【Daily Question 20090320】What is the main purpose of the AC withstanding voltage test for electrical equipment?

To ensure the safe operation of power systems, it is necessary to assess the insulation condition of electrical equipment. AC withstand voltage tests are used to simulate the ability of these devices to withstand overvoltage during operation, thereby enabling the detection of insulation defects and weak points in the electrical equipment

What is the main purpose of the AC withstanding voltage test for electrical equipment? Answer: To ensure the safe operation of power systems, it is necessary to assess the insulation condition of electrical equipment. AC withstanding voltage tests are used to simulate the ability of these devices to withstand overvoltages during operation, thereby identifying insulation defects, weak points, moisture damage, cracks, and other issues. If it trips at the specified voltage withstand value, the test specimen shall be rejected and must not be put into operation.

To ensure the safe operation of power systems, it is necessary to assess the insulation condition of electrical equipment. AC withstanding voltage tests are used to simulate the equipment’s ability to withstand overvoltage during operation, thereby enabling the detection of insulation defects, weak points, cracks, and moisture damage in such equipment. The main purpose is to ensure the safe operation of the power system.

AC withstand voltage test: It is conducted at 2.5 times or more of the voltage of the equipment under test; from the perspective of thermal breakdown due to dielectric loss, it can effectively detect local partial discharge defects and weaknesses in insulation aging. Since voltage division mainly occurs via capacitance under alternating voltage, it is possible to effectively expose insulation defects in the equipment. However, the destructive effect of AC withstand voltage on insulation is greater than that of DC, and since the test current is a capacitive current, large-capacity testing equipment is required.

The power frequency alternating current withstand test is a test that evaluates the ability of electrical equipment’s insulation to withstand power frequency voltages. It is also used for electrical equipment of 220 kV and below to assess the insulation’s capacity to withstand switching overvoltages and transient overvoltages. During the test, the specimen is connected to the test circuit as specified, and the voltage is gradually increased to the rated power-frequency withstand voltage value specified by the standards; this voltage level is maintained for 1 minute, after which the voltage is rapidly and evenly reduced to zero. If the insulation of the test specimen does not experience breakdown or surface flashover within the specified time, it is considered to have passed this test. The voltage applied in the power-frequency AC withstanding voltage test is higher than the rated operating voltage of electrical equipment. This test can reveal many insulation defects, and it is particularly effective for detecting local defects. Its drawback is that it may cause some damage to the insulation during the test; therefore, the power-frequency AC withstanding voltage test should only be carried out after tests such as insulation resistance and dielectric loss factor have given satisfactory results.

The power frequency alternating current withstand test is a test that evaluates the ability of electrical equipment’s insulation to withstand the effects of power frequency voltage. It is also used to test the ability of electrical equipment of 220 kV and below to withstand operational overvoltages and transient overvoltages in terms of insulation. During the test, the specimen is connected to the testing circuit as specified, and the voltage is gradually increased to the rated power-frequency withstand voltage specified by the standards; this voltage level is maintained for 1 minute, after which the voltage is rapidly and evenly reduced to zero. If the insulation of the specimen does not experience breakdown or surface flashover within the specified time, then the specimen is considered to have passed this test. The voltage applied in the power-frequency AC test is higher than the rated operating voltage of electrical equipment; this test enables the detection of many insulation defects, and it is particularly effective for detecting local defects. Its drawback is that it may cause some damage to the insulation during the withstand voltage test. Therefore, the power-frequency AC withstand voltage test should only be carried out after tests such as insulation resistance and dielectric loss factor have given satisfactory results. The generation of power-frequency high voltages primarily relies on test transformers. The working principle of these transformers is similar to that of power transformers; they raise lower voltages to the desired high levels through electromagnetic coupling. Test transformers have high voltages, relatively small capacities, short operating times, and limited insulation safety margins. They require strict control over output waveform distortion and partial discharge. Test transformers are usually powered by voltage regulators in order to produce adjustable power-frequency high voltages. Before conducting the test, the capacity of the test transformer required should be estimated based on the capacitance of the test specimen and the test voltage. Due to limitations on volume and weight, the rated voltage of a single test transformer should not be too high. To obtain higher voltages, several transformers can be connected in series to form a series test transformer.

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