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 action of electric field, temperature and mechanical vibration for a long time. It includes overall deterioration and partial deterioration, forming 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, and can find some defects respectively, reflecting the insulation condition. However, the test voltage of other test methods is often lower than the working voltage of power equipment, which is not strong enough as a guarantee for safe operation. Although the test voltage of DC withstand voltage test is relatively high, some insulation weaknesses can be found, because most of the insulation of power equipment are combined dielectrics, and the voltage is distributed according to resistance under the action of DC voltage, the weaknesses of AC power equipment under AC electric field may not be found by DC test, For example, the slot defect of generator is not easy to be found under DC. The AC withstand voltage test conforms to the electrical conditions borne by the power equipment during operation. At the same time, the AC withstand voltage test voltage is generally higher than the operating voltage. Therefore, after passing the test, the equipment has a large safety margin, so this test has become an important means to ensure safe operation.
However, the test voltage used in the AC withstand voltage test is much higher than the operating voltage. Too high voltage will increase the loss of insulation medium, heat and discharge, and accelerate the development of insulation defects. Therefore, in a sense, the AC withstand voltage test is a destructive test.
Before the AC voltage withstand 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 contains defects. If any existing problem is found, it shall be handled in advance. The AC withstand voltage test can be carried out only after the defect is eliminated, so as to avoid insulation breakdown, expand insulation defects, prolong maintenance time and increase maintenance workload during the AC withstand voltage test.