1. Partial discharge test At present, there are 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 amount of 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 over-voltage during operation. The partial discharge excited by the pre-excitation 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 field strength of the charged and grounded electrode surface, 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. When the power frequency withstand voltage is used as the pre-excitation voltage, the duration of the partial discharge test voltage is generally shorter, about 1 to 5 minutes. Extending the duration of the partial discharge test voltage is more severe to the insulation, sometimes causing destructive damage. When Um is used as the pre-excitation voltage, the partial discharge test voltage lasts longer. The standard requirement is 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 ground 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 test sequence, partial discharge test should be placed after all insulation tests, in terms of test type. One of the long-term induction zone partial discharge test or the short-term induction zone partial discharge test shall be used as the transformer factory test. In terms of the Um grade of the transformer, the existing standard requires a partial discharge test starting from Um≥252kV, and the revised IEC76-3 requires a partial discharge test starting from Um≥126kV. In terms of specific core structure, a transformer with a three-phase five-leg core structure cannot saturate the magnetic flux density in the upper and lower yokes during partial discharge tests. In terms of insulation structure, it should be able to withstand the requirements of the three-phase method for partial discharge test.

2. The cut wave impact test is generally a wave truncated by the wave tail, which can be cut off with IEC standard rod-shaped gaps, or with a multi-pole ignition cut-off device. When cutting with a multi-pole ignition cut-off device, a more accurate cut-off time can be obtained, and there is a problem with the cut-wave impact test results. It is not easy to judge whether it can pass the test from the difference of the cut-off time with the rod-shaped gap. When the cut-wave test voltage is 110 full-wave test voltage, if the cut-off time is less than or equal to 3μS, the two strengths are the same. For transformers connected with GIS, chopping test should be considered. The cut-wave test*** and the full-wave test alternate. Generally, negative polarity chopping is used.

3. Full-wave impulse test The IEC76-3 standard that is being revised has listed the full-wave impulse test as a factory test item for Um≥126kV transformers. For transformers to undergo sudden short-circuit test, a full-wave impact test shall be performed after the short-circuit test.

4. Operating wave test The IEC76-3 standard that is being revised has listed the operating wave test as a factory test item for Um≥252kV transformers. Since the phase-to-phase insulation of Um=252kV transformers without operating wave test is determined by the full-wave impulse test or the test of long-term induction with partial discharge measurement. When the operation wave test is to be performed, the phase-to-phase insulation size of the external air gap must be determined by the operation wave test voltage, which may be larger than the external air gap when the operation wave test is not assessed. From the above analysis, it can be seen that when evaluating the insulation performance of transformers according to the new IEC76-3 standard, many design principles must be reconsidered, the processing plan must be changed accordingly, and the test workload will be greatly increased.