Generator Collector Ring arcing and pitting problem
Generator Collector Ring arcing and pitting problem
(OP)
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.
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.





RE: Generator Collector Ring arcing and pitting problem
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?
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
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?
RE: Generator Collector Ring arcing and pitting problem
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.
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
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
RE: Generator Collector Ring arcing and pitting problem
rasevskii
RE: Generator Collector Ring arcing and pitting problem
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.
RE: Generator Collector Ring arcing and pitting problem
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
RE: Generator Collector Ring arcing and pitting problem
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Generator Collector Ring arcing and pitting problem
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.
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
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!
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
htt
RE: Generator Collector Ring arcing and pitting problem
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
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
www.hydropower-consult.com
RE: Generator Collector Ring arcing and pitting problem
They have changed all the brushes and put in new constant pressure brush holders. I was there last night and it was just barely arcing on one of the brushes. We have a chart recorder hooked up for Field Volts, Field Current, Pos to Ground, and Neg to Gnd. It still seems strange to get pitting in the shape of the brush usless the arcing is synchronized to something that is once per revolution. It only ocuurs on the outside brushes and they have tried swapping polarity.
RE: Generator Collector Ring arcing and pitting problem
Good to hear that the problems eased.
I may have chosen a wrong term by using the word "polish". If a slipring is perfectly polished, brushes tend to squeak and rattle. I rather meant to improve the surface quality by removing the brush marks. Be careful to use grinding paper or similar because particles remaining on the slip ring surface will deteriorate brush wear.
Wolf
www.hydropower-consult.com
RE: Generator Collector Ring arcing and pitting problem
I will probably be corrected by the materials and mechanical purists, but I understand the difference to be that polishing is an abrasive process while burnishing is a forming process where the material is displaced on a microscopic level rather than removed completely.
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
They use the recomended stone for cleaning the Slip Rings. The process removes the pitting caused by the Arcing. The pictures show the discoloration and once the light stoning is complete the arcing seems to go away for a period of time but it always comes back.
They are going to take the end apart and we are going to inspect the connections from the Slips Rings to the conductors that pass through the rotor into the Main Generator Field.
RE: Generator Collector Ring arcing and pitting problem
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
Very wise - your eyes are harder to fool than your ears!
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
rmw
RE: Generator Collector Ring arcing and pitting problem
True but can we make a case that "not having the Turbine End of the rotor shaft grounded" could cause arcing and pitting on the outside slip ring only.
RE: Generator Collector Ring arcing and pitting problem
On your last visit to site did you actually check that the new excitation system has NO ground anywhere else in the circuitry? There must be an excitation transformer which was supplied new as part of it. Are you absolutely sure that there is not a neutral or star point ground in that supply somewhere? Why not post us a dwg of the new system?
There can be unpleasant surprises when someone has modified or retrofitted something old.
There is still a possibility of a ground fault in the rotor that only shows up during running of the unit. That together with another solid ground elsewhere could cause such a problem.
rasevskii
RE: Generator Collector Ring arcing and pitting problem
Just a wild guess:
What if the outside slip ring was improperly heated by welding torches during the heating/shrinking process of the slip rings to the generator shaft? If this was the case the mechanical/metallurgical properties of the surface may have been affected unequally thus generating these spread surface marks.
Also, the sliprings are having spiral grooves. These grooves should be chamfered.
Wolf
www.hydropower-consult.com
RE: Generator Collector Ring arcing and pitting problem
I had thought about the Slip Rings having high resistance of some sort but that doesn't really explain the pitting looking like a picture of the brush. You would think it would be more generally spread out over the ring. We are going to take a DLRO and measure the resistance of the various connections. The spiral groves look good and there are no raised edges.I am not sure when the Slip Rings were installed. I don't believe they were disassembled prior to installation at this site.
RE: Generator Collector Ring arcing and pitting problem
Unless you have a double ground fault: one in the rotor, and one solid ground somewhere in the excitation supply: result: a hundred amps (or so) flowing in the loop..also via the shaft grounding brush.. A similar scenario has happened at times during commissioning in other places, where, however, both rings were more or less ruined by current applied at standstill.
rasevskii
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
RE: Generator Collector Ring arcing and pitting problem
My first reaction is that the current trace is not truly showing what is happening, and that the ringing is a result of rectifier commutation interfering with the transducer. There is simply too much inductance in the field for such rapid changes in current to occur without a very powerful source. The ringing occurs at too high a frequency to be influenced by the AVR itself. The logical conclusion is that the ringing is a measurement artifact.
What are you using to measure the current? In my experience many of the LEM Hall-effect type transducers are pretty good but some are susceptible to inteference caused by high dv/dt on the conductor passing through the transducer. I normally fit a copper foil electrostatic shield in the bore of the transducer and connected to frame earth which really supresses the capacitively coupled interference. It's a cheap and easy fix - worth a try if you find yourself questioning your instruments.
Most AVR's I have worked on use a shunt to measure field current and you could tap in to this but BE CAREFUL if you connect directly to the shunt, and use an isolating probe or a Scopemeter. Your voltages are well within the capability of the Scopemeter type instruments, so that would be my first choice in this instance.
This is developing into an interesting problem!
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
Probably Basler can supply a rotor ground fault relay. ANSI Device reference 64 if I recall.
rasevskii
RE: Generator Collector Ring arcing and pitting problem
Yes exactly what we going to propose.
Thanks ScottyUK,
This was my thought also. We are using a DATAQ 730HV digital 8 channel recorder/oscilloscope. It seems to show up at very high data rates 37,500S/S.
Today is the day they are going to take the emd covers off so we can see the field connections.
RE: Generator Collector Ring arcing and pitting problem
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting problem
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. www.dataq.com
RE: Generator Collector Ring arcing and pitting problem
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If we learn from our mistakes I'm getting a great education!
RE: Generator Collector Ring arcing and pitting 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.
RE: Generator Collector Ring arcing and pitting problem
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.