Insulation resistance test on power transformer 1

[Powerykh]

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.

 

[buzzp]

What kind of test equipment are you using for the test? The HV bushings are tied together in the transformer after the windings (assuming three phase transformer). When you test all of them hooked up, you are testing all of them at once. If you get a failure, you would have to test them separately to see which winding was bad. However, none of this may apply until I know what your using to do the test.

 

[prc]

I believe just to save time! Otherwise you have to measure for each phase. Then the IR you will be getting will be three times the value you get after shorting all phases,

 

[waross]

There may be some confusion here.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

I would guess the parallel connection reduces the chance of applying a high DC voltage across the winding and magnetizing the core.

 

[jfpe]

We don't jump H1/H2/H3 when performing insulation resistance testing on transformers. The windings are a dead short to DC and the test set (Megger MIT1025) ramps the applied voltage slow enough that I'm not worried about magnetizing the core.

JFPE

 

[vijaypandiyan]

No need to apply jumpers between phases. Anyways they are interconnected inside. If any phase has bad insulation, your IR value will be less.

Thanks,
Vijay

 

[Powerykh]

Hello guys, thanks for your reply. I attached a document regarding the requirement of linking all the Tx HV bushing terminal, ie. L1 , L2 and L3 together during the insulation resistance test.
Dear buzzp: I was using normal insulation resistance tester (kyoritsu 3123A)
Dear Waross: I am checking the insulation resistance with a megger type test

 

[ScottyUK]

It doesn't specify use of an external test link, just that they must be connected together. The star point fulfills that role more than adequately

 

[krisys]

Powerykh,

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!

 

[unclebob]

My guess is this test will evaluate to dielectric strength of the winding, and only that, even if it's open in some place. The ratio and winding res. tests will evaluate if a winding is open. So, open or not, the whole winding is tested with the megger test, if all the windings are shorted together.

For a Doble test, not shorting the windings may lead to awkward results.

 

[Powerykh]

Thank you ScottyUK and Krisys.
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.

 

[prc]

With doble test for power factor of bushings, the same voltage bushings must be shorted otherwise the capacitive current to other bushings will show a higher PF value.(maximum for middle phase bushings). There is a 1995 IEEE paper by Doble engineers explaining this phenomenon. This will not be true for IR as DC voltage is applied. I shall check this on some actual transformers to see any difference in measured values. The following facts may be of interest to transformer engineers.
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.