What are the reasons for unqualified insulation of power equipment during withstand voltage test?

In the withstand voltage test, the possible reasons for unqualified insulation of power equipment are: deterioration of insulation performance. Such as moisture in transformer oil, moisture in solid insulation, insulation aging, etc. , will lead to the decline of insulation performance, and the withstand voltage test may be unqualified.

The test method and voltage measurement method are incorrect. For example, in the transformer test, the untested winding is not short-circuited to the ground, and the untested winding may discharge to the ground, which is misjudged as unqualified.

For another example, when testing a large-capacity sample, the voltage is still measured at the low-voltage side. Due to the volume rise effect, the voltage actually applied to the sample exceeds the test voltage, which leads to the breakdown of the sample and misjudges it as unqualified.

Extended data:

Ac withstand voltage test: the voltage of the tested equipment is more than 2.5 times. From the thermal breakdown angle of dielectric loss, local free defects and insulation aging weaknesses can be found effectively. Because the voltage under AC voltage is mainly divided by capacitor, it can effectively expose the insulation defects of equipment. However, AC withstand voltage is more destructive to insulation than DC, and because the test current is capacitive current, large-capacity test equipment is needed.

To sum up, DC withstand voltage test and power frequency AC withstand voltage test can effectively find insulation defects, but each has its own characteristics, and the two methods cannot replace each other. If necessary, they should be carried out simultaneously and complement each other.

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