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VT pressure test

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

You can test VT by two methods:
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

(OP)
Exactly what I was planning to do but the neutral is only a stub bushing and will only take around 10 to 12 kv .

RE: VT pressure test

About 040358 tests

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

Or, if you can generate at a higher frequency, i.e. 120-150 Hz or so, you can apply the PFWV to the HV winding with the neutral terminal grounded and the secondaries open. Although, this is typically only done in the factory, as gen-sets are normally not available in the field.

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