Loss of Excitation

[pwrengrds]

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.

 

[waross]

I would take a look at the end rings on the damper winding for signs of overheating.
The ammortisseur winding may not be designed for continuous operation as an induction machine.


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

 

[ScottyUK]

Not sure there will be an amortisseur winding on a machine that size.

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.

 

[edison123]

I would check the rotor retaining rings, rotor wedges, rotor iron and rotor winding for any overheating marks. A two pole machine can survive a short term loss of excitation unlike multi pole hydro machines.

How did they rewind a machine for twice the original capacity?

Muthu

http://www.edison.co.in

 

[pwrengrds]

It wasn't actually twice, from 6250 kVA to 8163 kVA; 5000kW to 8000kW.

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.

 

[ScottyUK]

Did it actually maintain synchronous speed, or did it drop to a sub-synchronous speed? It makes a big difference to the machine. Have you got a Historian on your turbine controller which records speed?

 

[edison123]

Agree with Scotty. A small change in speed affects the synchronous machine reactions drastically.

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

http://www.edison.co.in

 

[pwrengrds]

The operators said it did, the data is missing due to the loss of DC

 

[waross]

As for the upgrade: Is this a very old machine?

http://wiki.vintagemachinery.org/Motor%20Frame%20Size.ashx[/URL]]THREE GENERATIONS (pre 1952,1952-1964, 1964-present)...
Over the years, motors' horsepower ratings have continued to increase. This is in part due to improvements made in insulating materials. As a result of this improved insulation, motors can be run much hotter than they could in the past.

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

 

[ScottyUK]

Many generators are insulated as Class F machines, but designed to operate at the Class B limits. Operating a Class F machine at the Class F limits would certainly yield a decent increase in output, albeit at the expense of operating life.

 

[pwrengrds]

It's a mid 1960's GE machine. In 2000 it had a stator ground fault was was rewound in 2001 by GE which re-wound and re-rated it.

 

[waross]

Thanks for the information, pwrengrds.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

The speed could not have been synchronous as the excitation was off.
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