The partial discharge test is a non-destructive test item, and there are currently two types of test methods.

One is to use the power frequency withstand voltage as the pre-excitation voltage and drop to the partial discharge test voltage for a few minutes to measure the partial discharge. The other is to use Um as the pre-excitation voltage and drop to the partial discharge test voltage for 1 hour to measure the amount of partial discharge.

The latter is used by transformers. The pre-excitation voltage is to simulate the overvoltage during operation. The partial discharge excited by the pre-excited voltage should not be continued by the partial discharge test voltage. The concept is that the partial discharge excited by the overvoltage on the system will not be continued by the long-term working voltage This method is to make the transformer or transformer have no partial discharge under the long-term working voltage of Um/√3 to ensure the safe operation of the transformer, so that the partial discharge initial voltage and the partial discharge extinguishing voltage can be higher than Um/√3 .

Therefore, the insulation structure design of the transformer, the processing and processing of the insulating parts, the surface field strength of the charged and grounding electrode, and the field strength of the insulating medium must be considered to make the partial discharge less than the specified value. It cannot be based on whether the main and longitudinal insulation are discharged. based on.

When the power frequency withstand voltage is used as the pre-excitation voltage, the duration of the partial discharge test voltage is generally short, about 1 to 5 minutes. Extending the duration of the partial discharge test voltage is more severe to the insulation and sometimes causes destructive damage. As the pre-excitation voltage, the partial discharge test voltage lasts longer, and the standard requires 1 hour. How long it can withstand is related to the volt-second characteristics of the insulation structure.

The amount of partial discharge is generally related to the electric field strength on the surface of the grounding electrode, and has nothing to do with the frequency of the power supply. The background noise of the test site should be small, and the partial discharge of the power supply should be isolated.

In terms of the test sequence, the partial discharge test should be placed after all insulation tests. In terms of the type of test, one of the long-term induction partial discharge test or the short-term induction partial discharge test should be used as the transformer factory test. In terms of grade, the existing standard requires partial discharge test starting from Um≥252kV, and the revised IEC76-3 requires partial discharge test starting from Um≥126kV.