Interconnection of the electric power systems is essential from the economical point of view and also for reliable and Parallel Operation. Interconnection of AC power systems requires synchronous generators to operate in parallel with each other. In generating stations, two or more generators are connected in parallel. The alternators are located at different locations forming a grid connected system.
They are connected parallel by means of transformer and transmission lines. Under normal operating conditions all the generators and synchronous motors in an interconnected system operate in synchronism with each other. A machine has to be adjusted for optimum operating efficiency and greater reliability if the generators are connected in parallel.
As 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 of Parallel Operation
Alternators are operated in parallel for the following reasons:
- Several alternators can supply a bigger load than a single alternator.
- One or more alternators may shut down during the period of light loads. Thus, the remaining alternator operates at near or full load with greater efficiency.
- 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.
- Number of machines can be added with disturbing the initial installation according to the requirement to fulfill the increasing future demand of the load.
- Parallel operation of the alternator, reduces the operating cost and the cost of energy generation.
- It ensures the greater security of supply and enables overall economic generation.
Necessary Conditions for Parallel Operation of the Alternator
Most synchronous machines will operate in parallel with other synchronous machines. The process of connecting one machine in parallel with another machine or with an Infinite Busbar system is known as Synchronizing. The machine carrying load is known as Running Machines while the alternator which is to be connected in parallel with the system is known as the Incoming machine.
The following condition should be satisfied for parallel operation are as follows:-
- The phase sequence of the Busbar voltages and the incoming machine voltage must be the same.
- The Busbar voltages and the incoming machine terminal voltage must be in phase.
- The terminal voltage of the incoming machine and the alternator which is to be connected in parallel or with the Busbar voltage should be equal.
- The frequency of the generated voltage of the incoming machine and the frequency of the voltage of the Busbar should be equal.