Synchronization ofSynchronous generators

We often have to connect two or more generators in parallel to supply a common load. For example, as the power requirements of large utility system build up during the day, generators are successively connected to the system to provide the extra power. Later, when the power demand falls, selected generators are temporarily disconnected from the system until power again builds up the following day. Synchronous generators are therefore regularly being connected and disconnected from large power grid in response to customer demand. Such a grid is said to be an infinite bus because it contains so many generators essentially connected in parallel that neither the voltage nor the frequency of the grid can be altered.
Before connecting a generator to an infinite bus (or in parallel with another generator). It must be Synchronized. A generator is said to be synchronized when it meets all the following conditions:
1- The generator frequency is equal to the system frequency.
2- The generator voltage is equal to the system voltage.
3- The generator voltage is in phase with system voltage.
4- The phase sequence of the generator is the same as that of the system.