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Bearing Temperature rises with increase in Excitation

168 Views 4 Replies 4 Participants Last post by  Corded, 
Moiz Farooqui
One of our 24MW,13.8KV STG was tripped due to the stator earth fault causing the blackout of the plant and resulting in the damage of the stator windings(U,W).
Our grid is and island grid with 3 STGs connected to the grid (2 of 24MW and one of 10MW)

The stator winding were repaired and one turn of each phase (U,V,W) was bypassed and new rotor was installed.
Rotor healthiness was tested through RSO test. However, stator windings was checked by megger test and Hi-Pot test at 17KV for 1 minute and was found ok.

At startup, we decided to check the generator through open and short circuit test. While performing open circuit test , the AVR voltages were increased manually with some steps. When the terminal voltage (Vt ) of the generator reached 11KV , the NDE (non-Driven End) Bearing Temperature increases to 425 C resulting in the tripping of turbine.

While, same test was repeated, and the same result was obtained but this time we decreased the excitation voltages which decreases the temperature of the NDE bearing protecting the turbine from tripping.

Kindly suggest what can be the real causes on the issues on the generator and what checks need to be taken before attempting another start-up??
I've attached the open circuit test check sheet

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OrlyP
Don't know anything about it. That's a big boy generator and compared to that, mine's a toy.

That said, you should maybe run another RSO test. According to the document below:

Operating the rotors with shorted turns may limit the loads due to higher excitation energy, abnormal vibration behavior, bowed rotors and more thermal dissipation. Catastrophic effects such as earth faults and bearing damages will happen.
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Good luck!
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A
Try this forum." International Generator Technical Community". They deal with large power generation systems.
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AKirkland said:
Try this forum." International Generator Technical Community". They deal with large power generation systems.
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I suspect that you still have a stator problem since it is requiring more field voltage and field current to achieve the required output voltage.
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C
Sounds like someone found a bad coil group and cut it out, they did the same on the other phases to balance things out ( questionable choice )

How old is the unit?
If its over 30 years the insulation may have issues.
I would give it a poke with a hi pot at about 1.5 times the RMS voltage rating to ground and see how much leakage there is... I am confortable with 15 kvdc being a non destructive test for 13.8v systems.

Test results less than 20 meg its dying quickly...
Less than 10 meg its already the walking dead if it runs at all.

Earth fault due to insulation failure.
Your buggered chum.
Not much to do but get the machine rewound or replace
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