It keeps the gen voltage the same at the set value
irrespective of variation in speed , load and grid voltage
after synchronization. It is useful whenever there is grid
failure as the gen speed suddenly increases to an extent
depending on the load interrupted.Then AVR does not allow
for rise in voltage.It is also very helpful while
synchronizing as you do not have to vary the excitation
every now and then ,once you have matched the voltages.If
you desire to keep the pf constant after synch and loading
,you have to switch over to APF mode from AVR ,both being
available with Auto channel.
Once the generator connects with the grid the system voltage depends upon to the grid referrence.the active power varies depends on the load.so load varies mean time reactive power also variey.so avr will maintaine power factor after synchronization.
After synch,AVR keeps the gen voltage constant at the set
value. If the grid voltage is more than the gen voltage, the
pf becomes leading and lagging if the grid voltage is less
than the gen voltage.Then, in AVR mode the operator has to
reset the gen voltage so that the pf is slightly lagging.In
APF mode ,the pf always remains at the set value
irrespective of variations in grid voltage.