The Principle of low-frequency method for measuring the excitation characteristics of current transformers

What's the principle of low-frequency method for measuring the excitation characteristics of current transformers?

Excitation characteristic test is the most commonly used and effective method for detecting the performance of protective current transformers.

The composite error of current transformers can be measured through the excitation characteristic curve, the accurate limit coefficient ALF of P class and PR class transformers can be estimated, the secondary time constant Ts of transformers can be calculated, and the relationship curve between the accurate limit coefficient of transformers and secondary load can be drawn. These are important performance indicators of transformers, indicating the importance of current transformer excitation characteristic testing.

Current transformers used in high-capacity and high-voltage systems often have high excitation saturation voltages, some of which can reach over 20000V. There are two main problems in conducting excitation characteristic tests under traditional power frequency conditions:

1. Applying such a high voltage to the secondary winding cannot guarantee the safety of the test object and may damage the insulation between the secondary winding or secondary terminals.

2. Large capacity regulating and boosting equipment is required.

Since the purpose of the excitation test is to measure the magnetic performance of the iron core, it can be carried out at low frequencies. This is because the excitation impedance Zm=jwL is proportional to the frequency. Therefore, reducing the frequency can reduce the excitation impedance, so that the same excitation current can be obtained at lower excitation voltages. Then, the low-frequency measurement results can be converted to a voltage of 50Hz. Assuming that the excitation characteristic test is conducted at frequency f1, and the excitation current I1 and excitation voltage U1 are measured, then the magnetic flux linkage of the iron core at this time φ For:

φ=  (√ 2 * U1)/(2 π f1)

So the excitation voltage U at 50Hz is:

U=√ 2 * π f φ= U1f/f1

When using a low-frequency variable frequency power supply for the excitation characteristic test of current transformers, the test method is consistent with the conventional method. However, due to the relatively low frequency, the monitoring ammeter and voltmeter should be instruments based on the principle of measuring the average value. For example, a TP class current transformer with an excitation saturation voltage of 10000V at a power frequency of 50Hz. If the frequency is reduced and the test is conducted at 5Hz, the maximum excitation voltage only needs to be raised to around 1000V to complete the test. If the excitation current is 0.5A, the required test equipment capacity is only 500VA.

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