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.

 

[electrageek]

ScottyUK,

We have a few of them and I really like it. It's an 8 channel unit and good for up to 1000Vdc per channel and it has 6 ranges. 1000, 100, 10, 1, .1, and .01 full range. It has a chart recorder mode and an Oscilloscope mode. They have the parallel port, USB and Ethernet. The Ethernet can be time synched between multiple units.

http://www.dataq.com

 

[ScottyUK]

Hmm, I will ask Santa if I can have one for Christmas. Or at least, I'll see if we have any surplus in the year-end budget.


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

 

[electrageek]

Well the saga continues. We shutdown today to inspected the connections from the slip rings to the leads to the generator field. Turns out they are in good condition and they were even soldered. We could not find any obvious problems and the resistance measurements looked good. The Turbine end shaft grounding brushes are installed, there are two of them. I found the GE instruction books and noticed that the original design called for 8 brushes per slip ring. We have four brushes per slip rings so at some point in the past someone decided we did not need 8 brushes. I checked with some other customers with similar machines and they all have 8 brushes per slip ring. It doesn't explain why the inside ring does not seem to have the same problem.

We also were told that after about 1 year of normal operation they had an incident that involved a tremendous amount of arcing that caused a ring of sparks that eventually caused a loss of excitation trip. The operator said that he observed the field voltmeter increase from approximately 90vdc to over 300vdc before the trip. Possible we have a problem with the resistance of the slip ring itself along with not nearly enough brushes to carry the load.

The customer is going to add more brushes to the outside slip ring and I am going to try and find out how the metallurgy might have changed.

 

[ShaikhSahab]

Please check the thickness of slip ring , it might be reduce due to aging or grinding in provious days.

1. Get the drawing of slip rings in which thickness at the time of commissioning should be mentioned and compare it with today's thickness of slip rings.

2. Check the distance b/w brush and slip rings , it should be within permissible limits defined by vendor.

3. Monitor the temperature of Slip rings for 24 hrs, if it is beyond 80 than these reason will be sure.

4. Check the chemical composition of brushes.