There are a couple of points here. One is mentioned above. The other is the type of generator. A synchronous alternator is designed to generate power at a given frequency when rotated at a given speed by some external means such as a steam turbine or diesel. It is separately excited via DC and can provide its output alone (no need to be connected to the main grid). This is in contrast to an asynchronous generator such a a double fed induction generator often used in wind turbines which must be connected to an established grid in order to be able to generate power and will run slighlty above the synchronous speed by the slip necessary to deliver the required power.