What are the test methods and precautions for transformers？

The transformer AC withstand voltage test is a destructive test in the insulation test. This test must be carried out after other non-destructive tests (such as insulation resistance and absorption ratio test, DC leakage test, dielectric loss correction cut and insulating oil test) are qualified. After passing this test, the transformer can be put into operation.

The AC withstand voltage test is a key test. Therefore, the preventive test regulations stipulate that the transformer should be 10kV and below for 1 to 5 years, 66kV and below after major repairs, after the winding is replaced, and when necessary, the AC withstand voltage test must be carried out. Apply a sinusoidal power frequency AC test voltage that exceeds a certain multiple of the rated voltage and continue the test for 1 min. Its purpose is to use a test voltage higher than a certain multiple of the rated voltage to replace the atmospheric overvoltage and internal overvoltage to assess the insulation performance of the transformer. An effective method to identify the insulation strength of a transformer is also an important experimental item to ensure the safe operation of the transformer and prevent insulation accidents. The AC withstand voltage test can find the main insulation of the transformer damp and concentrated defects, such as the main insulation crack of the winding, loose winding displacement, The lead insulation distance is not enough, and there are defects such as dirt attached to the insulation.

Transformer test method

A) Test wiring

AC withstand voltage test wiring is applied outside the small and medium-sized power test transformers below 35kV. All windings should be tested. When testing the transformer, the lead wires of each phase winding should be shorted together, and the lead wires with lead wires at the neutral point should also be shorted together with the three phases.

Two) Test voltage

The handover test specification stipulates that transformers with a capacity of 8000kV or less and a winding rated voltage of 110kV or less shall be subjected to an AC withstand voltage test in accordance with the test voltage specifications listed in Appendix 1 of the specification.

The preventive test regulations stipulate: the test voltage value of oil-immersed transformer is detailed in the regulation table (regular test is according to the voltage value of partial winding). When all windings of dry-type test transformer are replaced, the value of the factory test voltage shall be used; for partial winding and regular test, press 0.85 times the value of the factory test voltage.

Precautions

In addition to the general AC withstand voltage test precautions, the following points should be noted according to the characteristics of the transformer:

1. The test transformer must be equipped with an over-current maintenance trip device.

2. The AC withstand voltage test of the three-phase transformer does not need to be carried out in phases. However, it is necessary to short-circuit all the lead wires of the three phases of the unified winding before performing the test, otherwise it will not only affect the accuracy of the test voltage, but may even endanger the main insulation of the transformer.

3. The preventive test regulations point out that for fully insulated transformers below 66kV, when the site conditions are not available, only the external construction frequency withstand voltage test can be performed.

4. For power transformers whose neutral point insulation is weaker than other parts or graded insulation, the above external AC withstand voltage test cannot be used, but an induction withstand voltage test of 1.3 times the rated voltage should be used.

5. It must be tested after being filled with qualified oil and resting for a certain period of time.

6. The voltage level is 35kV medium and small capacity transformers, allowing the test voltage to be measured on the low-voltage side of the test transformer. For power transformers with larger capacity, in order to make the measurement accurate and reliable, a voltage transformer or electrostatic voltmeter should be used on the high-voltage side. Directly measure the test voltage.

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