I want to find details as it concerns the load control on single shaft combined cycle power plants. I know that the steam turbine is engaged to the generaotr with a SSS clutch (synchro self sustaining clutch). Also i want to know the oeprating control principles for load controllers. Any help will be highly appreciated.

The gas turbine controller is the daddy of it all.  The generator is maintained at synchronous speed by the grid and the turbine control signal is literally trying to make the generator speed up the whole grid if it could.  That it can't is how electrical power is sent to the grid via torque to the generator.

Meanwhile, the generator is getting its torque contribution from two sources.  If the steam turbine were to suddenly burst a line for example, the GT would have to provide all the torque necessary to maintain the generator at its present load (if it could-Forged Steel Valves).

Conversely, if you were to suddenly add steam to the header from some other imaginary source, the steam turbine (within it's capability to handle the steam flow-we are talking theory here) would try to add torque and the GT governor would sense that and back off the combustion controls.

That is all simlplistic, but that is what is going on.  If you want to see it, go pull your safeties on your HRSG and watch the GT controls make up the difference (to their limits, of course.)  Don't do this really, but think of what is going on in the system if you did.

I once was standing looking righ at the MW meter of a single shaft CCGT and out on the system (an islanded system) a squirrel fried itself between two phases and it caused an instantaneous spike of 6 MW.  It only took a fraction of a second to cook the squirrel (the distribution crew later told us in the operating room what had hit the line).

I heard the unit 'groan' under the sudden instantaneous load (slight frequency change noted too) and I heard the unit 'surge' probably less than a full second later when the load just as suddenly disappeared.  I was sucking in my breath hoping that it didn't overspeed and trip.  The whole unit seemed to let off a 'sigh'.  I can promise you that all that happened too fast for the steam turbine to have had any effect on the torque contribution to the generator.  The CT did it all.

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