Flux Probe monitoring on 4 pole, 1800 rpm generator

[TheCairn]

We are installting a 25 MW 4 pole, 1800 RPM generator made by Converteam. Our electrical department has requested that we look into having a flux probe sensor installed in the unit however the manufacturer has indicated that there is very little value on a 4 pole 1800 rpm machine of this size.

Does anyone out there have any experiences with or comments on the value of this monitoring on this type of generator?

 

[Skogsgurra]

I think you should let the electrical deparment have its way. Do they comment on your mechanical arrangements? Or question the need for them?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[TheCairn]

I would tend to agree with you however our electrical department does not get into generation issues on a regular basis and I have a very proficient power systems engineering consultant agreeing with the supplier on this one. While I don't want to get into the politics of project management and discipline boundaries, suffice it to say we get along very well here. I am merely trying to sort through conflicting information perhaps with some unbiased experience. Electrical still makes the electrical decisions.

So, back to the question, how useful is the flux sensor on our type of machine?

 

[electricpete]

I presume the purpose of the flux probe if to detect shorted turns. There is a dramatic difference in progression of shorted turn fault in an ac winding (like motor stator) compared to dc winding (like generator rotor). The ac winding short immediately brings local high currents in the shorted loop due to auto transformer effect, which causes rapid escalation of the fault. On contrast the dc winding has no auto transformer effect and the current change is only a result of impedance change which usually is small, especially with several could in series. So we expect a lot of time to detect the shorted rotor winding and schedule repairs before the fault disrupts the machine. Fault can be seen from increased vibration, and possibly change in field current for a given operating condition.

That's not a definitive rbasis

just some thoughts fwiw.I'm not a big generator guy.


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(2B)+(2B)' ?

 

[electricpete]

My apologies. I'm typing on a tiny phone screen and having some problems

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(2B)+(2B)' ?

 

[ScottyUK]

Hey Pete, you managed fewer typos on that little screen than I manage on the big one.

In my opinion unless you have the rotor out for some reason then it would not be worth the effort of unthreading it specifically to fit an air-gap search coil. If the opportunity is there then it is certainly worth considering as the cost of installation isn't all that high and a 25MW set is still a fairly pricey piece of plant.


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

 

[TheCairn]

As the generator is not yet built we could have an opportunity to have the generator manufacturer install a sensor at the factory. I've looked into the price of the Iris sensor (~$5k) which seems high for the little black dodad with some wire and a JB. It is what it is though.

 

[Skogsgurra]

"Seems high"? I would say an insignificant addition to the cost of a 25 MW machine. Why are you at all worrying about this?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[TheCairn]

I really should not have brought up the cost because the sensor is only the tip of the iceberg and it is not terribly relevant. It will cost whatever it costs.

I want to know if anyone with in the field experience has seen flux probes provide a real benefit to a ~25 MW 4 pole machine? What were they? Did you catch a failing winding or something else? is there someone out there who wishes they had flux monitoring as it would have saved a headache or two?

It's about utility. I could probably pay to get pin striping put on the generator. It would look nice but it doesn't change the performance or reliability so what 's the scoop? Is this flux monitoring pinstriping or practical?

Forget I mentioned pinstriping. I'll probably hit a nerve with the hotrod crowd.

 

[ScottyUK]

It's a means of detecting a shorted turn on the rotor. A shorted turn on the rotor will probably not hurt anything in the short term, but at high excitation you may develop a thermal bend in the rotor which will cause an imbalance and maybe a vibration trip. The benefits of having it on a small unit are no different to the benefits on a large unit.

Really it is like any protection - an insurance policy against something you hope will never happen. How critical is the generator to your business? If you identify a fault you can plan a repair at your convenience. An undetected fault will probably announce itself in a more aggressive manner at some future date. Can you afford the downtime?


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

 

[pkelectengr]

I would suggest that you people should have excitation system monitoring on your generator.
which will comprise on dc voltmeter showing output dc voltage of avr (input to exciter stator) and current (to the exciter stator).

If you have not installed them, get them installed first.

But if you have already installed the above mentioned meters for monitoring, you should get flux probe installed as well it will also give you a plus in condition monitoring.

But I have a question that does any one knows the method to get main rotor current, exciter rotor current and voltage on meters??? as they are rotating parts. So, I wonder how to get them????

Thanks

 

[iwright]

I would install the flux probe. At the company I work for we have installed flux probes on all of our 2 and 4 pole generators from ~25MVA up to 360MVA, and we continue to install probes on new units. We use the GeneratorTech version as we started installing these before the Iris model was on the market.

There are other ways to detect a shorted turn but a flux probe is the most accurate online method. The flux probe can tell you there is a shorted turn long before you have vibration problems. If you know about the problem before it is severe you can plan to fix it when it is convenient.