Polarity measurement of current transformers

The current transformer should be tested for polarity before and after handover and overhaul to prevent the polarity from being wrong during wiring, causing the protection device to malfunction and fail to measure correctly in the relay protection circuit and the metering circuit. Therefore, the polarity test must be done before commissioning. There 

　　are many ways to measure the polarity of the current transformer. We often use the following three test methods at work: ① DC method; ② AC method; ③ instrument method. 

1 DC method 

　　See Figure 1. Use a 1.5~3V dry battery to connect its positive pole to the primary coil L1 of the transformer, L2 to the negative pole, the secondary side K1 of the transformer to the positive pole of the milliammeter, and the negative pole to K2. After the wires are connected, close K and the milliammeter pointer is positively biased. After opening, the milliammeter pointer is negatively biased, indicating that the terminal of the transformer connected to the positive pole of the battery and the terminal connected to the positive end of the milliammeter are of the same polarity, that is, L1 and K1 are of the same polarity, that is, the transformer is of subtractive polarity. If the pointer swings in the opposite direction to the above, it is a plus polarity. 

Figure 1 DC method to measure current transformer polarity

 2 AC method 

　　See Figure 2. Connect the primary and secondary coils L2 and the secondary side K2 of the current transformer with a wire, pass an AC voltage of 1 to 5V (using a small range) on the secondary side, and use a voltmeter below 10V to measure the values ​​of U2 and U3. If U3=U1-U2, it is a subtractive polarity.

Figure 2 AC method to measure current transformer polarity

 　　U3=U1+U2 is the addition polarity. Note: During the test, the input voltage should be as low as possible to prevent the coil from being damaged by too much current. For clear readings, the voltmeter should be as small as possible. When the conversion ratio is below 5, it is simpler and more accurate to use the AC method for measurement. Do not use this method to measure the transformer with a conversion ratio of more than 10, because the value of U2 is small and the values ​​of U3 and U1 are close, and the reading of the voltmeter is not easy to distinguish the size, so it is difficult to distinguish during measurement. Generally, this method should not be used to measure polarity. 

3 Instrument method 

　　General transformer calibrators have polarity indicators. Before measuring the current transformer error, the instrument can check the polarity in advance. If the indicator does not indicate, it means that the polarity of the current transformer under test is correct (minus polarity).