Taping of 11kV transformer stator bar 2

[forensiclab]

I have a situation where the failure of an 11kV transformer stator bar is being attributed to poor repair at a previous failure. Specifically it seems that the (repair) taping insulation was wound with a 10-20% overlay, instead of the manufacturer’s normal 50% overlay. I would appreciate any advice I could be given as follows.

1. Is it a cardinal rule that all insulation taping for high voltage applications should be carried out with a 50% overlay? If so, is there a specification for this or an authority that can be quoted.

2. Are compromised stator bars ever rehabilitated by manually retaping, or is it best to use new bars?

 

[ScottyUK]

Your post is confusing. Rotating machines have stator bars and rotor bars. Transformers have windings. Are we talking about a rotating machine - motor, generator - or a transformer? The design standards and operating conditions are completely different.

 

[forensiclab]

Sorry. Would like to blame secretary, but it was my mistake. It is a 11kV generator. Thanks Scotty

 

[electricpete]

9.2.2.8 Turn Insulation for 2.3 and 4.0 kV motors with ratings up to 3000 hp shall be
heavy enamel with a Dacron glass fiber overlay, or mica tape. 2.3 and 4.0 kV
motors rated above 3000 hp and all motors with voltage ratings of 6.0 kV and
above shall have dedicated mica tape turn insulation. For voltage ratings up to 7.0
kV this shall be a minimum of one 1/2-lap layer, or 2 butt-lap layers. For voltage
ratings above 7.0 kV this turn insulation shall be a minimum of two ½-lap, or 4
butt-lap layers. Turn insulation shall be reinforced at the coil knuckles. The
conductor stacks shall be fully consolidated in the straight sections of the coil
legs before application of the ground-wall insulation....

9.2.2.10 Coils with B-Stage (resin-rich) Groundwall Insulation - Coils are to be insulated
with necessary layers of ½-lap epoxy resin-rich mica tape supported by glass
cloth, polyester mat, or floc.....

9.2.2.12 Individual, or Global Vacuum Pressure Impregnation (VPI) Coil Insulation
The coils are to be insulated with necessary layers of ½-lap mica tape with
materials compatible with the VPI epoxy, or polyester resin to ensure proper
penetration and bonding....

I think 25% overlap on a given layer would be called "butt lapped". You can see this spec allows butt lapped on turn insulation, but number of "layers" required is twice as much... which sort of makes sense. A layer is one pass of taping... so a layer of 1/2 lap tape gives roughly two tape thicknesses everywhere, while a layer of 1/4 lap has about two thicknesses at half the locations and only one thickness in the other half of location ( a given tape piece is overlapped in the first and last 25% for total of 50% of its width).

You can also see this specification does not allow butt lapped for groundwall insulation. I can speculate the reason is to maximize the distance that must be traveled to get through the insulation travelling along edges of tape rather than through the tape.

I also know that OEM's place a lot of attention to how the "seam" lines up between one layer and the next. If you overlay a half-lap layer on top of another half-lap layer where the seems line up, you line up the weak points of one layer with the next. I think it is preferred to stagger by 1/4 width between layers.

I'm inclined to think it is more important for B-stage than for VPI, but don't know for sure. If I was investigating a failure blamed on this, I'd be interested to know about the post-rewind electrical testing. If winding was bad from the beginning, how did it pass the ac factory test?

There are probably others more knowledgeable than me. If you don't get many responses here you may want to ask on the motors forum.

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[electricpete]

Is it a cardinal rule that all insulation taping for high voltage applications should be carried out with a 50% overlay?

In the EPRI world... for groundwall insulation...YES. Anything else would not meet the above specification.

http://books.google.com/books?id=4-...e&q=butt-lapped groundwall insulation&f=false

Above figure shows half-lapped and butt-lapped. So I used the term butt-lapped wrong in my post above. I should have used the term edge-lapped (instead of butt-lapped) in my post above... edge-lapped would involve some small overlap like 10-20-25%

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[electricpete]

If I was investigating a failure blamed on this, I'd be interested to know about the post-rewind electrical testing. If winding was bad from the beginning, how did it pass the ac factory test?

I wasn't intending to suggest a conclusion based on this (many motors do pass ac factory hi-pot and later fail for reasons that can be traced to manufature). I was just pointing out that this is one (of many) aspect to look at during your investigation (what tests were specified and done... what were the results... what about later testing).

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[ScottyUK]

I can't quote a specific standard right now - I'll have a look at IEC 60034 when I get to work if I have time - but I agree with ePete that it is a 'cardinal rule' to achieve a minimum 50% overlay. Why do I say 'minimum'? At the endwindings where the bar takes on a more intricate shape the taping may approach 80-90% overlap.

A bit more detail might help. How big is this machine? Is the failure mode inter-strand, or bar-core in the slot, or even bar-bar in the endwinding region?

I have never seen a Roebel bar have inter-strand insulation repaired manually. I'm not even sure it would be possible even though the insulation demands are not high. Manual re-taping in the accessible endwinding region is not uncommon for machines where PD is a problem - this is usually done with the machine assembled as a 'patch' to keep the generator in service until a planned major outage when something more permanent can be arranged and budgeted for. The cost of a new bar is fairly small compared with the cost of removing a large generator and transporting it, then stripping it down. I wouldn't even consider re-taping a bar for an in-slot failure on a medium / large generator - it's too small an overall saving against a relatively large risk. 'Medium' for context being from about 20MW up to say 200MW.

 

[forensiclab]

To Scotty
Thanks for your comments. I would be grateful if you could if IEC 60034 does apply. The machine is 11kV, 200MW. The failure was in-slot.

 

[forensiclab]

To electricpete:
Appreciate the detail of your reply. In your second post you refer to a figure, but when I click on the link I am taken to a Google page featuring a book, Handbook of Electric Motors, by Hamid A. Toliyat, Gerald B. Kliman. I may well buy the book, but this will take time, so could you attach the figure that you refer to.

 

[ScottyUK]

IEC 60034 didn't turn up much in the way of useful detail. At least you didn't have to buy it to discover that.

You might want to locate a copy of "Electrical Insulation for Rotating Machines." by Stone, Boulter, Culbert, and Dhirani. ISBN 0-471-44506-1, pub. 2004 by Institute of Electrical and Electronics Engineers. It's a very detailed reference and I found it highly informative.

The fact that it is a pretty big machine further reinforces my comment that a bar repair is not likely to be economic sense: with consideration to the cost of unthreading the rotor to allow access to the stator, plus the cost of downtime, the cost differential between a repair to the bar and a new replacement bar will be a small fraction of overall costs.

 

[electricpete]

In your second post you refer to a figure, but when I click on the link I am taken to a Google page featuring a book, Handbook of Electric Motors, by Hamid A. Toliyat, Gerald B. Kliman. I may well buy the book, but this will take time, so could you attach the figure that you refer to.

Attached powerpoint slide 1 is figure from “Handbook of Electric Motors” by Toliyat, Kliman showing half-lap, butt-lap

Attached powerpoint slide 2, I created a powerpoint showing half-lap, butt-lap and edge-lap.

Edge lap is what you described in your orignal post.



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[forensiclab]

Many thanks to you two blokes. You have been of great help.