Loss of Excitation
Loss of Excitation
(OP)
A 5MVA (rewound to 10MVA), 13.8kV steam turbine generator was operating normally. Some of the control screens started operating oddly and the operators decided to trip it off, so the power was lowered to 0.5MW and the stop valve tripped. It turned out the control screen issues were due to low DC voltage. The stop valve opened, the exciter tripped, the generator breaker did not trip, the DC trip coil started smoking and eventually burned up. The operators called an off site plant engineer for advice, were told to trip the breaker manually, saw the smoke and would not open the cabinet door. The off site plant engineer came to the site and tripped the generator breaker off manually after 45 minutes of reverse power. After some testing of the rotor and exciter and repair of the DC system the generator is back on line without apparent ill effects. (currently running for the second day) The excitation system is a Basler SSE half wave bridge with a fly back diode. The generator had about 5MVAR and 300kW going into it for the 45 minutes it was in reverse power.
Would any damage be expected on the stator? It was at 50% FLA current rating.
What amount of current was going through the rotor? Any damage? I think, not much due to it operating near sync speed, only needing enough induction to keep it near sync speed (3600RPM).
Advice and comments appreciated.
Would any damage be expected on the stator? It was at 50% FLA current rating.
What amount of current was going through the rotor? Any damage? I think, not much due to it operating near sync speed, only needing enough induction to keep it near sync speed (3600RPM).
Advice and comments appreciated.





RE: Loss of Excitation
The ammortisseur winding may not be designed for continuous operation as an induction machine.
Bill
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"Why not the best?"
Jimmy Carter
RE: Loss of Excitation
If there is damage it will almost certainly be on the rotor. In the first instance look for burnt paint on the rotor. As it is a small set and power levels involved are fairly low I think you'll probably be OK. A similar set of events on a much larger set would likely have caused some problems.
RE: Loss of Excitation
How did they rewind a machine for twice the original capacity?
Muthu
www.edison.co.in
RE: Loss of Excitation
My question was more about what would be induced into the rotor on a running machine. Since it's at sync speed should't the induced current be relative small? Only enough to keep it at speed, which was a few hundred kW. I thought initially that the currents would be high, but on reflection, since it's at speed rotor currents would be low since there is a very low load.
RE: Loss of Excitation
RE: Loss of Excitation
As for uprating, 5 MW to 8 MW is still a big jump requiring more than double the original copper cross section for the same temp rise. Unless of course, the machine was originally designed for a higher capacity and then was derated for some reason.
Muthu
www.edison.co.in
RE: Loss of Excitation
RE: Loss of Excitation
Comparing an old pre 1952 motor frame with a post 1964 T-frame motor, the HP has approximately doubled for most given frame sizes.
Bill
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"Why not the best?"
Jimmy Carter
RE: Loss of Excitation
RE: Loss of Excitation
RE: Loss of Excitation
That would be a reasonable ratio of increase from a pre 1964 standard motor (U Frame) to a post 1964 standards motor (T frame).
Bill
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"Why not the best?"
Jimmy Carter
RE: Loss of Excitation
In an induction motor, there would be slip but considering that the load is very small, probably the slip was also quite small and may not have caused much circulating currents in rotor.
How about the turbine side itself?
Low load or no-load running results in turbine blade tip overheating. Any such ill effects on turbine?
It is likely that the turbine is back pressure type and hopefully, not affected by the incident.
Rompicherla Raghunath