Jr has correctly answered, and I wrote quite a bit about this (coal plant operation from the control system side) several years ago here. But if there are further questions please ask and I'll elaborate.
I have an unanswered question. Let's say there's a power plant that's produces 100 MWatts of electricity to match a demand load of 100 MWatts. As the demand tapers off to say MWatts and the generator continues to produce 100 MWatts what happens to the other 50 MWatts being produced by the generators? There has to be a conservation of energy going on here. I'm thinking reduced load (less current draw) would result in much higher voltages and possibly higher Hz? Where/how is that extra energy dissipated?
This seems to be where people are confused.
As the load tappers off, the generator follows and does not continue to to produce 100 MW.
As the load drops, and with the turbine still pushing as hard, the generator will want to speed up since the force holding the generator back is now less the force pushing it faster.
However.......
The controls notice the very very slight increase in speed and close the steam valves a bit. As the load continues to drop the control system keeps closing the valves and and the coal/gas/oil feed to boiler.
Thus nothing to dissipate. The generator and turbine follow the load.
The generator always runs at a speed that gives 50/60 Hz depending on your system. Voltage is not controlled by speed, it is controlled by alter-ex/gener-ex/exciter which provides DC to slip rings on the rotor that provides the magnetic field. Stronger field gives higher voltage.
Load is a function of how hard you have to "push" the generator to stay at the right speed.