Sparking at Carbon Brushes of Main Exciter of Generator. One Thing is Requested to be Explained. !!

[Naumaan]

Hi Everyone!
Our Generator is rated at 330 MWe. Its excitation current is provided through a Permanent Magnet Generator (58 KW) and Main Exciter(1360 KW).
The excitation to Main Exciter is provided by Permanent Magnet Generator using 3 carbon brushes per pole and overall 6 carbon brushes (Ie=60A, Ve=70V). The excitation to Generator is provided by Main Exciter using 40 carbon brushes per pole and overall 80 carbon brushes (Ie=1800A, Ve=460V).

It so happened that during operation sparking stared to occur at the negative pole (negative slip ring) of Main Exciter at all the 3 carbon bushes at it. The initial remedial actions included spring replacement, brush replacement and brush facing. The current distribution among brushes was found normal. The vibration of generator shaft was although a bit higher from previous records but it was well within normal range. The sparking was sometimes intermittent and sometimes it continued for days and hours at a single brush or at all 3 brushes of negative pole.

Our generator went to shutdown due to grid problem which provided us opportunity to look at the slip ring in stationary state. At the negative pole, we observed "GHOSTMARKS" or "FOOTPRINTS". We did a little bit of polishing at negative slip ring to remove GHOSTMARKS and make generator go online as early as possible. We were unable to remove these marks completely. As soon as the generator started operation, there was severe sparking at the negative pole. We again checked springs, brushes and current distribution which was OK. But after operation of 4 weeks sparking has ceased and there is no sparking at brushes for four months as generator has continuous operation.

[highlight Red]WHY SPARKING HAS STOPPED NOW?[/highlight]

In theory, I have read that once ghost marks appear and cause sparking they keep on making more damage but now everything is OK. Can there be any other problem?

Hoping for some expert to look at it. Thanks in advance!

 

[ScottyUK]

Have a look at

https://www.mersen.com/uploads/tx_mersen/5-carbon-brush-technical-guide-mersen.pdf

Do you recognise any of the tell-tale patterns or description?

Generally polishing a slipring or commutator is a bad idea because it strips away the protective patina from the surface, which results in sparking. When (if) the patina re-forms the sparking subsides.

My guess to the underlying problem is that ring is distorted. A slipring is a relatively small component on a massive shaft, so relying on vibration data only may not reveal the extent of the distortion.

 

[Naumaan]

Most of the cases shown by Mersen are for motros and I had unsuccessfully tried to match these earlier.

If polishing has caused sparking, then it should always spark whenever a generator starts operation after overhauling. In our case, there was no sparking at brushes at the start of operation but first of all it had started after an operation of nearly 2 months.

The part of your suggestion that it might be due to distorted ring, I also agree with it partly.
The distortion might be due to ghostmarks or out of roundness.

The question is that if the ring is really distorted, then sparking should not stop and the problem should become more and more severe with time. But it has not. The sparking is gone.

 

[ScottyUK]

Do you have any photographs of the 'ghostmarks'? I'm not 100% clear on what you are describing.

 

[Naumaan]

No, I was unable to take photographs. On negative pole slip ring, three marks were present of brush face dimensions having the spacing of carbon brushes in between them. These were also a bit spread out.

 

[electricpete]

On negative pole slip ring, three marks were present of brush face dimensions having the spacing of carbon brushes in between them. These were also a bit spread out.

Two possible interpretations:
1 - damage while stationary (I think the most common interpretation of this pattern). For example moist environment and different corrosion pattern on the covered area vs non-covered area or else corrosion just around the periphery of the brush. Probably the pattern would be symmetric around the brush, not showing any preference for leading or trailing edge.

2 - damage due to severe one-time electrical transient or mechanical shock while rotating. Actually I haven't really heard of that but seems credible. At any rate in this case I'd think you might see the pattern would be asymmetrical with respect to the brush. For example more damage on the trailing edge.

Just my two cents. I don't deal much with generator slip rings.
Photo might give other clues of course...


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

 

[Naumaan]

As I have described earlier, the marks had appeared while generator was in operation. So, these had not appeared at stand still.

My question is that if once these marks appear and cause sparking, then how can sparking vanish later while these marks were at slip ring.

Look at the article at web.

http://www.lfc554.ru/download/be16gb/ae1644gb.pdf

It suggests that once these appear, the damage to ring increases more and more.

But in my case, sparking has stopped.
How can this phenomenon be explained?

 

[Naumaan]

And sorry, I was unable to take photographs>

 

[low1]

Do you have a regular maintenance interval in which you swap polarity of the field connections? The polarity must be swapped regularly to balance the formation of the patina mentioned earlier.

 

[Naumaan]

We actually have two units of 325 MWe at our site (different plants). One of the units is working since 10 years and other has operated for 5 years. it is the older one which has problem of sparking.

We have never made it a part of our maintenance routine to swap the polarity of field connections. Although it equalizes the condition of the pair of slip rings.

As you see, the problem is only on one of the units, the older one.