Abstract: This paper analyzes the common problems in laboratory calibration of current transformer errors: reverse polarity, wrong ratio, secondary open circuit, abnormal error display, etc. Corresponding analysis and solutions are given for these problems.

This paper analyzes the common problems in laboratory calibration of current transformer errors: reverse polarity, wrong ratio, secondary open circuit, abnormal error display, etc. And gives corresponding analysis and solutions to these problems.

introduction

Current transformers are mandatory products for verification under the Measurement Law. According to the verification regulations, current transformers for measurement must be verified every two years. Therefore, it is of great significance for the value transfer, traceability, and electric energy measurement of current transformers to verify the error of current transformers in laboratories and assess whether their errors are qualified. It is very necessary to analyze the common problems in the process of verifying the error of current transformers and propose corresponding solutions for the laboratories to correctly verify the current transformers and ensure the fairness, accuracy, and reliability of the verification results.

1. Verification method and circuit of current transformer

When the current sensor is calibrated in the laboratory, the current transformer under test is compared with the standard current transformer with the same current ratio. The current booster supplies the same primary current to the standard current transformer and the current transformer under test. The secondary current of the standard current transformer passes through the standard circuit of the transformer calibrator. The difference between the secondary currents of the current transformer under test and the standard current transformer flows into the differential circuit of the transformer calibrator. Then the error data is read out by the transformer and the calibrator.

The error ε of the current transformer is composed of the current error (i.e. ratio difference) f and the phase difference (i.e. angle difference) δ, namely:

Where: fp, δp are the ratio difference and phase difference read out by the current transformer tester; fx, δx are the ratio difference and phase difference of the current transformer under test; f0, δ0 are the ratio difference and phase difference of the standard current transformer.

The current transformer verification circuit is shown in Figure 1. In the figure, T1 is a voltage regulator, which is used to adjust the output voltage to adjust the output current. SL is a current booster, which cooperates with the voltage regulator to provide the current transformer CTx under test with a rated load. CT0 is a standard current transformer, and its current ratio is the same as that of the current transformer under test. HE is a transformer tester, which is used to test the ratio of the differential current to the secondary current phase, that is, the error of the current transformer under test relative to the standard current transformer.

Figure 1 Current transformer verification circuit.

2. Common problems in laboratory calibration of current transformers:

(1) Reverse polarity; (2) Wrong transformation ratio; (3) Secondary open circuit; (4) Abnormal error reading of the calibrator.

3 Problem Analysis

3.1 Polarity Reversal

When calibrating a current transformer, it is required to connect the polarity ends of the primary and secondary sides of the standard current transformer and the current transformer under test. Polarity reversal is caused by the reverse polarity of the primary or secondary of the standard current transformer or the current transformer under test. Generally, transformer testers have a polarity reversal alarm function. When the tester displays a polarity reversal alarm, just swap the polarity ends of the standard current transformer or the current transformer under test.

3. 2 Transformation ratio error

When calibrating a current transformer, the current ratio of the standard current transformer and the current transformer under test is required to be the same. If the ratio is wrong, it means that the current ratio of the standard current transformer and the current transformer under test is different. At this time, the reading of the transformer tester will exceed the error limit of the current transformer of the corresponding accuracy level. If the current ratio is greatly different, it may also exceed the display range of the transformer tester of the corresponding accuracy level. At this time, you should carefully check whether the current ratio of the current transformer under test is consistent with that of the standard current transformer, and ensure that the current ratio of the two is the same.

3. 3 Secondary open circuit

When calibrating the current transformer, the secondary circuit of the current transformer is strictly prohibited to be open. If the secondary circuit of the current transformer is open, the secondary circuit of the current transformer will generate high voltage, endangering personal safety. If the secondary circuit of the standard current transformer or the current transformer under test is open (or the secondary circuit is open) when calibrating the current transformer, the transformer calibrator will show that: although the output voltage of the voltage regulator increases rapidly, the verifier's dial meter increases slowly, generally not exceeding 5% of the rated current. At this time, the voltage regulator should be returned to zero, and the calibration circuit should be carefully checked to ensure that the wiring is correct.

3.4 The error reading of the calibrator is abnormal

Abnormal error readings of the calibrator can be manifested in the following two situations: (1) the reading is zero; (2) the reading exceeds the error limit of the accuracy level of the current transformer being tested.

For situation 1, it is usually because there is no differential current in the differential circuit of the calibrator, that is, there is no differential signal in the K and D circuits. At this time, you should check whether the differential circuit of the calibration line is short-circuited or open-circuited.

For situation 2, the various possibilities of this situation should be carefully analyzed.

(1) The transformer tester is out of tolerance.

When the transformer calibrator is out of tolerance, the transformer calibrator should be verified by the overall calibration device to ensure the transformer calibrator has a level 2 accuracy. Or use the following circuit diagram 2 for verification:

Figure 2: Error loop calibration circuit of transformer tester.

(2) The standard current transformer exceeds the tolerance.

The out-of-tolerance of the standard current transformer is generally caused by the burning of the compensation element, which causes the error of the standard current transformer to exceed its error limit. At this time, the self-calibration circuit of the current transformer can be used to verify whether its error is qualified. The circuit is shown in Figure 3, or another qualified standard current transformer can be used to calibrate to determine whether the standard current transformer is out of tolerance.

Figure 3 Current transformer self-calibration circuit.

(3) The impedance of the secondary circuit of the standard current transformer or the secondary circuit of the current transformer under test exceeds the tolerance.

When the secondary circuit resistance of the standard current transformer is out of tolerance, the error of the standard current transformer will increase. If the error of the standard current transformer is two levels lower than the accuracy level of the current transformer being measured, the error of the standard current transformer will be added to the error of the current transformer being measured. From equations (2) and (3), we can see that fx = fp + f0, δx = δp + δ0. At this time, f0 and δ0 cannot be ignored.

The increase of the secondary circuit impedance of the measured transformer can directly lead to the increase of the error of the measured transformer. At this time, the impedance measurement circuit of the secondary circuit of the current transformer is used to verify whether the secondary circuit impedance of the two is out of tolerance. As shown in Figure 4 and Figure 5.

Figure 4 Impedance measurement circuit of the secondary circuit of the current transformer under test.

Figure 5 Standard current transformer secondary circuit impedance measurement circuit.

4 Conclusion

The above analysis is about the problems that may be encountered when calibrating current transformers in the laboratory, and the corresponding solutions are proposed. In the daily calibration work, we should master the calibration principles and circuits of general current transformers. The commonly used transformer testers, standard current transformers, and current load boxes should be calibrated regularly to ensure the correctness of the calibration results.