During the load operation of the generator, the insulation will gradually deteriorate under the action of electric field, temperature and mechanical vibration for a long time, including overall deterioration and partial deterioration, resulting in defects. The withstand voltage test of generators is an effective and direct method to identify the insulation strength of generators, and it is an important content of preventive tests. Therefore, Hipot test is also an important means to ensure the safe operation of the generator.

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? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?HV Hipot GDVLF Series 0.1Hz Programmable Ultra-low Frequency(VLF) High Voltage Generator

The operation method of the ultra-low frequency withstand voltage test for the generator is similar to the operation method for the cable above. The following is a supplementary explanation of the different places.??
1. This test can be carried out during handover, overhaul, partial replacement of windings and routine tests. The withstand voltage test of the motor with 0.1Hz ultra-low frequency is more effective for the insulation defect of the generator end than the power frequency withstand voltage test. Under the power frequency voltage, since the capacitive current flowing from the wire rod causes a large voltage drop when it flows through the semiconductor anti-corona layer outside the insulation, the voltage on the insulation of the wire rod at the end is reduced; In the case of ultra-low frequency, the capacitor current is greatly reduced, and the voltage drop on the semiconductor anti-corona layer is also greatly reduced, so the voltage on the end insulation is higher, which is easy to find defects.??
2. Connection method: The test should be carried out in phases, the tested phase is pressurized, and the non-tested phase is short-circuited to ground.??
3. According to the requirements of the relevant regulations, the peak value of the test voltage can be determined according to the following formula:

Umax=√2βKUo In the formula, Umax: is the peak value of the 0.1Hz test voltage (kV) K: usually takes 1.3 to 1.5, generally takes 1.5

Uo: the rated value of the generator stator winding Voltage (kV)

β: Equivalent coefficient of 0.1Hz and 50Hz voltage, according to the requirements of our country’s regulations, take 1.2

? For example: for a generator with a rated voltage of 13.8kV, the calculation method of the test voltage peak value of ultra-low frequency is: Umax=√2× 1.2×1.5×13.8≈35.1(kV)
4. The test time is carried out according to the relevant regulations
5. In the process of withstand voltage, if there is no abnormal sound, smell, smoke and unstable data display, it can be considered that the insulation has withstood the test of the test. In order to better understand the insulation situation, the surface state of the insulation should be monitored as comprehensively as possible, especially for air-cooled units. Experience has pointed out that appearance monitoring can find abnormal generator insulation phenomena that cannot be reflected by the instrument, such as surface corona, discharge, etc.