AC withstand voltage test is the most effective and direct method to identify the insulation strength of power equipment.
During the operation of power equipment, the insulation will gradually deteriorate under the effect of electric field, temperature and mechanical vibration for a long time. It includes overall deterioration and local deterioration to form defects. For example, because the local electric field is relatively concentrated or the local insulation is relatively fragile, there are local defects. Each preventive test method has its own advantages. It can find some defects and reflect the insulation condition.
However, the test voltage of other test methods is often lower than the working voltage of power equipment, which is not enough to guarantee safe operation. Although the test voltage of DC withstand voltage test is relatively high, some insulation weaknesses can be found. Because the insulation of power equipment is mostly composite dielectric, and the voltage is distributed according to resistance under the action of DC voltage, the DC test may not be able to find the weaknesses of AC power equipment under AC electric field, for example, it is not easy to find the defects of generator under DC.
At the same time, the AC withstand voltage test voltage is generally higher than the working voltage. Therefore, after the test, the equipment has a large safety margin, so the test becomes an important means to ensure safe operation.
However, the test voltage used for AC withstand voltage test is much higher than the working voltage. Too high voltage will increase the loss, heat and discharge of insulation medium, and accelerate the development of insulation defects. Therefore, in a sense, AC withstand voltage test is a destructive test.
Before the AC withstand voltage test, various non-destructive tests must be carried out in advance, such as measuring the insulation resistance, absorption ratio, dielectric loss factor TG ￠, DC leakage current, etc., and comprehensively analyzing the test results to determine whether the equipment is damp or defective. If problems are found, they shall be handled in advance. The AC withstand voltage test can only be carried out after the defects are eliminated, so as to avoid insulation breakdown during the AC withstand voltage test, expand the insulation defects, extend the maintenance time and increase the maintenance workload.
