generator bar material between conductor and groundwall 1

[electricpete]

Attached is a photo of a stator bar cross section from a 1000+MW 25KV Westinghouse hydrogen/water-cooled generator with Thermalastic insulation system. This is a normal healthy bar cut apart for inspection long ago when machine was wound.

What is the white material between copper and groundwall tape on top and bottom ?


=====================================
(2B)+(2B)' ?

 

[desertfox]

Hi electricpete

Could it be some kind of adhesive or sealant?

desertfox

 

[ScottyUK]

I may be wrong - I seem to be making a habit of it today - but I think it is likely to be an epoxy resin. Doesn't Thermalastic use at least two stages of VPI resin impregnation to ensure very low voiding?

It would be virtually impossible to get a mica tape to conform tightly to the shape of the composite bar, especially at the bottom where it is concave, so without the resin there would be a significant void which would result in a loss of mechanical strength and perhaps allow movement or vibration of the individual strands within the composite bar.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Thanks.

The Thermalstic insulation system definitely includes epoxy resin and that was my thought also. However, look at the shape of the boundary between the white material and tape at top... it is very straight and regular. I'm trying to imagine a process that would result in resin fill in this very regular shape and I can't.





=====================================
(2B)+(2B)' ?

 

[ScottyUK]

The only (possible) explanation I can offer is that mica tape is fairly stiff and adopts that shape of its own accord as the composite bar is wrapped. Assuming that to be the case, the epoxy just fills whatever void is left. Is it possible that the coil is formed in a 'mould' of some type?

I think edison123 might be well-placed to comment on this as he has a rewind shop: even if he doesn't use the exact process he's a lot more familiar than I am with the materials and how they behave as they are worked.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Maybe it has something to do with irregular profile of conductors at top/bottom due to the Roebel twist? It still leaves a question of how they build it that way.

=====================================
(2B)+(2B)' ?

 

[jpts]

Maybe electricpete is right, in Roebel-bars top- and bottom surfaces are filled with mastic material to smoothen the surface and remove the voids

Figure 6:

http://www.vonroll.ch/downloads/CPIRHVM.pdf

Figure 1:

http://www.vonroll.com/downloads/NMTPCRB.pdf

 

[ScottyUK]

Not impossible, although whether a mastic would be able to wick in between the strands to the extent that the white material has managed is open to question. That was part of the reasoning for suggesting it was an epoxy impregnant.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Thanks for all the thoughts and comments. I haven't heard of that mastic before.

whether a mastic would be able to wick in between the strands to the extent that the white material has managed is open to question.

Yes good question. I'm guessing the mastic would be applied and the coil consolidated in a press before taping, which could push the mastic deep inside? I doubt that would be a desired feature since it would seem to inhibit vpi penetration into the inter-strand area which leaves the possibility of undesired voids (although not as undesirable in this interstrand area as in the groundwall)

I do have the coil section available here at my desk. With my fingernail I thought the large white-material area at top/bottom felt softer than the resin-impregnated tape area. However it may have been my imagination: scratching a scissor point accross the coil created a similar scratch in both areas.

=====================================
(2B)+(2B)' ?