No-one here has addressed the more fundamental question: Where does unused energy go? Or, more practically, why do grid operators go to such lengths to carefully balance power flow to ensure that there is no excess energy to go anywhere?
Answered early on in the thread. The energy comes from the fuel. If you don't need that load level, you don't put the fuel into the boiler to start with and don't generate excess energy.
If you had no grid operators, a system of generators with changing loads will follow the overall system load. That is as long as your control system keeps each unit spinning at 3600RPM. (This is why any movie or tin foil hat person that talks about hacking the power plant/system computers is an idiot. Plants can and do run just fine without gird operators or computers of any kind.)
Grid operators are balancing several things. One is you can't overload feeder lines. Another big one is overall system efficiency. You have a bunch of units, built at different times, and they all have a different curve for heat rate vs. load. In fact it is not the operator that does this but a routine that runs abut once a second that computes the best load for each unit and then bumps one ups 1MW and another down the same amount. Overall the total generated power is the same, but now it is more efficient overall.
The other thing they do is sell or buy power. Some it short term, some longer term contracts.
Say you have a city with several units generating power for it. Outside of your city you have tie lines to other utility companies. Just sum up the power flow on the tie lines with outbound being negative and in being positive. When it sums to zero, then you know that what you are generating is the load of your city. If someone is buying power from you, then all you do is bias the equation, rather than zero, the amount you are buying or selling. Then about once every tens seconds you make up for any over or under that flowed on the tie lines.
Now in terms of sudden load loss. Yea that happens, like when a unit trips and the generator breaker opens. The control system is very very fast, and the hydraulics that move the valves are to be respected. Not much overspeed is tolerated by the controls. The emergency all mechanical trip is set at 110% percent of rated speed if I remember right.
Normally you have the main steam feed split into two "stop valves". As the name implies they are meant to stop the flow, right now. Either open or closed. Pressure is required to keep them open, and large springs and the steam flow wants to push them closed. Pull the top off of one and take the stuff out and you can sit inside one. From that point it goes to 4 control valves. These modulate the steam flow. Once a unit trips these 6 along with the reheat control and stop valves slam closed
Or if you want to lose your job there is a big red button on the front of the turbine that is an emergency trip. Just push it.
That button does get tested, also the turbine overspeed trip is tested, and then the backup mechanical overspeed trip is tested. This happens after an outage, (once a year for 2 or more weeks) after the turbine is buttoned back up and just before it is put back online. Of course during the test the generator breaker is open, so the trurbine is free to run at any speed. The "load" is just the frictional losses.