What is synchronization of alternators?

The frequency of a large electric power distribution system is established by the speed of rotation of many powerful alternators all connected by various tie-lines in the total network. The collective inertia and power of these generators is so great that there is no single load or disturbance which would be large enough to change their speed of rotation. The frequency of an electric system is, therefore, remarkably stable. An alternator can only deliver power to an existing electric power system if it operĀ­ates at the same frequency as the system. A system whose frequency is 50 Hz cannot receive power from an alternator operating at 50.01 Hz. They must both operate at exactly the same frequency. This is not as difficult to achieve as may appear at first, because (when an alternator is connected into an existing system) automatic forces come into play to keep its frequency constant. Synchronization of an alternator with a large utility system, or "infinite bus' as it is called is comparable to matching a small gear to another of enormous size and power. If the teeth of both gears are properly synchronized upon contact, then the matching will be smooth. But should the teeth edges meet shock would result with possible damage to the smaller gear. Smooth synchronization of an alternator means first that its frequency must be equal to that of the supply. In addition, the phase sequence (or rotation) must be the same. Returning to our example of the gears, we would not think of trying to mesh two gears going in opposite directions, even if their speeds were identical. The next thing to watch for when we push gears together is to see that the teeth of one meet the slots of the other. In electrical terms the voltage of the alternator must be in phase with the voltage of the supply. Finally, when matching gears we always choose a tooth depth which is compatible with the master gear. Electrically, the voltage amplitude of the alternator should be equal to the supply voltage amplitude. With these conditions met, the alternator is perfectly synchronized with the network and the switch between the two can be turned on. Ref aus lab script