Impending Generator Failure

[BigJohn1]

Have a 12MW utility-parallel, enclosed generator that is suddenly producing a very significant "hot electrical" smell.

All instruments are reporting "normal" values for temperature, current, voltage, etc. No protection has operated.

Did winding resistance and insulation resistance on both windings, plus surge on the stator. The only unexplainable reading is the rotor resistance was about 25% low based on nameplate voltage and current. The two caveats are the resistance is NOT temperature corrected, and this is a 28 pole machine so one shorted pole would not account for that much difference.

The rotor is basically inaccessible. You can barely even see it, so there's no way to do pole-drop.

Any suggestions at all for further testing? Something is going to fail here, and I have no idea how else to find it.

 

[edison123]

Do you have on-line vibration monitoring for the unit? If the vibration changes drastically from rotor unexcited to excited, then you may have pole inter-turn shorts.

Muthu

http://www.edison.co.in

 

[BigJohn1]

It does have vibration as well as field ground and loss of field. Nothing has tripped. We could have a failing rotor pole, but I have no idea how to prove it from outside the machine.

 

[itsmoked]

That should cause a periodic vibration to occur that's related to the number of poles and how often the failing pole passes an armature winding.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BigJohn1]

I follow where you're coming from, but can say that I have found shorted rotor poles on machines before that did not have obviously deficient vibration analysis results. It's worth a try, but in my experience that is not a guaranteed symptom.

 

[MikeHalloran]

All I can think of is recording an IR video of the rotor in operation.
Better if you can mark the poles uniquely, and sort of sync the frame rate to rotation.
The idea is to get a slow motion video, in focus and not blurred, of the poles passing, and marked in such a way that you can identify which is which.
There will be some challenges.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

An air-gap search coil in the rotor bore should be able to see field assymmetry, although I haven't heard of one being fitted to such a small machine. Might be worth talking to the OEM and see if they have ever fitted such a coil, of if they can recommend someone who has done so. I would imagine that installation is easier on a salient pole machine where the bore is short relative to the diameter and where the clearances are a little more generous compared to a turbo machine but in your case this sounds quite a tricky design to work on.

 

[BigJohn1]

Not familiar with what a research coil is? I'll have to do some reading.

Also, to be clear, I'm looking at the rotor because it's the only test that is slightly "off" but I have no solid evidence that's the location of the fault.

We've discussed infrared, but you can't see the stator, and can barely see parts of the rotor: It's another case of just because we can't see s problem doesn't mean it isn't there.

 

[itsmoked]

I think it's called a search coil not a research coil.

I have a FLIR thermal camera and I'd scream at a 1 degree temp difference but I'm not sure how fast its framerate is so I'm not sure how it would perform on your spinning rotor. Be nice if you could borrow one and try it. You might see one rotor bar that's significantly hotter.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

John,

Have a look at this paper from GE:

https://powergen.gepower.com/conten...ger-2668-interturn-short-circuit-detector.pdf

In my opinion the Mk. 1 human eyeball and an oscilloscope is still the best way to observe the trace, but some manufacturers have added a computer and attempted to use software diagnostics to recognise a shorted turn as an online monitoring system. I remain unconvinced about this based on the ones I've seen, but one day someone will get it right.