There are two methods for measuring the DC resistance of a current transformer; the first is to simply measure with a multimeter; the second is to use a loop resistance tester or a current transformer comprehensive calibrator.
I. Simple measurement with multimeter
The DC resistance of the secondary side of the current transformer is often very small, less than 1 ohm, so it is best to use a digital multimeter when measuring, and measure the DC resistance between the secondary side S1 and S2. If the error is not large, it means there is no problem. Compared with other transformers in the same group, there is not much difference between the three phases.
The measurement accuracy with the loop resistance tester is relatively high, which can reach the level of mΩ. The measured data is compared with the original value to determine whether there is a problem with the transformer.
2. Measure with current transformer calibrator
According to the requirements of the current regulations, high-voltage current transformers must be periodically tested for characteristics to ensure the accuracy of measurement and protection. Common items include DC resistance, volt-ampere characteristics, and polarity. The test results should be consistent with the factory data or the previous The test data is compared to ensure that the accuracy and characteristics of the transformer meet the requirements.
The working principle of the high voltage current transformer is the principle of electromagnetic induction. During operation, the primary coil of the current transformer is directly connected to the main circuit, and the number of coils on the secondary side is more.
According to its principle, if it is a single transformer that has not been connected to the line, you can directly measure it with a multimeter, and measure the resistance of the primary side and the secondary side respectively.
If it is a transformer in series in the circuit, the primary side cannot be measured directly during the measurement, the voltage is high, the danger is increased, and the measurement cannot be performed; during the secondary side measurement, the circuit must not be opened. By measuring the voltage across the transformer, the current can be calculated to a resistance value, and then the primary resistance value is calculated using the primary ratio. This resistance value will have a little resistance value inside, so the actual resistance value should be appropriately lower than the resistance value measured in this way.
The actual value is still a single transformer, the most accurate value measured with a multimeter or other instrument, or with a constant DC current.