Simply how AVR (Automatic Voltage Regulator) works is as follows :
AVR continuously checks the voltage at the output of the generator set.
Then controls the excitation system of the generator accordingly.
If Voltage at the output is low, then increase current for excitation to achieve the correct voltage. And inversely, if the output is high, then AVR lowers the excitation accordingly.
This is the simple regulation logic of the system.
Of course how it is working in details is a very complex matter as you can see in the drawing, in the message before this one. Thanks to the sender.