Before conducting
Current Transformer error tests, tests such as polarity and demagnetization are usually required.
1, polarity check
The current transformer primary winding marks are P1 and P2, and the secondary winding marks are S1 and S2. If P1 and S1 are the same name, then this flag is called polarity reduction. The primary current flows from P1 and the secondary current flows from S1. The polarity check is very simple. In addition to checking on the transformer calibrator, DC test can also be used.
2, demagnetization check
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 primary winding is current-passed, ramping from zero to the allowable large current of the primary winding of the transformer, and then ramping to zero, thus repeating 2-3 times. The core may not be fully demagnetized due to the load. Due to the limitation of the large current of the primary winding, 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.
3, accuracy check
The transformer error test generally uses the measured transformer to compare with the standard transformer. The secondary current difference between the two transformers is the measured transformer error. This method of inspection is called the comparison method. The standard transformer requires two levels higher than the measured transformer, and the standard transformer error is negligible. If the standard transformer is only one level higher than the transformer under test, the error of the test result should be considered plus the standard transformer error.
The secondary current difference between the measured transformer and the standard transformer is generally measured by a transformer calibrator. The ratio difference fx (%) and the phase difference δx(') are read 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. The level of transformers that can be verified is basically determined by standard transformers.
