Roebel Generator Bars

[ronnie497]

Hi Everyone,

I have been involved in the post mortem of a 60MVA H2 cooled turbo generator failure.

The machine was rewound. Originally it had "straight" bars, overhang clip transpositions. This was to be replaced with 360 deg roebel. Due to space constraints, 180 deg roebel was employed together with overhang clip transpositions. (96 slots, single circuit)

A top coil limb leading to the Blue interphase vapourised for over 30cm. (Very short time on load). Dissection has shown serious inter-adjacent strand arcing in the limbs leading to the other interphase connections.

After tracing the transpositions through the winding I have developed a theory that the 180 deg roebel can cause a voltage difference to appear between adjacent strands that start about the centre of the bar as they will depart from each other and only recombine at the bar end (Left Up, Right down). All the bars are of the same twist (Clockwise). As the stacks are reversed at the series (interphase) connection, this voltage increases from the star point up to the series connection and then reduces towards the line connection.

Not having any design experience I am unable to qualify this theory. Does the main flux density vary with depth or is it only the quadrature flux? (How much, can I guesstimate it?).

All suggestions and help are welcome.

Thanks

 

[edison123]

60 MVA, 2 pole machine with straight bars and transpositions only in end winding !!. That is unusual. Were all end connections transposed ? I have seen a 78 slot, 60 MW m/c with 540 deg roebel and a partial transposition in the end winding in the middle of each pole-phase group (7th coil only).

With 180 deg transposition, you are not careful, the end winding transposition may nullify your slot transposition.

With the machine capacity of 60 MW, I would expect a core length of about 3.5 m. With that length, 360 degree roebel is possible and better. Could you post the no. of conductors and the core length ?

Right now, I am making in my repair shop 360 deg Roebel bars for a 6.6 KV, 15 MW m/c with the core length of only 1550 mm with 2 x 24 conductors.

Btw, the eddy current losses are caused by the leakage flux across the slots. Deeper the slot, less is the flux across the bottom most layer of conductors.

You may refer “The Performance and Design of A/C machines” by M.G. Say under “Armature Windings” and slot leakages under “Magnetic Circuits” for further study.

 

[edison123]

correction, the 60 mw m/c I have cited had 360 deg transposition not 540 dge.

 

[ronnie497]

The machine design could have been before Roebel? Made by AEI. (Ran for 50 years). They had a straight (2 knuckle) evolute in the NCE with a two knuckle, transposed cross - over evolute in the CE. (Two clips per stack). Worst distribution was 81%, then 87%, 92%, etc.

The new transpositions work out fine (89%,91%,94%, etc).

The bars were changed from 2 stacks of 32 to 2 stacks of 24 conductors with an increase in section, they kept the evolute layout with the two clips per stack. Copper 2.8 x 3.2mm. Core 2800mm long with 30mm slot width. The 360 deg was abandoned due to scissor damage causing interstrand faults.

 

[edison123]

Very interesting old winding. Would love to see that kinda winding.

btw, how did you work out the distribution as 89%, 91% etc ?

With 2x24 conductors and 2800 mm core length, 360 degree transposition is easily achievable. I am doing the same 2 x 24 with 1550 mm length (conductor 8.3 x 1.9 mm).

Your copper size (2.8 x 3.2)mm doesn't seem right. Am I missing something here ?

 

[ronnie497]

A pity that I was called in at the end of the thing, so no photo's etc. Sorry for the Cu size, blonde moment! It should be 2.8 x 10.8mm.

The distribution% comes from an anaylisis of the summed strand locations. 89% says that the strand starting at that position occupies 89% of the bar's slot position throughout the wind.

 

[Skogsgurra]

ronnie and edison,

I seldom get close to machines of that size - and I never get under their skins. So what you are discussing is like greek (or chinese) to me. But I have enjoyed every word, and not understood much of it. Pure poetry, I would say. Shows how far from each other we can be, even if we are in almost the same business.

 

[jbartos]

Suggestion: Reference:
M.G. Say "Alternating Current Machines," John Wiley & Sons, 1978,
Chapter 3 "Windings"
and

http://www.engr.usask.ca/classes/EE/444/notes/VERY-IMP-01016841.pdf

for: ...Roebel Molding Bar Injection...