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Trying to find a company to evaluate our generator.
2

Trying to find a company to evaluate our generator.

Trying to find a company to evaluate our generator.

(OP)
Hello all

I've got white marks on the end turns on one of our generators.  I think it may be corona.  It is a Brush BDAX 7-290ERJT, 71176 KVA.  We have been going through Brush for the last week, had a rep from them out for the last couple days and no answers.  I think the white marks are an issue and want a second opinion.

So does anyone know of a company that that can take a look at our generator in California? And hopefully do repairs as neccesary.

Thanks all.

RE: Trying to find a company to evaluate our generator.

What part of California?

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Trying to find a company to evaluate our generator.

(OP)
Bay Area, San Jose area.

RE: Trying to find a company to evaluate our generator.

http://www.natlmech.com/about.html

Look over their website.  Ask for Rudy.  They go far and wide.

rmw

PS: my $0.02: you got corona, sorry.

 

RE: Trying to find a company to evaluate our generator.

Corona isn't fatal.  Actually it is fairly common.  Fixing it is fairly straight forward.  But, you need someone who knows what they are doing to fix it.  The problem is that the air gap isn't sufficient at that point and corona is the result.  Beware of someone who only wants to just gob on more tape and varnish and thereby further reducing the air gap.

rmw

RE: Trying to find a company to evaluate our generator.

(OP)
the problem is there is corona, or atleast white marks, between several windings in a row.  from what I understand it should be between phases not between 4 or 5 windings in a row.  We are more concerned about that, and brush says its nothing.

RE: Trying to find a company to evaluate our generator.

Quote:

from what I understand it should be between phases not between 4 or 5 windings in a row.  We are more concerned about that, and brush says its nothing.
Typically pd tends to occur where there are larger phase to phase and phase to ground voltages, often at the line end coils.  Typically not at every coil.  If you have pattern that occurs on every coil and does not seem to correlate with voltage, then consider non-corona causes. One non-corona cause would be chafing due to excess vibration.  Another non-corona cause may be some kind of external contamination.

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

RE: Trying to find a company to evaluate our generator.

What voltage is your machine?
Is the location of the mark approx 1" from the slot?

If > 6kv and at that location, sounds like degadation at the overlap between the slot-section carbon/semicon and the grading.   

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

RE: Trying to find a company to evaluate our generator.

Also, it may help to know if this is air-cooled or hydrogen cooled.
 

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

RE: Trying to find a company to evaluate our generator.

Brush (England) makes nothing but air cooled.  A 7-290 may or may not have been built in England - some were/are made in the Czech Republic - but the 7-290 is an air cooled machine.

rmw

RE: Trying to find a company to evaluate our generator.

(OP)
It's 12kv and it's about 5 or 6 inches from slot. The rep from brush said it's at the edge of the shielding/semiconductor. Official Brush report said nothing.  

RE: Trying to find a company to evaluate our generator.

I'm not familiar with the term "shielding" in this context. For me: Semiconducting = carbon tape or paint is used in the slot and only and inch or so outside. Grading (silicon carbie paint or tape) overlaps that just a little, and extends maybe 6" out before it stops.   That must be the outer edge of the grading tape.  I have never heard of burning at that location.   Sounds like you are doing the right thing to bring someone in OEM... warranty?) help to evaluate.  

Meanwhile, just thinking out loud, since grading layer is supposed to slope the voltage gradually from ground voltage at core end to line voltage at outer end, maybe the effective resistance of that grading layer is way too low for some reason, resulting in outer edge of grading being too near ground.  

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

RE: Trying to find a company to evaluate our generator.

The bigger BDAX-9 operating at 15.5kV suffered continuous problems with low / medium levels of PD and repairs were carried out on at least an annual basis, almost always in the endwinding and phase break areas. I'll see if I have any photos. PD normally leaves a pure white deposit - that looks fairly yellow. The winding looks darker, maybe a change in manufacturing process and/or location, or perhaps the whole image is just dark?
  

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

RE: Trying to find a company to evaluate our generator.

(OP)
I think 'shielding' was just an example used to explain some of the layers in the insulation to me.  

Some of the spots look like, from what I understand, classic corona. White to a little yellow, starting at a point and a 'V' pattern away.  

Other spots just look like a patch of white, almost pure white.  

Electricpete, thats the best idea I've heard so far.
 

RE: Trying to find a company to evaluate our generator.

I thought corona is a problem with insulation breakdown and moisture issues.  If your in the bay area do you have problems with salt moisture in the same room as this machine?  Why it would be yellow, contanimation of some kind in the air?

RE: Trying to find a company to evaluate our generator.

I think there is a lot of evidence pointing to the fact that the pd is occurring at the outer edge (endwinding edge) of your grading tape/paint. Here are 3 factors that point in that direction:

1 – It does not appear to be a tight area.   Typical pd due to coils too close occurs at tight areas where coils cross over each other, where jumper is adjacent to coil, where blocking bridges coils etc... none of these appear to be the case.

2 – It is about 6" outside the slot where the end of the grading would be.  And your OEM rep said something like it was the end of the "shield" .... can't think what else he would be referring to.

3 – It appears to be a generally circumferential pattern (*although perhaps you can check this better for yourself than we can check from the pictures*).  Traditional pd due to close spacing does not follow a circumferential pattern. But circumferential pattern is expected at edges of stress treatments if they malfunction in a manner to create voltage difference at that edge.
 
The more typical malfunction of a stress treatment occurs at the junction of the semicon = carbon and the grading = SiC..   It gives a circumferential band about 1" outside the slot.  This degradation  can progress in 2 stages (as well detailed in "Electrical Insulation for Rotating Machines" by Greg Stone, Ian Culbert, etc al):
Stage 1: Burning occurs due to surface current flowing from endwindings to the slot.   In the endwindings, the circuit to the conductor is completed through the winding capacitance.  (I think endwinding conductive contamination will tend to increase this capacitance by the way).  So we have current flowing and it is relatively small, but the heating occurs at a local high-resistance area which is the interface where the current must transition from one layer to another.   Eventually the heating from this current burns the stress treatment open to create an open circuit, which leads to stage 2:
Stage 2: A voltage difference appears across the open circuit created by stage 1. Surface partial discharge occurs due to this voltage difference.


Again all of the above is well documented for the interface from semicon to grading, but I have never heard of anything like it at the outer edge of the grading (the endwinding side).  We could reasonably guess that there are 2 possible mechanism for damage at this location (outer edge of the grading) that resemble the 2 possible mechanisms of stage 1 and stage 2 above.   The two possibilities are:
1 – burning due to current which is passing from the outer surface of the endwinding, possibly through a coating / paint layer, down into the grading layer.  This type of burning could discolor the coatings to create the yellow or other colors.
2 – surface partial discharge at the outer edge of the grading.  It is harder to come up with a reason for this since it is not really analogous to stage 2 above.  The best I can come up with is what I had suggested in the previous post.... if effective resistance of grading is too low than you could end up with a surface voltage different at the outer edge of the grading which creates partial discharge.

If it were one of our critical machines, I would call Dave Scherer of IPS LaPorte for his opinion.  We have paid him several times to travel to other repair shops to provide his opinion on various visual partial discharge symptoms to help us figure out what we should do.     

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

RE: Trying to find a company to evaluate our generator.

I am not an EE but I understand the concepts.  Pete has hit on what the "shielding" is, although I am not familiar with that term.  As I remember it, it extended out about ~4" from the edge of the iron.

I can tell you that Brush England speaks a different English than USA folks.  Same language, different words or different meaning for the same words.  I can't remember what they called this conductive tape.  If I heard it I would remember it.  Shielding doesn't ring a bell.

Ask your "rep" what those metal things on the end of the rotor are.  If he says 'end caps' he is a Brit.  If he says 'retaining rings' he is an American.  If he says 'end caps' with an american accent, be suspicious of his/her background.

In either case, as I am acquainted with the rep that used to cover that area for Brush, I can state that this 'rep' must be a fairly recent addition to the Brush crew.

See if you can find out who did the report from the home office.

What vintage is this BDAX unit?

rmw

RE: Trying to find a company to evaluate our generator.

(OP)
The rep from Brush was actually a sub-contractor out of Canada and he has been doing this for Brush for about 15 years.  I think he thinks there is an issue, but he isn't allowed to say anything to us directly.  The Brush report for our generator says there is no problem, we just need to pull the rotor and clean it.  

The moisture isn't really an issue here.

The generator was manufactured in 2002, and been in service since 2005.   

RE: Trying to find a company to evaluate our generator.

Is the first initial of his first name "J" or "H"?

rmw

RE: Trying to find a company to evaluate our generator.

(OP)
"H"

RE: Trying to find a company to evaluate our generator.

This is getting intriguing.  I will have to call "J" and see if I can find out why "H" was there on Brush's behalf.  Until I find out more, I'd prefer not to say more about "H" at this time other than to say that he knows his stuff and I trust his judgement completely.  The mystery to me is why he was the one who came out.

I will say that if I had your problem, he would be one of my first choices to call.

rmw

RE: Trying to find a company to evaluate our generator.

(OP)
"H" happened to be near by (Southern CA).  I trusted him, he was hamstringed by his agreement with Brush.  I guess the insulation engineer in England did not see the issues he did.  We asked him if he'd come back independently, but he would not. He didnt want to ruin his working arrangement with Brush.

We may call him back for future generator issues though.

RE: Trying to find a company to evaluate our generator.

OK, I can see that you completely know who he is and who he is with, and yes, he does (or as of the last time I spoke with him did) have a relationship that he would need to be careful to protect.

I almost recommended him (his firm) to you when I made the recommendation above, but thought that he might be too far away from your location for you to consider so I didn't.  Ironic that you encountered him anyway.  He is a solid individual.

The firm that I did recommend would be his competitor.

I am a little surprised by Brush's approach.  Is the Insulation Engineer you refer to named "H" too, the same name with that first letter; second name begins with "L"?  If so, he pretty well knows his stuff, but remember that he is a factory guy and as such has a different view of things than someone like "H" who does nothing but fix-em day in and day out.

rmw

 

RE: Trying to find a company to evaluate our generator.

(OP)
The 'H' here was very frustrated that he could not get 'H' there to see what we were seeing here.  He was taking it as a failure on his part to show them what we saw here.  We sent them about 2 dozen boroscope pictures showing patches of white on the windings.  They still said we didnt have a problem, but the rotor needed to be pulled out and cleaned.  Then quoted us $200,000 to pull rotor and clean!   

RE: Trying to find a company to evaluate our generator.

Let me get this straight.  They are saying that pulling the rotor and cleaning it will cure white stuff on the stator end windings?????  When I read your words several posts back "...pull the rotor and clean it" I read it several times and thought it meant 'pull the rotor and clean "it" the deposits or white stuff' with the rotor out of the way.

Your last post made it clear that it is the rotor that they are recommending be removed and cleaned.  Is it dirty or oily?

I'll come do it for 200K.  (see tongue in cheek.)  You would be well on your way to buying a new one at that price.

rmw

RE: Trying to find a company to evaluate our generator.

(OP)
It is oily. We were told thats not an issue for this insulation.  The quote is just for pulling the rotor and a general cleaning of the machine.  All competitive quotes, since we are a municipality, were between 60,000 to 80,000.  Which include insulation repair.  I dont think Brush will back on site.

My manager and I considered doing it for $200,000.

RE: Trying to find a company to evaluate our generator.

Oil dripping form the dolls eyes isn't an uncommon 'feature' of the BDAX machines. We were in the process of renewing the shaft oil seals with a modified design when I left the station. Check the ventilation hoses to the chamber between the inner and outer generator end plates. If the plenum pressure is wrong due to air leaks then it contributes to the oil problem. the hoses weren't particularly robust on our version.
  

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

RE: Trying to find a company to evaluate our generator.

what do you mean by "the dolls eyes"?

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(2B)+(2B)'  ?

RE: Trying to find a company to evaluate our generator.

I'll let Scotty answer to make sure he is referring to the same thing but I remember the first time I heard the term and especially with a thick english accent, it sounded like "does eyes" (the thing you aim your rifle at) to me.

The reason I won't answer is that I have seen oil dripping from so much of the entire endwinding area that I hardly noticed any on the dolls eyes.  And... I actually don't know what the piece is called in USA English.

Justatech, depending on the driver, oil leakage can be caused by problems with the vent mist elimination system as well.  One particular popular model uses a filter that will cause the generator bearings to puke oil oint the endwindings if it is not serviced regularly.  Based on the generator model you quoted in the OP, it is highly likely that you have one of these.

rmw

RE: Trying to find a company to evaluate our generator.

ePete,

It's the funnel-shaped region of the endwinding outside the stator core. Viewed end-on from a distance the endwinding has a vaguely woven appearance, with a hole in the middle where the rotor would sit. I guess that's where the name comes from. Interested to know if rmw's recollection is the same - it's a phrase I've picked up along the way and have never seen it referred to in a text.
 
  

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

RE: Trying to find a company to evaluate our generator.

Let's see if I can do this.  Remember that this is from a ME who only knows how to cut things with a torch and weld it with a stinger and whack it with a hammer.

In the end winding area, the end windings are tied to (non conducting - bakelite?) braces with string (cord, twine) and the string is tied through those brackets and braces with holes in them.  There are 4-6 (been a while) large flat brackets that are attached to the end of the stator that stick out axially from end of the frame which are shaped at an angle that follows the angle of the winding as it leaves the stator.  That bracket has a row of holes in it into which is threaded the twine that is wrapped around the end windings to bind them to the bracket.  The holes in those brackets and the other various pieces like it to which various parts of tne end windings are secured are referred to as the "dolls eyes".

That is why I didn't quite understand your use of it with respect to the oil dripping, as the oil drippings I had seem were all over the end windings not limited to just the bracketry with the dolls eyes.

I don't know if that is a Brush term or an industry term, but I suspect it is the former.

It does blow my mind as a ME that a machine as powerful as a generator would be held together by string and tape.  We certainly don't build the turbines that drive them that way.

rmw
 

RE: Trying to find a company to evaluate our generator.

We can do Corona Inspections and repairs, we provide world wide services. We are based out of San Diego,CA.

www.qinetiq-na.com

RE: Trying to find a company to evaluate our generator.

(OP)
Thanks all for the info.  

Looks like we are going with Electrical Maintenance Consultants (EMC) out of Rocklin, CA. They've done a lot of work with the other small municipal utilities in the area.

I've added all the other companies info to a list we have going around here.  

Thanks again

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