The AC withstand voltage test is the most effective and direct method for identifying the insulation strength of power equipment.
During the operation of power equipment, insulation will gradually deteriorate over a long period of time under the influence of electric fields, temperature, and mechanical vibrations. It includes both overall degradation and local degradation to form defects. For example, due to relatively concentrated local electric fields or relatively fragile local insulation, there are local defects. Each preventive testing method has its own advantages. It can detect some defects and reflect insulation conditions.
However, the testing voltage of other testing methods is often lower than the operating voltage of power equipment, which is not sufficient to ensure safe operation. Although the test voltage for DC withstand voltage testing is relatively high, some insulation weaknesses can be identified. Due to the fact that the insulation of power equipment is mostly composed of composite dielectrics, under the action of DC voltage, the voltage is distributed according to the resistance. Therefore, DC testing may not be able to detect the weak points of AC equipment under AC electric fields, such as defects in generators under DC electric fields.
Meanwhile, the AC withstand voltage test voltage is generally higher than the operating voltage. Therefore, after testing, the equipment has a large safety margin, making testing an important means to ensure safe operation.
However, the test voltage used for AC withstand voltage testing is much higher than the operating voltage. Excessive voltage can increase the loss, heat, and discharge of insulation media, accelerating the development of insulation defects. Therefore, in a sense, the AC withstand voltage test is a destructive test.
Before conducting the AC withstand voltage test, various non-destructive tests must be carried out in advance, such as measuring insulation resistance, absorption ratio, dielectric loss factor TG ￠, DC leakage current, etc. The test results should be comprehensively analyzed to determine whether the equipment is damp or defective. If problems are discovered, they should be dealt with in advance. The AC withstand voltage test can only be conducted after the defects have been eliminated to avoid insulation breakdown, expansion of insulation defects, extension of maintenance time, and increase in maintenance workload during the AC withstand voltage test process.
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