Generator Collector Ring arcing and pitting problem

[electrageek]

We have a 30 year old GE 20MW 13.8kV generator runinng at 10MW with a Power Factor or 1.0. The field is rated at 250VDC at 199Adc maximum. The excitation is provided by a Basler Static Full wave unit. We run at approximately 90Vdc at 100Adc most of the time. We consistently have arcing on the outside collector rings. It is not there all the time it seems to come and got but it is generally related to the amount of field current. There is never any arcing on the inside rings.

We have tried:

1. Comparing the hardness between the inside and outside rings. They are the same.
2. Increase the brush pressure with new brush holders.
3. There is no vibration or out of roundness.
4. Swapped the polarity of the power going to the rings.
5. Checked and reworked the connection from each embeded copper cable coming from each brush to the ring.
6. Cleaned and resurfaced the ring to remove the pitting.

I am now thinking it might be a metallurgy problem. Something has happened to the outside ring metal that has increased it's resistance. The photo I attached shows an interesting thing. It almost appears that there is a pitted area about the shape of the brush itself. Could it be an pulsation from the excitation system happening at the same time each revolution? But why only on the outside ring?

I am looking for some new ideas and possible tests to run.

 

[ScottyUK]

Have you checked the output of the static exciter? Do you have conduction in all thyristor bridge elements? You will need a clamp-on current probe or a Rogowski coil.

Out of curiousity, do you have a ground fault detection on the system? Is it operational? Is there any degradation on the bearing metal or journal?


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

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

 

[electrageek]

I did check the AC current on all three phases while I was there but naturally the arcing was not occurring at that time. It was balanced at the time (56, 56, 58) Also put an oscilloscope on the output and it appeared to be balanced and correct. But again it was not arcing at the time.

I have asked the same question about the collector end bearing isolation and the rotor ground fault monitoring.

Out of curiousity how would that affect only the outside collector ring?

 

[ScottyUK]

Not 100% certain - possibly dependant on where the earth faults occur, e.g. mid-point of winding, on the pole, etc. It would be possible to have very little voltage to earth on one ring and a significant voltage on the other.

The damage appears to be intermittent: are there six (or twelve?) equally spaced damaged areas around the ring periphery? Is it a two-pole or four-pole machine?

Hopefully Gunnar will see this thread - it doesn't look like the high frequency damage from a VFD which he usually deals with but I'd be interested to hear his opinion. Wolf and Rasevskii will no doubt share their knowledge too.


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

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

 

[electrageek]

I have attached a few more photos. The other markings look more like a shadow image or a brush with a trailing pitting effect.

 

[electrageek]

Here is the other one.

 

[ScottyUK]

I can't see the collector brushgear - how does the length of the damaged area compare with the size of the brush?


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

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

 

[electrageek]

Hard to tell exactly without measuring or comparing directly. The unit was running while I was there. I would say that the imprint is slightly larger then the actual brush itself. I would stay the brush itself was 1" by 1.25". It is a two pole generator rotor and there are four brushes mounted on each collector ring. It is possible we have a regulation problem in the exciter. I am going to check the time constants for the AVR regulator. However I can't understand why that would affect only the one set of rings.

 

[rasevskii]

Did you check to see if there is an earth fault in the field winding? It may be intermittant, and close to the problem slipring as Scotty has said. Megger the winding at 500V with the brushes llfted on both rings. That will still not rule out something happening when the machine warms up.

Is there an earth-fault relay on the excitation system? Is this a retrofit of an old unit or all-around new? Is there any other earth connection in the excitation system, such as an excitation trafo with a neutral or star point earthed. The excitation circuits must be floating and ungrounded throughout. Therefore an earth fault relay has to be provided to detect any earth fault.

Is there any possibility that oil from the bearing is getting onto the slipring? Is this an enclosed unit where cooling air flow may be drawing oil out of the bearing housing (when nobody is looking..)

What is the RPM of the unit? GT. Steam or hydro...Is the bearing pressure lubed (splash from bad gasket) or what..

just some possibilities..

rasevskii

 

[rasevskii]

OK your previous post said it was a 2 pole machine while I was writing my msg. Therefore a turbogenerator with pressure lube bearings. Check for oil leakage onto the ring. But likely both rings would suffer in that case..

rasevskii

 

[electrageek]

Thanks to ScottyUK and rasevskii.

I will check into the excitation Xfrm configuration. Only see the three leads coming into the Exciter though. The collector ring assembly seems to be fairly well isolated from any oil contamination and the other ring never has had a problems, they are in the same area. The problem does seem to come a go but I cannot say for sure that it is based on the amount of excitation current. After I left they shut down to check a few things and when they started back up it was there again, very noticable. The arcing occurs on the outgoing edge of the brush and the sparks apparently fly pretty good.

 

[byrdj]

you state you have no vibration , have you actually measured. For a two pole machine, cutting the rings at speed to reduce vibration, result in an out of round ring.
to measure the brush vibration I use a dry wooden rod to hold a siesmic type detector in line with the brush.

the other problem I have run into is the gap between the holder and the ring. by the gap getting greater, the brush to ring friction puts a moment on the brush and brush hangs up in holder.

Is the brush fully on the ring at full load, you need to account for thermal growth of the turbine and the field

 

[Skogsgurra]

Thanks for your unfounded faith in my capabilities, Scotty. But I have nothing to add to what has been said. The rotor ground fault seems to be the most likely cause. Brush pressure? Perhaps, but holders and brushes have been changed.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electrageek]

Thanks byrdj.

I will check the actaully vibration on each ring. Maybe there will be a noticable difference between the two.

I did notice that the brushes on the problem ring are right on the edge of the ring. They do not cover the entire ring surface as I am use to. Normally I see shims placed to give the brushes a 50% offset to better us the entire surface for even wear. This application only has about a 1/8inch shim on two of the brushes.

 

[Gamaliel57]

One thing that cones to mind is electrolytic corrosion. Are the brushes left in place when the generator is shut down for extended periods? I have seen impressions left from this. Though why not the inner ring?

 

[panter]

Hello, Everyone !!
Electrageek if you have some thermal camera you should make collector rings picture during the work of generator. Sometime distribution of the current between brushes is not good and makes condition for similar problem.
Good hunting!

 

[pwrengrds]

I was on site today, took some traces using a chart recording. It was arcing a bit. Plan on going back to site on Monday to do some testing on rotor (Ductor, megger (their results were 75 M ohms at 500vdc), AC Impedance). Any comments?

 

[pwrengrds]

http://files.engineering.com/getfil...96a-87a6-082d51b4ceb8&file=Exciter_traces.pdf

 

[ScottyUK]

The rapid change in field voltage relative to ground is rather suspcious. Have you checked your shaft grounding brush? Is it in good condition? It should be clean, and on a clean shaft, with reasonable spring pressure and ideally connected to frame earth via a few ohms.


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

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

 

[wolf39]

Corrosion between brushes and slip ring or some current flow when generator was shut down are possibilities. I would recommend to polish the slip ring in question, or both. Replace all brushes and see what happens.

Does the photograph show a fan blade next to the outside slip ring? Are temperatures of both slip rings equal?

Wolf

http://www.hydropower-consult.com