# High voltage transformer winding repair



## CAUSA (Apr 3, 2013)

John b85 said:


> Can high voltage dry type transformer winding be repaired without rewinding the whole transformer if it’s the very outer winding


Only the manufacturer can really answer that question properly.

On thing to have it repaired. Another thing to have it tested. And another thing to put in to service….

Like did it get dented and the varnish is scraped?

Or did it fail.?


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## LGLS (Nov 10, 2007)

John b85 said:


> Can high voltage dry type transformer winding be repaired without rewinding the whole transformer if it’s the very outer winding


That's one big fat NO.


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## joe-nwt (Mar 28, 2019)

John b85 said:


> Can high voltage dry type transformer winding be repaired without rewinding the whole transformer if it’s the very outer winding


Sure! DIYers do it all the time!


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## SWDweller (Dec 9, 2020)

Why would you even ask such a question. If you are considering patching an high voltage transformer > 65K volts your crazy.

Just so we understand each other

High (HV), Extra- High (EHV) & Ultra-High Voltages (UHV) - 115,000 to 1,100,000 VAC.
Medium Voltage (MV) - 2,400 to 69,000 VAC.
Low Voltage (LV) - 240 to 600 VAC.


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## John Valdes (May 17, 2007)

While I agree trying to repair this is not a good idea, you could make a few calls and see if its possible. You may not have to replace the complete xfmr as the coils are separated and maybe all they have to do is repair the bad one.
I would call the manufacturer first. There are xfmr repair replacement shops too. Give these guys a call. If they cannot help you, they can point you in the right direction. 
Jenkins Electric | Electrical, Mechanical Repair & Machining I used them for custom xfmr's in the past. They sell and repair.


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## SWDweller (Dec 9, 2020)

T&R in South Dakota or SUNBELT SOLOMON SERVICES, Kansas. Used to give better warranties on rebuilds than you could get new. I once had a transformer that I waiting for a rebuild was going to take to long. Talking to my buddy at Sunbelt, Jed found out they had just finished one that would work for my need. Contacted the customer and made a deal. They shipped the rebuilt one and picked up the old one. Everyone was happy except for my 20 man crew that had to disconnect and reconnect in less than a week. This was 115kv to 69 kv. Got yelled at by the big boss for give the guys a 4 day weekend when the job was done. That was ok every one in the shop wanted to work on my projects.


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## John b85 (4 mo ago)

CAUSA said:


> Only the manufacturer can really answer that question properly.
> 
> On thing to have it repaired. Another thing to have it tested. And another thing to put in to service….
> 
> ...


 4 inches of the very outer winding got slightly dented and a little bit of the varnish and tape came off. This transformer is 25kv (7.65kv per phase)


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## John b85 (4 mo ago)

SWDweller said:


> Why would you even ask such a question. If you are considering patching an high voltage transformer > 65K volts your crazy.
> 
> Just so we understand each other
> 
> ...


Sorry it’s medium voltage 25kv (7.65 kv per phase)


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## kb1jb1 (Nov 11, 2017)

SWDweller said:


> Why would you even ask such a question. If you are considering patching an high voltage transformer > 65K volts your crazy.
> 
> Just so we understand each other
> 
> ...


What many call high voltage is really low voltage. I try to go by context rather than terminology. I bet 90% of electricians call 480/277 high voltage. Being that the OP asked the question, I doubt he works with 5000 volts or above. I commend him for asking because I was in a situation 30 years ago where we needed one on a Saturday night. There was a damaged one on site but we opted to wait until we could get a new one. What is 12 volts considered?


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## jonthefellow (Sep 3, 2021)

0-240VAC: hardly voltage


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## RUSKES (6 mo ago)

John b85 said:


> Can a dry type transformer winding be repaired without rewinding the whole transformer. 4 inches of the very outer winding got slightly dented and a little bit of the varnish and tape came off. This transformer is 25kv (7.65kv per phase)


AFIK ,it has to be one continuous wire


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## Southeast Power (Jan 18, 2009)

SWDweller said:


> T&R in South Dakota or SUNBELT SOLOMON SERVICES, Kansas. Used to give better warranties on rebuilds than you could get new. I once had a transformer that I waiting for a rebuild was going to take to long. Talking to my buddy at Sunbelt, Jed found out they had just finished one that would work for my need. Contacted the customer and made a deal. They shipped the rebuilt one and picked up the old one. Everyone was happy except for my 20 man crew that had to disconnect and reconnect in less than a week. This was 115kv to 69 kv. Got yelled at by the big boss for give the guys a 4 day weekend when the job was done. That was ok every one in the shop wanted to work on my projects.


I remember the last 13.2/ 13.8kv -480 volt transformer we changed cost more to ship and handle than it was worth to ship, repair, and send back. We had a transfer company move it in and out of place. It was about 7' tall, very heavy but, at the time, it was a stock item.

Ah, there is one on Ebay: Siemens 3 Phase Dry Transformer 3000/4000 KVA 13800-480 Volt DELTA | eBay


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## Slay301 (Apr 23, 2018)

John b85 said:


> 4 inches of the very outer winding got slightly dented and a little bit of the varnish and tape came off. This transformer is 25kv (7.65kv per phase)


sheeee dented how bad my guy ? Stand back and flip the switch and see what happens on that deal


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## John b85 (4 mo ago)

Slay301 said:


> sheeee dented how bad my guy ? Stand back and flip the switch and see what happens on that deal


I think that’s what is going to end up happening. Hard to tell unless I take off the varnish / tape.


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## Slay301 (Apr 23, 2018)

John b85 said:


> I think that’s what is going to end up happening. Hard to tell unless I take off the varnish / tape.


I wouldn’t. I’d see what happens with it you know u are already gonna need a new one buy that one off eBay for a backup

*Heroes Get Remembered, but Legends Never Die.* *Follow Your Heart, Kid, and You'll Never Go Wrong.*


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## micromind (Aug 11, 2007)

The thing with this transformer is even though it's 25KV, the voltage from one coil turn to another is not 25,000 volts. It's a couple hundred at most. 

If the bad spot looks like it can handle a few hundred volts to the adjacent windings, it'll be fine.


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## paulengr (Oct 8, 2017)

CAUSA said:


> Only the manufacturer can really answer that question properly.
> 
> On thing to have it repaired. Another thing to have it tested. And another thing to put in to service….
> 
> ...


BS from someone that doesn’t know how to do it/

It depends on the extent of the damage: if it passes standard transformer tests a little epoxy to clean up the damage is all you need.

Rewinding is usually only done on very large or unusual transformers where lead time is an issue. Removing the varnish or epoxy means heating to 550-650 at for several hours to burn it off. You can’t realistically do a small area.


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## joe-nwt (Mar 28, 2019)

John b85 said:


> I think that’s what is going to end up happening. Hard to tell unless I take off the varnish / tape.





paulengr said:


> It depends on the extent of the damage: if it passes standard transformer tests a little epoxy to clean up the damage is all you need.


Skipping this step has the potential to turn a relatively inexpensive repair into a very expensive repair.


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## CAUSA (Apr 3, 2013)

paulengr said:


> BS from someone that doesn’t know how to do it/
> 
> It depends on the extent of the damage: if it passes standard transformer tests a little epoxy to clean up the damage is all you need.
> 
> Rewinding is usually only done on very large or unusual transformers where lead time is an issue. Removing the varnish or epoxy means heating to 550-650 at for several hours to burn it off. You can’t realistically do a small area.


So, what part is BS?

Be specific….:

They are pretty general statements. So since your talking from a position of authority.

And let’s see. Once you modify/repair electrical equipment in Canada. It has to haRecertification Documentation. Depending on the provincial AHJ.

caution: bias sarcasm below…
I can go on but you should continue for me. Since you know from a position of authority.


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## paulengr (Oct 8, 2017)

Standard tests are Megger, TTR (turns ratio), ohms/inductance or tan delta, There are other more advanced tests but these tests look specifically for insulation and wiring damage. These are the same tests specified in NFPA 70B and NETA MTS. Nothing special except the TTR and ohms/tan delta test equipment costs several thousand and not something most electricians have. The TTR for instance gives you a very accurate turns ratio. Works great on 3 phase. On single phase it would be hard to tell anything except compared to name plate. Or just do a smoke test (run it…pass/fail)!and check temperature. This is one of those situations where it’s often hard to tell.

We have repaired a few when it’s really just 1 or 2 turns and easily accessible even if we had to crimp or TiG the wiring then reinsulate the repairs for moisture protection. If it gets to the point where it’s more than that though you are looking at rewinding. If you feel confident just get a couple of the year off packs of epoxy that comes with the popsicle stick. Mix it up on cardboard and smear a THIN coating on the wiring once the epoxy gets a little thick.


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## John b85 (4 mo ago)

paulengr said:


> Standard tests are Megger, TTR (turns ratio), ohms/inductance or tan delta, There are other more advanced tests but these tests look specifically for insulation and wiring damage. These are the same tests specified in NFPA 70B and NETA MTS. Nothing special except the TTR and ohms/tan delta test equipment costs several thousand and not something most electricians have. The TTR for instance gives you a very accurate turns ratio. Works great on 3 phase. On single phase it would be hard to tell anything except compared to name plate. Or just do a smoke test (run it…pass/fail)!and check temperature. This is one of those situations where it’s often hard to tell.
> 
> We have repaired a few when it’s really just 1 or 2 turns and easily accessible even if we had to crimp or TiG the wiring then reinsulate the repairs for moisture protection. If it gets to the point where it’s more than that though you are looking at rewinding. If you feel confident just get a couple of the year off packs of epoxy that comes with the popsicle stick. Mix it up on cardboard and smear a THIN coating on the wiring once the epoxy gets a little thick.


This is what it looks like


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## paulengr (Oct 8, 2017)

That doesn't look bad at all. The tape is just glass tape. Slice it open with a utility knife and check what the damage is, if anything. It looks like it shouldn't be a problem. Then rewrap tightly with glass tape and coat everything in epoxy. Getting glass tape started can be tricky...if it doesn't stick you can use a piece of vinyl tape (88/33+) or even just tie a knot in it back along where you have good insulation.

You can buy glass tape and epoxy yourself from EIS Sales or even McMaster-Carr. Or if you know of a local transformer shop or even a local motor shop they can probably sell you/give you the stuff very inexpensively since they buy it in bulk. Those bundles are groups of parallel flat wires...it will look like "ribbons". Once you verify that nothing has been seriously damaged then just reinsulate. Even if the insulation is damaged you can take strips of glass tape and lay between them then wrap the whole bundle. I'm probably giving away the shop secrets but once in a while things like this happen and every shop will make minor repairs as long as it doesn't compromise the product.


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## joe-nwt (Mar 28, 2019)

What the heck happened there? The frayed tape second row in on the top damaged row looks like there might be an interesting story there.


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## paulengr (Oct 8, 2017)

joe-nwt said:


> What the heck happened there? The frayed tape second row in on the top damaged row looks like there might be an interesting story there.


Glass tape doesn’t stick all that well,,,anything rubbing on it does that.


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## John b85 (4 mo ago)

paulengr said:


> That doesn't look bad at all. The tape is just glass tape. Slice it open with a utility knife and check what the damage is, if anything. It looks like it shouldn't be a problem. Then rewrap tightly with glass tape and coat everything in epoxy. Getting glass tape started can be tricky...if it doesn't stick you can use a piece of vinyl tape (88/33+) or even just tie a knot in it back along where you have good insulation.
> 
> You can buy glass tape and epoxy yourself from EIS Sales or even McMaster-Carr. Or if you know of a local transformer shop or even a local motor shop they can probably sell you/give you the stuff very inexpensively since they buy it in bulk. Those bundles are groups of parallel flat wires...it will look like "ribbons". Once you verify that nothing has been seriously damaged then just reinsulate. Even if the insulation is damaged you can take strips of glass tape and lay between them then wrap the whole bundle. I'm probably giving away the shop secrets but once in a while things like this happen and every shop will make minor repairs as long as it doesn't compromise the product.


Would a glass cloth electrical tape like this work: https://www.amazon.ca/3M-Glass-Clot...=1663631309&sprefix=glass+tape,aps,150&sr=8-4
or a ACRYLIC GLASS FILAMENT TAPE like this be better 


https://www.itape.com/-/media/files/tds/elec/elec_4238_tds.pdf


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## joe-nwt (Mar 28, 2019)

paulengr said:


> Glass tape doesn’t stick all that well,,,anything rubbing on it does that.


The first layer doesn't look too bad. I was wondering how the second layer in got frayed.


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## LGLS (Nov 10, 2007)




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## just the cowboy (Sep 4, 2013)

John b85 said:


> Can a dry type transformer winding be repaired without rewinding the whole transformer. 4 inches of the very outer winding got slightly dented and a little bit of the varnish and tape came off. This transformer is 25kv (7.65kv per phase)


First question I have is who are you, you did not fill out your profile are you a Contractor, Owner of equipment, Project manager?
The reason is each one will have a different reason for asking this question.
As the owner/customer my answer is no, not worth the risk.
As the PM my answer is no, the delay is better than the risks.
As the EC my answer is hell no, if it has an issue later on not worth the risk of you being on the hook for lost down time once building is running.

Even if it tests and powers up now once it start vibrating later it may wear thru or after it is fully loaded and heats up it may fail.

Your choice but whoever is footing the bill is paying for NEW equipment, not a repaired piece.

Cowboy


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