Intermittent Vibration on 4.16KV Generators.

[dedi]

Units: 4160 V, 4750 KW, 60Hz, 1800RPM; 0.8pf

We have a few of the above unit that start vibrating once they are put back in service after a recondition in a manufacturer certified shop. The vibration is intermittent; it can happen as often as 3 times in a day or once in 3 weeks. The vibration amplitude is also inconsistent; it can range from 1mil to 5 mils, and last anywhere from 2 minutes to 10minutes. The generators are set to alarm at 4 mils and trip at 5 mils. We've had a couple of trips. The vibration only occurs at 1x, but we can't find a reason for it. It doesn't appear to be instrumental, since on one unit all the Bentley Nevada probes were cleaned &/or replaced, but the vibration returned. We are trying to figure out if it is electrically induced or due to a mechanical (or physical) fault in the rotor.

Does anyone have an idea of what might be going on? So far the repair shop and manufacturer are just as clueless as we are.

 

[byrdj]

I would suggest verifing actual vibration is occuring with siesmic detectors on the bearing caps. The vibration magnitude will be much less, like 25% but when the excertion occurs it should be proportional.

If actual, you will need to get phase to help identify the problem. Do you have access to a"TurboBalancer" for logging magnitude and phase. It can accept B-N along with siesmic like IRD.

If it were bearing instablity (oil whipping)due to change in bearing geometry or alignment changing bearing loading, it should be 0.6 to 0.8X

A change in the mounting of the B-N could allow the probes to go into resonance giving errounous readings. If the probes are long (over a couple inches) the mounting should have bushings near the shaft to damping out probe resonace.

Shaft voltages my generate errounous readings(and be causing damage). I would expect the driven end bearing has a grounding brush and the bearing insulated.

Is the excursion related to changes in excitation current? If the field bar's expansions are restricted
can cause problems.

 

[RRaghunath]

It is also worthwhile checking that the rotor windings do not have ground fault.

 

[ScottyUK]

Ground fault probably wouldn't give a problem - one fault won't cause vibration, and a second one will announce itself rather more dramatically. A shorted turn on the rotor could cause vibration though, by causing a thermal bend in the rotor. Does the vibration get worse with increased excitation? This is a typical signature for a thermal bend caused by a shorted turn.



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

 

[buzzp]

I believe this is called 'rough zone' operation. Maybe not. I am no expert on this phenomenon but it is something we worry about on our hydro generators. I would think it is a similar event but maybe I am way off. Do a google on 'generator rough zone' and you will get a lot of relevant hits.
Hopefully I am not way out in left field with this reply.