Testing transformers with a megger
Testing transformers with a megger
(OP)
Recently had a boss tell me to look over a procedure. he had us testing using a megger with red and black leads. We are asked to use the red lead to ground and the black lead to the power connection for each phase.
My recommendation was to switch the leads locations so the black was on ground and red for the power connections. Stating that altho it wouldnt change the test results it did enable good habits for any later tests, since we are not working with electricians doing these tests.
He came back saying a test was done in France on this. They actually proved that changing the leads so the black was on the power, or phase connections, and the red was on the ground connection gave a significantly better reading. it had to do with aligning the electrons and providing a secure field.
Has anyone heard of a test like this or any kind a validity to it?
My recommendation was to switch the leads locations so the black was on ground and red for the power connections. Stating that altho it wouldnt change the test results it did enable good habits for any later tests, since we are not working with electricians doing these tests.
He came back saying a test was done in France on this. They actually proved that changing the leads so the black was on the power, or phase connections, and the red was on the ground connection gave a significantly better reading. it had to do with aligning the electrons and providing a secure field.
Has anyone heard of a test like this or any kind a validity to it?





RE: Testing transformers with a megger
TTFN
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RE: Testing transformers with a megger
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Testing transformers with a megger
thread237-90352: megger test
One excerpt from the SD Myer's Transformer Maintenance Institute book posted in that thread was:
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RE: Testing transformers with a megger
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(2B)+(2B)' ?
RE: Testing transformers with a megger
I'd appreciate it if you would explain this one... how would the polarity of water affect measurement in one direction and not the other?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Testing transformers with a megger
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Note this survived beyond the draft and remains in the 2006 version of IEEE43.
Why does it happen? I lost my original reference, so I don't have any unique insights on that question. It does not seem unreasonable to me that the diffusion of a polar molecule can be influenced by an electric field (does that seem unreasonable to you... are you looking for atomic type explanation ?). I wouldn't have any idea of the timescale of the diffusion, but I gather it can be enough to affect results from the above quote and my vague memories of that older reference.
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RE: Testing transformers with a megger
If conductor is negative electrode (and tank/core is positive), then water collects directly around the conductor (configuration A)
If conductor is positive, (and tank/core is negative), then water tends to be move either toward the outside of the insulation (configuration B) or perhaps more uniformly distributed (configuration C) since the negative electrode / ground plane may be less well defined.
So A, B, C all represent different distribution of water within the insulation and we might expect they would all have different insulation resistance.
Any of those explanations seem reasonable to me, but I'm just offering why it seems plausible to me ....and apparently observed directly by other.
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RE: Testing transformers with a megger
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Testing transformers with a megger
ht
http://www
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(2B)+(2B)' ?
RE: Testing transformers with a megger
It might be possible that the presence of water causes dissolution of materials in the insulation that results in conductive ions being in solution with the water. These would result in a measurable reduction in resistance, and if they have work function differences with the electrode materials in the circuit, there might be a noticeable difference in resistance with reversed voltage.
TTFN
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RE: Testing transformers with a megger
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RE: Testing transformers with a megger
What do you think?
RE: Testing transformers with a megger
TTFN
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RE: Testing transformers with a megger
The atomic level is messing with us all the time. Hold a clip on a resistor lead on one side and the resistance goes up. Hold the other side and the resistance goes down. That thermoelectric effect is easily seen on any meter over 3 1/2 digits.
RE: Testing transformers with a megger
But I still fail to see how that would cause a difference in measurement from one direction to the other. Seems to me if it's applied in one direction for 5 seconds, then swapped around for another 5, the measurements should be the same.
Sorry guys, but it just doesn't make sense to me. Now I'm wondering if electroendosmosis is what is really to blame, or if someone chose the wrong word to describe what's going on.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Testing transformers with a megger
I think all the references including IEEE are consistent in suggesting that you get a lower reading when applying negative to the conductor and positive to ground. I suspect what may be a point of confusion may the word "normal polarity." in paragraph 3.3. IEEE is using the word normal polarity to mean negative to conductor. (See section 5.5: "Insulation resistance tests are usually conducted at constant direct voltages of 500–10,000 V having negative polarity")
That's up to you and your conscience. (I'd think the difference probably won't be big except for a very wet winding with paper or asphalt insulation). The logic of the test equipment industry's approach seems to be to always test it the same way so valid comparisons can be made. One could argue the acceptance criteria are based on the standard polarity (although really in most cases we shouldn't feel that much different about an insulation resistance reading 10% above an acceptance reading than 10% below... the acceptance criteria are somewhat aribtrary binary cutoff in a continuous world).
The rule was geared to testing to ground. If both windings are floating I'm not sure how you could decide which winding to play the role of ground and which one to play the role of conductor. I would tend to say it does not apply, or at least not worth worrying about for that special situation.
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(2B)+(2B)' ?
RE: Testing transformers with a megger
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(2B)+(2B)' ?
RE: Testing transformers with a megger
Thanks everyone I have the answers I needed this time around. I will certainly look up the information provided and pass on what I learn.
Jonathon
RE: Testing transformers with a megger
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If we learn from our mistakes I'm getting a great education!