What is VLF ?
VLF is short for Very Low Frequency . It is normally 0.1Hz, even frequency as low as 0.01Hz is also called VLF.
What is VLF testing ?
VLF testing is a method to test Medium voltage and high voltage cable for verifying the cable insulation . VLF diagnostic testing is a withstand voltage test usually carried out at a frequency of between 0.1 Hz and 0.01 Hz rather than 50/60 Hz.
Why do choose VLF test equipment for cable ?
The common withstand voltage test for HV cables can be divided into AC withstand voltage test and DC withstand voltage test. At present, AC withstand voltage test mainly uses variable frequency resonance test system. As cable is a large capacity load, the corresponding series resonance test The capacity of the device is also large, such as some wind power lines, photovoltaic lines, and heavy series resonance equipment are difficult to transport, and the test is time-consuming and laborious. Therefore, when the test conditions are not allowed, we can use the ultra-low frequency high-voltage generator to withstand voltage on the cable. test. After all, because of the low test frequency of the ultra-low frequency high-voltage generator, the device capacity can be greatly reduced. Compared with series resonance, the device is much easier. So that you can choose a compact and lightweight VLF withstand voltage test system. VLF withstand voltage test system adopts very low frequency for testing . The capacity can be largely reduced and test system can be more lightweight .
How do you do a VLF Hipot Tester for cable ?
Generally, VLF hipot test system can be carried out the withstand voltage test for cable as per the bleow steps.
1. Firstly, unconnect VLF hipot tester to the test cable .
2. Use a 10kV megohmmeter to test the insulation resistance of each phase of test cable, and record the test value.
3. Peak test voltage: Umax=3Uo, Uo is the rated working voltage between the cable conductor and the ground or the metal shield. For example: the rated voltage is 10KV cable, the single-phase rated voltage Uo: Uo=10/√3kV, so the test voltage peak value is: Umax=3Uo=3×10/√3kV=√3×10kV=17.32kV 4. Test time: 3 minutes.
4. It can be tested by phase separation. When the capacitance value of the test cable is within the load capacity of the test equipment, the three-phase cores of the test cable can be connected in parallel, and then the withstand voltage test can be carried out at the same time.
5. After connecting VLF high voltage booster and the test cable according to the method shown in the figure above. You can turn on the power supply and set the test frequency, time and voltage, as well as the overcurrent protection value and overvoltage protection value of the high voltage side, and then start the increasing voltage test. During the boosting process, the high-voltage circuit should be payed close attention to veryify whether there is abonomal noise. When the test voltage is raised, the test time is recorded and the test voltage value is read.
6. After the test time ended, the instrument will automatically stop. If there is no destructive discharge occurs during the test, it is considered to pass the withstand voltage test.
7. During the increasing voltage of withstand voltage test, if the current increases abnormally, the voltage is unstable, the test cable has peculiar smell, smoke or abnormal sound or flashing, etc., the boosting should be stopped immediately, and the reason should be inspected after the shutdown. If these phenomena are caused by the weak insulation of the test cable, the withstand voltage test is considered to be unqualified. If it is determined that the test cable is caused by air humidity or surface contamination, etc., the test cable should be cleaned and dried before testing.
8. During the test, if the insulation defect of the non-tested cable causes the overcurrent protection of the instrument, after the cause is identified, the full-time connection withstand voltage test should be performed again. Only “make-up time” trials shall not be performed
During the test, if the overcurrent protection of instrument is caused by non-insulation defect of test cable . it need to perform the full time test of withstand voltage test after finding out the reason.