VT pressure test
VT pressure test
(OP)
I have a 22KV 3 phase VT with the star point going to a small bushing before being Earthed. I need to do a pressure test on the VT . It has been some years since I have done this and cannot remember how. I seem to remeber leaveing the earth conected and just putting the HV on the 3 bushings shorted together with the secondary shorted and earthed but cannot remember what value we used . If the star point is not earth it will flash over anyhow at not to high a voltage . I would appreciate any advice.






RE: VT pressure test
A. HV test:
1. Short all the primary terminals together including nuetral (remove the neutral from the ground).
2.Short all the secondary terminals and ground.
3. As per IEC, for 24kV rated equipment, power frequency withstand voltage is 50kV.
4. Apply 75% of 50kV i.e. 37.5kV for 1 minute (field test)
5. If there is no disruptive discharge, then VT is passed.
B. Ratio Test
Apply rated secondary voltage on secondary winding while primary is open circuit.
Increase voltage 1.5 times of rated sec. voltage.
RE: VT pressure test
RE: VT pressure test
A is an external source voltage test which will require a hipot test kit. During this test, you isolate the primary neutral lead from the bushing and apply the test voltage between the HV windings and the VT tank and secondary windings. This tests only the ground insulation of HV winding.
B. is an induced overvoltage test for which HV neutral need not be isolated and a hipot kit is not needed. This tests the ground as well as the turn insulation of both HV & LV windings. However, because of core saturation at above rated voltage, you will need a test source of hgiher frequency (say at twice the rated voltage, you will need twice the rated frequency for the input voltage).
RE: VT pressure test