How can control power factor in power plant ?

When the synchronous generator is pushing reactive power into the electrical system we say the machine is over-excited. When the synchronous generator is absorbing reactive power from the electrical system, it is under-excited. So, the reactive power output of the generator is associated with the generator field current, provided by the excitation system. 

But note that we do not control reactive power output of a generator, at least in terms of automatic control. Synchronous machines are usually operated on automatic voltage regulator (AVR) mode, so we are actually controlling voltage. The reactive power output of the generator is thus automatically adjusted as needed to maintain the specified voltage.

Conversely, if you try to control reactive power output or power factor in a generator, you will not be able to control voltage.

And we should operate the synchronous generators on AVR because of stability, the AVR is a proven solution to several stability issues (steady state stability and transient stability).