Shorting transformer windings for PI, why is it done?

[test11]

I got stuck at this problem only because some guy in my team wanted to skip shorting windings of transformer. I was happy to the discussion here. I found some useful information a HV testing technique book which was published in 1970. The reason why we consider shorting winding on each side is that the impulse voltage which caused by IR/PI testing will create voltage differece and current between three phases, which might cause potectial damage to insulation or saftey hazards.Attachment 318

It states for rapidly changing voltages. This would definitely apply to power factor testing, but not a stable DC voltage after a few miliseconds.

[Primepower1]

It states for rapidly changing voltages. This would definitely apply to power factor testing, but not a stable DC voltage after a few miliseconds.

So, I agree, you do not need to short the X and H windings for a DC PI test, but it is very important to short them for an AC Power Factor Test.

[ibeam01]

This was an interesting read. I am new to the field, and we do not short the windings when doing a 10kV megger/PI test. Just one H to one X.

We do "jumper" the unused windings to ground when doing AC hi-pot tests.

I hadn't put much thought into the "WHY" though.

[mfndoc]

Had an interesting discussion on the job today about shorting together transformer windings when doing a PI test. Every technical manual I've come across tells you to do it but I can't seem to find a clear answer why.

I've asked around and get different answers. Some guys argue it reduces stress on the winding and others have said you do it to reduce induction, which opens up a whole new topic of discussion.

Personally, I do it because that's the way I was always taught, but I do see how you can argue that its unnecessary because transformer windings are normally connected together anyway. I also know that using jumpers can reduce your actual reading because I've seen it happen.

What do you normally do in the field? Looking for opinions on whether or not you use jumpers and why.

I would say that your megger has an AC power supply, no inverter makes a perfect DC signal, it is normally more of a ripple DC, which is enough to introduce an AC component, which would make inductive reactance a factor, all be it probably a small one, but a factor none the less.

[testguy]

Responses from our LinkedIn Page:

Frank says: "Been a while but back in the day we were taught that “shorting” the windings “ nulled out the resistance of the windings because you were only interested in the insulation resistance. Consider a single phase power transformer instead of a 3 phase internal connected. Megger instructions still have you short the single winding. Old sets even had a null switch somewhat like a Doble UST. With the windings shorted the test voltage is applied across the insulation and not the windings. Just a side note don’t ever try to measure the resistance of a winding with a AC DLRO just to test the theory. It can’t handle the field collapse and will let the smoke out while your hand is still on the test switch. Use a winding resistance set instead.
Happy Testing"

https://www.linkedin.com/feed/update...47695355905%29

Jeff says: "Glad to see this debate. I too short the windings because the guys who had been doing it longer than me taught me that way. Likewise, that's how I teach it at PHe Services but I do wonder. I've seen it done both ways. Given what we know about multiple resistors in parallel, I find it’s easy to affect the test results negatively using jumpers if your not careful."

https://www.linkedin.com/feed/update...19127896064%29

[backpack96]

Same potential at each point. BTW a PI on oil filled equipment is useless no matter what NETA states. In the instruction manual from Megger (Biddle was the expert on DC testing) states that a PI on an oil filled is useless. Dry type equipment such as motors, generators, dry type transformers, cables and etc. is a very good test to perform before applying a high voltage.

[cycle61alt]

Same potential at each point. BTW a PI on oil filled equipment is useless no matter what NETA states. In the instruction manual from Megger (Biddle was the expert on DC testing) states that a PI on an oil filled is useless. Dry type equipment such as motors, generators, dry type transformers, cables and etc. is a very good test to perform before applying a high voltage.

I was curious about this, and have performed PI tests on many, many oil filled transformers, so I went looking for a source. Found the following quote in Megger's "A Guide to Diagnostic Insulation Testing above 1KV"

It is also interesting to note that many people have tried
to use the PI test on oil-filled transformers and cannot
understand why a known good transformer gives them
results close to 1. The answer is simple. PI testing is not
appropriate for oil-filled transformers. The concept
depends on the relatively rigid structures of solid
insulating materials, where absorption energy is required
to reconfigure the electronic structure of comparatively
fixed molecules against the applied voltage field.
Because this process can go to a theoretical state of
completion (at “infinite time,” which obviously cannot
be achieved in the practical field, but can be reasonably
approximated), the result is a steady diminution of
current as molecules reach their “final” alignment.
Because the PI test is defined by this phenomenon, it
cannot be successfully applied to fluid materials since
the passage of test current through an oil-filled sample
creates convection currents that continually swirl the oil,
resulting in a chaotic lack of structure that opposes the
basic premise upon which the PI test rests.

[RedBrick]

I was curious about this, and have performed PI tests on many, many oil filled transformers, so I went looking for a source. Found the following quote in Megger's "A Guide to Diagnostic Insulation Testing above 1KV"

Also if you have access to IEEE C57.152 Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators and Reactors the following is said:

"The polarization index method should not be used to assess insulation condition in new power transformers."

"The polarization index for insulation liquid is always close to 1. Therefore, the polarization index for transformers with low conductivity liquids may be low in spite of good insulation condition."

[LHammermeister]

That is another excellent point! But there still could be many other variables that won’t allow you to compare apples to apples.

Humidity could be different causing lower readings. I have yet to see any correction factors for humidity. And I think we all have to agree that humidity is a major factor.

Was the transformer turned off the night before you arrived? Or the morning of? We’re there any delays in getting started with your work or mine for that fact? Temperature of windings may not have been properly documented on my part or yours.

But I do like your response. For some reason I am looking for a technical answer. But you could be 100% right with your responses. Everybody seems to look at technicality for reasoning. If you give a technical answer that makes sense, it’s easier for someone to buy into it. Rather than for you to give a basic response and people blow you off.

I want there to be a technical answer, but am afraid there might not be one.

Thanks for contributing to discussion. Not many people offer much feedback, more like a 1 response and done. Hopefully we can change that

Hello,

As is standard practice with transformer testing you always complete your AC tests before DC as with DC you will magnetize the transformer core in such a way that the AC tests will have problems. While we all know the an insulation resistance tests will have a very low current however there is still current flow through the windings to ground or the oppossite winding. I believe this is why you would short them all to ensure each winding is at the same potential.

A recent example of a megger causing me some greif was when we were completing maintenance transformer testing work. I was running the Doble PF set and an electrican of mine without telling me started doing Polarization index tests on the transformers ahead of me. I did not have an issue with PF(on any of the xfmrs) or excitation on the first transformer as he had not tested this one yet. However, when I went to complete excitation testing on the following transformers my numbers were way out of whack. After an hour of troubleshooting the worker told me that he had started testing ahead of me and was not jumpering the bushings of each bushing together (all highs together, x's together). I did notice every time I ran my excitation test the results would get slightly better on the winding under test. I confirmed with Doble that the xfmr was most likely magnetized unevenly for lack of a better term although the megger being such a low current doesn't typically cause magnetization issues.

Not quite a technical reason but a piece of recent field experience.

[RabbleRabble]

Had an interesting discussion on the job today about shorting together transformer windings when doing a PI test. Every technical manual I've come across tells you to do it but I can't seem to find a clear answer why.

I've asked around and get different answers. Some guys argue it reduces stress on the winding and others have said you do it to reduce induction, which opens up a whole new topic of discussion.

Personally, I do it because that's the way I was always taught, but I do see how you can argue that its unnecessary because transformer windings are normally connected together anyway. I also know that using jumpers can reduce your actual reading because I've seen it happen.

What do you normally do in the field? Looking for opinions on whether or not you use jumpers and why.

I would think this is more of an equipment and safety issue. Charging up a big inductor will create an impulse as the EMF collapses. Using jumpers to short the winding instead of making the tool eat that impulse. That’s just a guess, I never jumper The windings for a PI test.