Well said David.
A few more comments:
Excitation; With a stand alone or islanded set the excitation or voltage adjustment controls the voltage.
When the same set is operated in parallel with the grid, the voltage adjustment controls the voltage in the ratio of
set capacity/grid capacity This ratio is so small that in practical terms there is no change in grid voltage. The excitation level or voltage adjustment does control the amount and direction of reactive power.
Real power; The throttle, gas valve or steam valve (power in to the prime mover) controls the speed and frequency of an islanded set. The throttle setting or power in to a grid connected set controls the real power delivered to the grid.
Expanding on David's comments, when the generator output is less than the local load, the grid supplies the power deficit. When the power output is more than the local load, the flow of power reverses and the excess power is delivered to the grid.
How do the sine waves interact?
The sine waves synchronize and lock together, almost. If the sine wave of the generator is slightly ahead of the sine wave of the grid, power will be transferred to the grid. If the generator sine wave drops slightly behind the grid the generator will become a motor and the grid will drive the machine.
Control; Co-generation sets are generally operated in droop mode. The utility will specify the droop setting. This gives good control. Droop allows the set to track with minor frequency variations and also acts to correct frequency variations. Droop control also gives an acceptable response to major system disruptions. Droop control prevents runaway should the set become disconnected from the grid.
Example; If the droop setting is 2% then a frequency setting of 102% or 61.2 Hz will result in 100% loading. A frequency setting of 101% or 60.6 Hz will result in 50% loading.
Bill
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"Why not the best?"
Jimmy Carter