Hydro Generator Brush Rigging Design

[bohdand]

Hi to all the Gurus,

I have a question that I cannot find an answer to and am hoping that someone can shed some light on this.

I am working on a 40MW vertical hydro generator upgrade project and we are replacing the existing brush rigging assembly and collector rings. The Amps Field Full Load (AFFL) is about 900Amps DC. There is a difference in the design of the brush rigging where the existing is a partial circle about 270 degrees around while the new design is a full 360 circle. We have been told by one consulatant that having a full 360 circle brush rigging will allow circulating currents in it and as a result of this circulating currents will be created in the main generator/turbine shaft. But, a major generator manufacturer says that it is not an issue, who is right? Does anyone know of any literature on this topic?

FYI - We do have shaft grounding brushes installed on an upgraded unit and we measured the current at 3ma. (This machine has a static exciter installed so the grounding brush is new).

Any insight would be appreciated - thanks in advance.

 

[crshears]

What is the consultant thinking would cause the circulating current? Induction via AC components in close proximity to the DC cables, or something else?

If the new brushgear rigging consists of a solid 360° ring mechanism, the only way to install it will be to lower it onto the top of the unit, then assemble everyting else around it and onto it, and in the event a portion of it becomes damaged, the only repair option will be to tear everything around it apart to lift it off...

If instead segmented rings were used, it wouldn't involve nearly as much dismantling to replace, say, one quadrant in the event of damage due to arc splash, since field forces could conceivably remove the inspection doors on the side of the cabinet and have at it.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

I'm not that familiar with vertical-shaft salient pole machines, but isn't the top bearing insulated from the frame? Turbo machines are built that way to prevent shaft currents, and a shaft grounding brush is provided to drain away windage-induced static. If the upper bearing is insulated then circulating currents in the shaft won't flow.

 

[MikeHalloran]

<tangent>
My ME class was standing on the top of the turbine volute,
looking up at the rotor, spinning steadily at 60rpm.
We were clustered loosely around the 30" diameter shaft,
so shiny and smooth that it almost looked stationary.
There was plenty of room for all of us,
since the rotor measured 12 feet in diameter at the air gap.
I remember seeing the rotor coils whizzing around,
and the stator coils just sitting there humming,
and an odd feeling in my hair,
possibly associated with the electric field.
Our guide mentioned that the normal output was 1200 VAC at 1200 amps per phase.

My good friend Newton, who affected a sort of farmboy act
to conceal his towering intellect, opined
"That would make a right good welder."

I don't remember seeing any brushes.
Maybe they were on the next floor up, with the upper bearing.

( That was almost 50 years ago. I doubt that you could get a college class into a major hydro dam today. Pity. )
</tangent>

Mike Halloran
Pembroke Pines, FL, USA

 

[davidbeach]

There are brushless excitation systems where there are essentially three machines on the same shaft so that the DC excitation never needs to leave/enter the rotating equipment.

 

[crshears]

Davidbeach: yes there are, and they're a great choice provided the slightly longer excitation response time isn't a problem, something the OP would have to answer...

MikeHalloran: [nostalgic sigh] thanks for the trip down memory lane...and you're almost certainly correct that the brushgear would have been found at a higher elevation, probably not far above the twelve-foot diameter rotor. Ours were twenty feet or so across, and turned in the neighbourhood of 94.7 rpm. I once took an eldery gentleman on a tour, and warned him that the electric fields could have an adverse effect on his cardiac pacemaker; they did, and he had to scooch back out from under there toot sweet.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bohdand]

crshears,

We have already purchased and designed a new static exciter so we are not considering going to a brushless design at this time - I'm not sure if the slower response time would be an issue. As a hydro machine running at just over 100RPM, the brush's are not really a maintenance problem.

I don't recall exactly why the consultant thinks circulating currents are a problem, but it had something to do with the field leads (actually copper bus bar) that run down the shaft when they are carrying current and the shaft is rotating. The magnetic field generated by these leads cuts the round brush rigging inducing a current (so he claims). I'm going from memory, but that is what I recall.

 

[ScottyUK]

You'll get pretty much total field cancellation because the two D-leads are in close proximity to each other and their currents should be in anti-parallel. If field current is passing down one D-lead and not coming back up the other one then induced current in the collector ring will be low on your list of problems...