By hvhipot 
During the operation of power equipment, insulation materials gradually degrade under the effects of temperature, electric field, and mechanical vibration over a long period of time, including overall and local degradation, resulting in defects. For example, due to local electric field concentration or weak local insulation, there may be local defects. Various preventive testing methods can detect defects and reflect the condition of insulation materials, but the testing voltage of these methods is often lower than the working voltage of power equipment, which is not sufficient to ensure safe operation. Although the DC withstand voltage test has a high test voltage, there are also some insulation defects. However, due to the fact that the insulation of power equipment is mainly composed of dielectric materials, the voltage is distributed according to resistance under the action of DC voltage. Therefore, the weak link of AC power supply is that equipment under AC electric field does not necessarily need to be tested with DC electricity. For example, under DC conditions, slot defects in generators are difficult to detect. The AC withstand voltage test meets the electrical conditions of power equipment during operation, and the voltage of the AC withstand voltage test is generally higher than the operating voltage. Therefore, after testing, the device has a high level of security, making this type of testing an important means of ensuring safe operation.
However, due to the fact that the test voltage in the series resonance test is much higher than the test voltage during operation, excessive voltage can increase dielectric loss, heat generation, and discharge, accelerating the development of insulation defects. Therefore, in a sense, the series resonance test is a destructive test.
Before the series resonance test, various non-destructive tests such as insulation resistance, absorption ratio, dielectric loss factor, DC leakage current, etc. must be conducted in advance, and the test results must be comprehensively analyzed to determine whether the equipment is wet or defective. If any problems are found, they must be dealt with in advance. In order to avoid insulation breakdown, expand insulation defects, extend maintenance time, and increase maintenance workload during the testing process, it is necessary to eliminate defects before conducting series resonance tests.