In practice, a very large number of 3-phase alternators operate in parallel because the various power stations are interconnected through the national grid. The electric power systems are interconnected for economy and reliable operation. This interconnection of power systems requires alternators to operate in parallel with each other.
In a generating station, two or more alternators are connected in parallel (as shown in Figure). Also, in an interconnected system forming a grid the alternators are located at different places and they are connected in parallel by means of transformers and transmission lines. Under normal operating conditions, all the alternators in an interconnected system operate in synchronism with each other. The parallel operation of alternators ensures greater security of supply and enables overall economic generation.
Because of the following reasons, the alternators are operated in parallel −
Continuity of service – When one alternator is taken out of service for its scheduled maintenance and inspection, the remaining alternators maintain the continuity of supply.
Efficiency – During the periods of light load, one or more alternators may be shut down and those remaining operate at or near full-load and hence more efficiently.
Several alternators operating in parallel can supply a bigger load than a single alternator.
If there is a breakdown of an alternator, there is no interruption of the power supply.
Load growth – In order to meet the increasing future demand of load more alternators can be added without disturbing the original installation of the power system.
Economy – The operating cost and cost of energy generated are decreased when several alternators operate in parallel.
The process of connecting one alternator in parallel with another alternator or with an infinite busbar is known as synchronization. Those alternators which are already carrying load are called running alternators, while the alternator which is to be connected in parallel with the system is called incoming alternator.
In order to connect an alternator safely in parallel with another alternators or busbars, the following conditions should be satisfied −
The phase sequence of the busbar voltages and the incoming alternator voltage must be the same.
The busbar voltages and the terminal voltage of the incoming alternator must be in phase.
The terminal voltage of the incoming alternator must be equal to the busbar voltages.
The frequency of the generated voltage of the incoming machine must be equal to the frequency of the voltages of the busbar.