Generator reactive power

After the generator connected to grid, the generator will be more stable than before connected to grid, because in this situation the frequency and voltage are fixed and controlled by the grid, not the independent generators. How much active and reactive power you can contribute to the grid depends on the grid requirement, such as when the grid shorts of active power, the frequency of the grid will drop, and then the grid will ask you or other generators to contribute more active power, and if short of reactive power, voltage will drop, then you could be asked to contribute more reactive power, and vice versa, which depend on the balance of power which is generated from generators and consumed by the users.

From generator side, the less reactive power, the better, as this power increase the VA and then the current to increase the losses on the transmission line which will be carried by the plant. But from grid side, as not too many equipment can generate the reactive power, the more contribution of the reactive power, the better.

At the full load operation of generator, the maximum contribution of reactive power should depend on the PF of the generator at full load (manufacturer provided for each generator). If your PF is too low and it could affect your active power transfers to the grid and will be punished by the grid. At the not full load situation of the generator, the PF could not be decided by the generator, if the grid does not need too much active power from you, but needs more reactive power and asks you to contribute more, PF could be more than 1 at the moment, but never over the Max reactive power calculated from full load.