Synchronous generator speed is set by grid. Governor valve just adjust the torque. So how Frequency control plays an important role to avoid alternator loss of synchronism.

Lets say alternator is at 3000 rpm (50 Hz) frequency now on addition of load the frequency decreases. How much time the alternator takes to respond the new frequency and accelerate and decelerates as per frequency change.

During that transient stage (during accelerating and deceleration) how rotor is said in synchronism. How rotor oscillations can be controlled.