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Shorting transformer windings for PI, why is it done?

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    Shorting transformer windings for PI, why is it done?

    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.

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    I've never seen that windings should be shorted for DC tests, and we never do.

    I could be mistaken, but that sounds like a misunderstanding of the requirement to short windings when doing AC tests like power factor or AC hipot. In those cases, you do it to null transforming effects.

    I agree with you, that's not a problem with static DC.

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    Quote Originally Posted by BigJohn View Post
    I've never seen that windings should be shorted for DC tests, and we never do.

    I could be mistaken, but that sounds like a misunderstanding of the requirement to short windings when doing AC tests like power factor or AC hipot. In those cases, you do it to null transforming effects.

    I agree with you, that's not a problem with static DC.

    I don't think its a misunderstanding, I've seen it in a lot of literature. This is the procedure Paul Gill lays out in his book:

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    I will post some more if I get a chance to dig through some megger manuals, I may have also seen it in "A stitch in time."

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    Winding Test

    Quote Originally Posted by SecondGen View Post
    I don't think its a misunderstanding, I've seen it in a lot of literature. This is the procedure Paul Gill lays out in his book:

    Click image for larger version. 

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    I will post some more if I get a chance to dig through some megger manuals, I may have also seen it in "A stitch in time."
    The tests are to measure resistance between the windings and ground, so would you not want to apply the test voltage equally across the device?

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    Quote Originally Posted by Relay1 View Post
    The tests are to measure resistance between the windings and ground, so would you not want to apply the test voltage equally across the device?
    Well, the argument against using jumpers has to do with the interconnections of the windings. Transformers are wired to where one winding jumps to another. If you look at a delta or why configuration, everything is already tied together. So some people have made the argument that you donít need to use jumpers when testing because they are already electrically continuous.

    But Paul gill and Megger and doble are not stupid people. Their procedure specifically spells out to jumper the windings. I just donít know the exact reason why

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    Quote Originally Posted by ElectricalTestTech View Post
    Well, the argument against using jumpers has to do with the interconnections of the windings. Transformers are wired to where one winding jumps to another. If you look at a delta or why configuration, everything is already tied together. So some people have made the argument that you donít need to use jumpers when testing because they are already electrically continuous.

    But Paul gill and Megger and doble are not stupid people. Their procedure specifically spells out to jumper the windings. I just donít know the exact reason why
    I am running a transformer resistance check, measuring High to Low winding. If I apply 10kV on bushing H1, what is the voltage at H2 and H3? By bonding the bushings together, I know 10kV is applied to the entire H winding. The same goes for bonding the X windings together.

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    Quote Originally Posted by Relay1 View Post
    I am running a transformer resistance check, measuring High to Low winding. If I apply 10kV on bushing H1, what is the voltage at H2 and H3? By bonding the bushings together, I know 10kV is applied to the entire H winding. The same goes for bonding the X windings together.
    I agree with that. But some can argue that a primary winding impedance of milliohms produces how much loss? Apply 10kv to H1 and read 9.999kv on h2 & h3. Donít get me wrong, Iím not disagreeing with you. But this is what was argued by others. I always apply jumpers because I want to apply the same voltage evenly to the windings.

    I just feel there is a more technical answer to why you jumper them other than being absolutely certain that voltage is applied evenly. I remember speaking to Paul Gil and he gave me a technical response. I just donít remember and I absolutely donít want to paraphrase and be incorrect

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    Quote Originally Posted by ElectricalTestTech View Post
    I agree with that. But some can argue that a primary winding impedance of milliohms produces how much loss? Apply 10kv to H1 and read 9.999kv on h2 & h3. Donít get me wrong, Iím not disagreeing with you. But this is what was argued by others. I always apply jumpers because I want to apply the same voltage evenly to the windings.

    I just feel there is a more technical answer to why you jumper them other than being absolutely certain that voltage is applied evenly. I remember speaking to Paul Gil and he gave me a technical response. I just donít remember and I absolutely donít want to paraphrase and be incorrect
    Hello ETT. Consistent is the term I would use. If you tested a device 5 years ago and I now have that task, if we are not being consistent in our connections and methods of testing, we may be comparing apples to oranges!

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    Quote Originally Posted by Relay1 View Post
    Hello ETT. Consistent is the term I would use. If you tested a device 5 years ago and I now have that task, if we are not being consistent in our connections and methods of testing, we may be comparing apples to oranges!
    That is another excellent point! But there still could be many other variables that wonít 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? Weíre 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, itís 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

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    Quote Originally Posted by ElectricalTestTech View Post
    That is another excellent point! But there still could be many other variables that wonít 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? Weíre 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, itís 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
    Sometimes the best we can do is follow our training and "Best-Practice Methodology" in testing equipment. We have no control over how it was done before, so we need to do all we can to insure our measurements can stand up to the scrutiny of others. Connections, temperature correction factors and weather all play a part in our accuracy.

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