PI test for transformers

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PI test for transformers

Postby Vilas Tamhane on May 11th, 2012, 7:17 am 

We were asked to take PI test (polarization test) on oil immersed power transformer winding. We got positive results for some but negative for other, though IR value was in G-Ohms. Is this test applicable? If not why not?
Vilas Tamhane
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Re: PI test for transformers

Postby CanadysPeak on May 11th, 2012, 8:28 am 

Vilas Tamhane wrote:We were asked to take PI test (polarization test) on oil immersed power transformer winding. We got positive results for some but negative for other, though IR value was in G-Ohms. Is this test applicable? If not why not?


I'm puzzled. Is your industry this far out of date? You're asking a question from thirty years back. Is this possibly a school question?

At any rate, are you familiar with IEEE 43-2000? If not, you need to come up to speed on those concerns.
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Re: PI test for transformers

Postby Vilas Tamhane on May 11th, 2012, 10:41 am 

CanadysPeak wrote:
Vilas Tamhane wrote:We were asked to take PI test (polarization test) on oil immersed power transformer winding. We got positive results for some but negative for other, though IR value was in G-Ohms. Is this test applicable? If not why not?


I'm puzzled. Is your industry this far out of date? You're asking a question from thirty years back. Is this possibly a school question?

At any rate, are you familiar with IEEE 43-2000? If not, you need to come up to speed on those concerns.


I would appreciate your direct answer. What could be wrong if the ratio is just aobut 1.05
Vilas Tamhane
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Re: PI test for transformers

Postby CanadysPeak on May 11th, 2012, 11:34 am 

Vilas Tamhane wrote:
CanadysPeak wrote:
Vilas Tamhane wrote:We were asked to take PI test (polarization test) on oil immersed power transformer winding. We got positive results for some but negative for other, though IR value was in G-Ohms. Is this test applicable? If not why not?


I'm puzzled. Is your industry this far out of date? You're asking a question from thirty years back. Is this possibly a school question?

At any rate, are you familiar with IEEE 43-2000? If not, you need to come up to speed on those concerns.


I would appreciate your direct answer. What could be wrong if the ratio is just aobut 1.05


Please read my previous answer.
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Re: PI test for transformers

Postby Vilas Tamhane on May 11th, 2012, 12:33 pm 

It is a new transformer but lying with us for a long period. We have filtered the oil. Insulation is new and so foreign matter contamination is most unlikely. Therefore I guess that insulation of the winding contains moisture. Hence the transformer will have to be dried in the oven.

This is a rectifier transformer with two LV windings. One is delta connected and the other is star. LV windings are disc windings and are wound on top of primary winding. PI ratio for HV and delta connected windings is about 2. There is a problem with star connected winding.
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Re: PI test for transformers

Postby CanadysPeak on May 11th, 2012, 2:28 pm 

If the unit has been lying about for many years, I'd suspect moisture as a first guess, but did you check the oil for oxidation?

Just heating the unit in an oven may not achieve the results you want. Have you considered pulling a vacuum on it?
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Re: PI test for transformers

Postby Vilas Tamhane on May 12th, 2012, 11:41 am 

CanadysPeak wrote:If the unit has been lying about for many years, I'd suspect moisture as a first guess, but did you check the oil for oxidation?

Just heating the unit in an oven may not achieve the results you want. Have you considered pulling a vacuum on it?


Many thanks for your reply. Engineers have dropped the idea of heating core and coil in the vacuum oven. Your doubt appears to be correct. There was more particulate matter in the oil than permitted. Oil is being filtered. They have shorted LV windings and rated current is being passed, while the oil is being filtered.
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Re: PI test for transformers

Postby CanadysPeak on May 12th, 2012, 7:20 pm 

Vilas Tamhane wrote:
CanadysPeak wrote:If the unit has been lying about for many years, I'd suspect moisture as a first guess, but did you check the oil for oxidation?

Just heating the unit in an oven may not achieve the results you want. Have you considered pulling a vacuum on it?


Many thanks for your reply. Engineers have dropped the idea of heating core and coil in the vacuum oven. Your doubt appears to be correct. There was more particulate matter in the oil than permitted. Oil is being filtered. They have shorted LV windings and rated current is being passed, while the oil is being filtered.


Vilas,

I welcome you here. I don't want to discourage you from posting, but this may not be the best forum for a question about transformers. I've hi-potted maybe a hundred transformers and those were small. You can find guys who have literally tested thousands on forums such as eng-tips.com or CR4.globalspec.com. Moreover, you'll find folks who live and breathe the IEEE specs. However, after you get info about your PI test, please come back and argue Physics. New voices are always good.
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