Generator problems 3

[samuelbb1]

We are having problems with a steam turbine generator rated 380V/1.25MW with brushless excitation.

Started 4 months ago with failing diodes (one or two) every 2 weeks (exciter/rotor/stator were megged and deemed ok), then every week (AVR was replaced with a spare) , then every day, then a a rewind of exciter rotor because it burnt (one of the phases overheated and shorted diodes were found). After rewind (4 days), remanent field was very hard to restore and sensing winding before turning on AVR sometimes showed 25V instead of usual 210V for this unit.

Synchronization is supposedly ok. Vibration levels are ok but operators feel it's different from say 1 year ago. Air is polluted with sawdust.

Most of the trips were over current or loss of field excitation. Logging system shows a reactive power positive peak (two times normal output) followed by a near zero down peak. Both peaks have a duration of less than 2 seconds, current shoots up during positive peak and usually trips. Peaks are more common during day time rather than night.

Specialists are suggesting a High-pot test (we don´t appreciate the idea of additional damage), a new rotor, or a new generator.

Our next step should be rotor removal and inspection. Any suggestions for a more precise diagnostic and less downtime?

 

[edison123]

Do a AC pole drop test and if possible, a Baker surge comparison test on the main rotor pole coils. I suspect turn shorts in the pole coils and the rotor has to be rewound.

Muthu

http://www.edison.co.in

 

[RRaghunath]

You say the plant is polluted with Saw dust.
Do you also have frequent faults in your 380V system??
Guess the generator is directly connected to 380V busbar.
The excitation getting pushed to twice its rated value could suggest the generator is maintaining its terminal voltage when there is an uncleared outgoing 380V feeder fault or 380V bus fault.
It would help understand the root cause if you also focus investigations in this direction.

 

[ScottyUK]

Likely you have a problem on the field winding, possibly one that will only show when the machine it at operating speed and/or temperature so you probably won't find it with a megger and ducter. Could be copper movement under the endbells where the windings have less support than in the slots. You might be lucky and be able to get a boroscope in among the windings to take a look.

 

[Hoxton]

Sorry Scotty, but at 1.25 MW it is likely to be a salient pole generator, probably 4 pole 1800 rpm. Not a 2 pole cylindrical rotor with end bells.

In any case I agree with everyone, likely to be a shorted turn in the rotor winding.

Increased or changed vibration levels support this, since the rotor magnetic field will be unbalanced with shorted turns.

At 1.25 MW it it likely to be open air circuit, so the windings could be contaminated with sawdust.

You could try taking the rotor out and getting a specialist repairer to clean and re bake it to dry it out. Do the same for the stator at the same time.

If that does not cure the problem, it is a rewind, or replacement.

What make is the generator and how old is it?

 

[ScottyUK]

Oops, missed the rating! Yes, agreed - it will be a salient pole machine.

 

[waross]

Likely you have a problem on the field winding, possibly one that will only show when the machine it at operating speed and/or temperature so you probably won't find it with a megger and ducter.

The first serious fault that I had to handle when I first started with generators was a brushed exciter that would only fail under working conditions. It would not fail when heated in an oven.
We had to run the machine until the exciter failed and then test quickly before it cooled down.
In the oven the windings and the core temperatures stayed close enough that there was no failure.
Under operating conditions the heat is generated in the windings and transferred to the core.
That caused enough differential movement to develop a short.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[samuelbb1]

Do a AC pole drop test and if possible, a Baker surge comparison test on the main rotor pole coils. I suspect turn shorts in the pole coils and the rotor has to be rewound.

Small update.

We chose to remove the generator rather than the rotor (effort was the same) and send to a repair center. A comparison test was done and at least 2 rotor poles were bad. The rotor is being rewinded with new copper and a higher temperature class insulation (but not uprated).

Will post all info once I get it.