Electric Power system are interconnected for economy and reliable operation. Interconnection of ac power system requires synchronous generator to operate in parallel with each other. In generating stations, two or more generators are connected in parallel. In an interconnected system forming a grid the alternator are located at different places. they are connected in parallel by means of transformers and transmission line. Under normal operating conditions all the alternator and synchronous motors in an interconnected system operate in synchronism with each other.

An arrangement of generators for parallel operation enables a plant engineer to adjust the machines for optimum operating efficiency and greater reliability.s the load increases beyond the generated capacity of the connected units, additional generators are parallel to carry the load. Similarly, if the load demand decreases, one or more machines are taken off the line as per the requirement. It allows the units to operate at a higher efficiency.

Reasons for Parallel operation
  • Several alternators can supply a bigger load than a single alternator.
  • During the period of light loads, One or more alternators may shut down and those the remaining alternator operates at near or full load and thus more efficient.
  • When one machine is taken out of service for its scheduled maintenance and inspection, the remaining machines maintain the continuity of the supply.
  • If there is a breakdown of the generator, there is no interruption of the power supply.
  • In order to meet the  increasing future demand of the load. more machines can be added with disturbing the original installation.
  • Parallel operation of the alternator, reduces the operating cost and the cost of energy generation.
  • Parallel operation of alternator ensures greater security of supply and enables overall economic generation. 

Necessary Condition for Parallel operation
Most synchronous machine will operate in Parallel with other synchronous machines and the process of connecting one machine in parallel with another machine or with infinite bus bar system known as synchronizing. those machines already carrying load are known as running machines, while the alternator which is connected in parallel with the system is known as incoming machine. Before the incoming machine is to be connected to the system , the following condition should be satisfied.

  1. The phase sequence of the Busbar voltages and the incoming machine voltage must be the same
  2. The Busbar voltages and the incoming machine terminal voltage must be in phase.
  3. The terminal voltage of the incoming machine should be equal to that of the alternator which it is to be run in parallel or with the Busbar voltage.
  4. The frequency of the generated voltage of the incoming machine must be equal to the frequency of the voltage of the live Busbar.


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