Insulation resistance test on power transformer
Insulation resistance test on power transformer
(OP)
Hello guys. I have a question and hope somebody can answer me as i tried google on this question, but seems not able to find an answer for it.
The question is:
When we conduct insulation resistance test on power transformer (let said vector group is Dyn11) high voltage winding against ground, we can clip the insulation tester '+ve' lead to any terminal of transformer HV bushing (either L1, L2 or L3 because these three terminal are linked together inside the transformer tank.) while '-Ve' lead clip to ground.
The question is : what is the reason for linking all the HV bushing terminal, ie. L1 , L2 and L3 together during the insulation resistance test, as i saw some people doing it, but they also cannot answer me the reason behind.
The question is:
When we conduct insulation resistance test on power transformer (let said vector group is Dyn11) high voltage winding against ground, we can clip the insulation tester '+ve' lead to any terminal of transformer HV bushing (either L1, L2 or L3 because these three terminal are linked together inside the transformer tank.) while '-Ve' lead clip to ground.
The question is : what is the reason for linking all the HV bushing terminal, ie. L1 , L2 and L3 together during the insulation resistance test, as i saw some people doing it, but they also cannot answer me the reason behind.






RE: Insulation resistance test on power transformer
RE: Insulation resistance test on power transformer
RE: Insulation resistance test on power transformer
Are you checking the insulation resistance with a megger type test or are you checking the resistance of the windings with a ductor type test?
Bill
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"Why not the best?"
Jimmy Carter
RE: Insulation resistance test on power transformer
RE: Insulation resistance test on power transformer
JFPE
RE: Insulation resistance test on power transformer
Thanks,
Vijay
RE: Insulation resistance test on power transformer
Dear buzzp: I was using normal insulation resistance tester (kyoritsu 3123A)
Dear Waross: I am checking the insulation resistance with a megger type test
RE: Insulation resistance test on power transformer
RE: Insulation resistance test on power transformer
The standard requirements normally cover a broad spectrum of situation, as they have to cover all the scenarios. If the attached standard is required to be applied to a transformer with delta connection using external links. Consider such transformers by any chance if you have kept the links open, then shorting all the three windings makes sense.
I have come across some contractors, who read the standard word by word and try to find some loopholes to raise the variation orders. They must have prepared the standard considering such contractors. Nevertheless, there is no harm in shorting the windings externally in addition to the internal links. No doubt it is superfluous!
RE: Insulation resistance test on power transformer
For a Doble test, not shorting the windings may lead to awkward results.
RE: Insulation resistance test on power transformer
Hi Unclebo: Agreed with you, the doble test (power factor test) will give a awkward result if we not shorting the HV (or LV) bushing terminal together during the test because I had seen it personally when the power factor test was carried out with and without HV (or LV) bushing terminal been shorted together. Maybe due to this, so the insulation resiatance test required winding to be shorted during the test.
RE: Insulation resistance test on power transformer
1) IR of oil filled transformers vary with temperature.Approximately reduce by half for every 10 C rise in ambient.
2)IR of a P/3 MVA single phase unit will be 3 times the IR of a P MVA 3 phase unit.
3) IR with out oil will be 10-20 times of IR with oil
4) IR of HV -LVE will be higher than LV-HVE,almost double in generator transformers.
5) IR value changes with test voltage.