What checks should be done before the current transformer error test?

- Feb 14, 2020-

1. Compare the accuracy of the tested transformer with the standard transformer, and the secondary current difference of the two transformers is 1. Accuracy inspection: generally, the error test of the transformer adopts the comparison between the tested transformer and the standard transformer, and the secondary current difference of the two transformers is the error of the tested transformer. This test method is called comparative method. The standard transformer is required to be two levels higher than the tested transformer. At this time, the standard transformer error can be ignored. If the standard transformer is only one level higher than the tested transformer, the test result error shall be considered plus the standard transformer error. The secondary current difference between the tested transformer and the standard transformer is generally measured by the transformer calibrator. Read the ratio difference FX (%) and phase difference δ x (') directly from the transformer calibrator. Because the current difference between the tested transformer and the standard transformer and the ratio of the secondary current are measured by the transformer calibrator, the requirements for the transformer calibrator are not high. It is basically determined by the standard transformer to be able to verify what level of transformer
2. Demagnetization inspection: when the current transformer suddenly drops, the core of the transformer may produce remanence. For example, the current transformer suddenly cuts off the power supply under the condition of large current, and the secondary winding suddenly opens. There is remanence in the core of the transformer, which makes the permeability of the core decrease and affects the performance of the transformer. After long-term use, all transformers shall be demagnetized. The transformer shall also be demagnetized before inspection. Demagnetization is to give alternating magnetic field to iron core through alternating excitation current of primary or secondary winding. Gradually increase the alternating magnetic field (excitation current) from 0 to make the core saturated, and then slowly reduce the excitation current to zero to eliminate remanence. For current transformer demagnetization, the primary winding is open circuit, and the secondary winding is connected with power frequency current, gradually increasing from zero to a certain current value (the current value is related to the upper limit of the design measurement of the transformer, generally about 20-50% of the rated current). It can be judged that if the current suddenly increases sharply, it means that the core enters into the magnetic saturation stage). Then slowly reduce the current to zero and repeat for 2-3 times. Before disconnecting the power supply, the primary winding should be short circuited before disconnecting the power supply. Core demagnetization completed. This method is called open circuit demagnetization. For some current transformers, the number of turns in the secondary winding is relatively large. If the open circuit demagnetization method is adopted, the open circuit winding may generate high voltage. Therefore, a large resistance (10-20 times of the rated impedance) can be connected to the secondary winding. The primary winding is connected with current, from zero to the maximum allowable current of the primary winding of the transformer, and then to zero, repeat for 2-3 times. It may not be demagnetized completely due to the connected load core. Due to the limitation of the maximum current of the primary winding, it may burn out the primary winding if it is too large. If the voltage of the secondary winding connected with load is not too high, the load resistance of the secondary winding can be increased. This can improve the demagnetization effect.
3. Polarity inspection: the primary winding mark of current transformer is P1 and P2, and the secondary winding mark is S1 and S2. If P1 and S1 are homonymous ends, this sign is called subtractive polarity. The primary current is from P1 and the secondary current is from S1. The polarity check is very simple. In addition to checking on the transformer calibrator, the DC check method can also be used.https://www.ctsensorducer.com/