the standard transformer is negligible at this time. If the standard transformer is only one grade higher than the measured transformer, the error of the test result should be considered to add the standard transformer error. The secondary current difference between the measured transformer and the standard transformer is generally measured with a transformer calibrator. Read the ratio difference fx(%) and the phase difference δx(’) directly from the transformer calibrator. Since the transformer calibrator measures the ratio of the current difference between the measured transformer and the standard transformer to the secondary current, the requirements for the transformer calibrator are not high. What level of transformer must be calibrated is basically determined by the standard transformer.

2. Demagnetization inspection: When the current of the current transformer suddenly drops, the core of the transformer may have residual magnetism. For example, the current transformer suddenly cuts off the power supply or the secondary winding is suddenly opened under the condition of large current. The core of the transformer has residual magnetism, which reduces the permeability of the core and affects the performance of the transformer. All transformers after long-term use should be demagnetized. Demagnetize the transformer before inspection. Demagnetization is the alternating excitation current through the primary or secondary winding to give the iron core an alternating magnetic field. Starting from 0, gradually increase the alternating magnetic field (excitation current) to make the iron core saturated, and then slowly reduce the excitation current to zero to eliminate residual magnetism. For current transformer demagnetization, the primary winding is open, and the secondary winding is supplied with power frequency current, which gradually increases from zero to a certain current value (the current value is related to the design measurement upper limit of the transformer, generally 20-50 of the rated current %. It can be judged this way, if the current suddenly increases suddenly, it means that the iron core has entered the magnetic saturation stage). Then slowly reduce the current to zero, and repeat this 2-3 times. Before disconnecting the power supply, short-circuit the primary winding before disconnecting the power supply. The iron core demagnetization is completed. This method is called open-circuit demagnetization. For some current transformers, the number of turns of the secondary winding is relatively large. If the open circuit demagnetization method is used, the open winding may generate high voltage. Therefore, a larger resistance (10-20 times the rated impedance) can be connected to the secondary winding. The primary winding is energized with current, which gradually changes from zero to the maximum allowable current of the primary winding of the transformer, and then to zero. Repeat this 2-3 times. The iron core may not be completely demagnetized due to the load. Since the maximum current of the primary winding is limited, if it is too large, the primary winding may be burnt out. If the voltage generated by the secondary winding connected to the load is not too high, the load resistance of the secondary winding can be increased. This can improve the demagnetization effect.

3. Polarity check: the primary winding of the current transformer is marked as P1 and P2, and the secondary winding is marked as S1 and S2. If P1 and S1 are terminals with the same name, this kind of mark is called minus polarity. The primary current enters from P1, and the secondary current exits from S1. The polarity check is very simple. In addition to checking on the transformer calibrator, the DC check method can also be used.