Multiple generators on a shaft

[bartb102]

I am working in an older hydroelectric powerhouse. Original generators had two sections, one at 800KW @4160V, the other at 1MW 250VDC. Exclusively powered an industrial plant. In '70s DC was removed, and AC section was rewound for 1.5KW @4160V. River is tidal at this location, and at low tide we have to limit power generation to prevent overheating the windings. We are looking for someway to extract the additional power available from the turbine, be it a DC winding or an additional AC winding. Some schemes may require a motor/generator set to convert to normal mains power (hard to get two AC windings on the same shaft syncronized).

 

[Skogsgurra]

A far fetched idea, perhaps. But what about an MV inverter regenerating back to mains? May be difficult if you are islanding, but will definitely work if you are tied to the grid.

That way, you can either reinstall a DC generator and skip the rectifier part of the VFD or put an AC generator there and have a complete rectifier/inverter do the regeneration.

The more I think about it, the more I like the idea. No MG set. Better efficiency. Less maintenance. More flexible control.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

How's your power factor? You can push your power factor up to 100% on the generator with external capacitors or a synchronous condensor.
Include protection to avoid a leading power factor in the event of a load loss.
If you are on a grid, try dropping the excitation to push some vars over to another part of the system.
If you are islanded, try power factor correction.
If you are tied to a grid, try reducing excitation and using power factor correction to supply the vars to the grid that the generator is shedding.
I assume that the figure for 1.5 KW should have been 1.5 MW.
At a pf of 80% the MVA rating will be 1.875 MVA. If you improve the power factor you can go to 1.875 MW if your turbine can turn it.

Option 2. Change transformer taps or use an autotransformer to reduce the voltage.
If you boost the voltage you can boost the output MWs and MVAs. Then use the taps or autotransformer to match the voltage to the system.

One advantage of a second winding on the same shaft is when you do get it synchronised it will stay synchronised.
You can use a heavy flange type coupling with slotted bolt holes. Use some method to compare the phase angles. When you get the position right you can synchronise without load and check for circulating currents. Then drill and ream a hole centered on the faces of the flanges and install a round key.
Adjustment of the voltage and/or power factor is probably cheaper.
respectfully

 

[bartb102]

Thanks waross. We are looking into raising the output voltage, success will depend on having enough excitation capability. This will probably be most cost effective way to boost output. We are already operating at 95%PF leading, so increasing to 100%PF won't buy us much current. Part of our problem is we do not have capability curves for the units, they were constructed in the field and to our knowledge were never tested. The plant only supplies the grid, cannot island.