700MW Rotor End Bells Damage

[Bekza]

Hi all,
I have a question regading 700MW generator rotor.
1. What kind of damage will the generator experience during switch on to stand still (SOS). I have seen pictures of rotor which suffered SOS but not sure exactly what else could go wrong?
2. If say 3000 rpm rotor is turning at lower speed, say 1500 rpm and is suddenly connected to the 5oHz network by mistake, what are the risks to the rotor and stator?
3. Rotor coil retaining rings (end bells), beside supporting the overhang, what else are their purpose? Others are shrink fit while others have end wedge gap?
4. How can one establish that the damage mark on the CRRs (end bells) are mechanical or electrical, how does a concept of fretting link to these?

Your advice will be appriciated.

 

[edison123]

If say 3000 rpm rotor is turning at lower speed, say 1500 rpm and is suddenly connected to the 5oHz network by mistake, what are the risks to the rotor and stator?

That machine would be blown away from the foundation, I guess.

 

[rheikaus]

Does it really matter what the failure mode is in the scenario you describe? It will be a motor load on the system rather than a generator and will fail very badly. You will incur significant downtime and in my experience it's not worth trying to stock spare parts in anticipation of this failure.

If this is a concern of yours, hopefully you have reverse power protective relaying on your system to minimize the damage.

 

[alehman]

You scenarios are too disastrous to contemplate. Make sure your protection is set up to prevent it.

 

[byrdj]

If the Generator stator is connected back to the grid with the rotor less than rated AND no field excitation, the unit becomes an induction motor and will roll to speed, The slip speed between driving stator frequency and rotor will cause induce current on the surface of the rotor. The design of rotor, with slots covers for the field copper will allow arcing of the induced surface current and cause localized heating. the localized heating will result in metal damage that will cause the development of cracks.

The unit will need to have the slot covers removed and any current damaged metal ground away. I have seen units that rolled from zero speed for >5 minute (don't recall how fast it got, say < 500 rpms )and the damage repairable with just removing and cleaning the slot covers.

If excitation, the driving force applied to the rotor will be much greater and mechanical damages will be severe

the purpose of the retaining rings IS to retain the copper end turns of the field. The forces to do so vary as per amount of copper and those the method to retain depends on force imposed. for a 700 MW, 2 pole will be much greater than a 100 mw 4 pole and higher strenght rings and the method of securing (shrink fit) are required.

determing the effect of any imperfection for a high strenght retaining ring requires an expert. and considerable concern when discovered