this posting should more appropriately go to Electric Power Engineering.
nevertheless, i will attempt an explanation in layman's terms...
the unit has different ways to operate.
1. isochronous: the unit will maintain the speed no matter what load is demanded - týpical form: island mode operation.
2. droop control: the unit reacts delivering more load when the grid frequency drops, and viceversa unloads when the frequency rises
3. partial pre-selected load: the unit maintains a certain load independent of the grid requirements
4. base load: the unit is at its nominal load (cannot admit any more steam) and will follow frequency changes.
Power output = torque x speed
torque being constant.
The operator can raise the load with the governor only if the turbine did not reach its base load (is at a partial load) - and there is load available in the grid.
how does the generator accept more load? it raises the excitation current.
so, if there is no more load available in the grid... and the operator keeps raising with the governor the following will happen:
if there is no ISOCHRONOUS DRIVER in the grid...
the grid frequency and voltage will increase.
if there is an isochronous driver, this driver will unload to keep the frequency constant.
to understand the effect on voltage it is easier to understand it when some of the generators trip off-line...
there are only some generators left, which cannot supply all the load required... the frequency drops (it actually works like a brake on the generator) and the voltage drops (lights go dimmer) - you've surely seen this before.
hope this helps.