Sorry waross, but I beg to differ.
A basic AVR will control the voltage in single running mode and can control the voltage droop when in parallel mode.
In parallel mode there are two (of many) modes you do not want to enter:
Over excitation – excess field current can overheat the rotor.
Under excitation – low field current can result in the generator rotor not producing sufficient magnetism to hold the rotor in synchronism and it could ‘pole slip’.
There are AVRs with these features available – at a price. It does add to system complexity and these features are often not used in simpler applications.
This is a simple answer to what can become a complex area.
I suggest that the OP googles some of the terms in this response, and searches AVR manufacturer's websites (BASLER, Comap, Deep Sea etc)