The operation of connecting an alternator in parallel with another alternator or with common bus bars is known as synchronizing. Generally, an n. a number of 3-phase alternators operate in parallel because the various power stations are interconnected through the grid. Therefore, the output of any single alternator is small compared with the total interconnected capacity. For example, the total capacity of the interconnected system may be over 50,000 MW while the capacity of the biggest single alternator may be 500 MW, therefore, we have to require n number of alternators in parallel.
An alternator connected to such a system is said to be connected to infinite bus bars. The electrical characteristics of such bus bars are that they are constant-voltage, constant-frequency bus bars. The alternators may be connected to or disconnected from the infinite bus, depending on the load demand on the system. The fig. is shown below the parallel operation of the alternator-
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Conditions for Paralleling Alternator with Infinite Busbars:
In order to connect an alternator safely to the infinite busbars, the following conditions are met:
(i) The terminal voltage (r.m.s. value) of an alternator must be the same as the busbar's voltage.
(ii) The frequency of the generated voltage of an alternator must be equal to the busbar's frequency.
(iii) The phase of the incoming alternator voltage must be identical to the phase of the busbars voltage.
(iv) The phase sequence of the voltage of the incoming alternator should be the same as the phase sequence voltage of the busbars.
Advantages of Parallel Operation of Alternators:
The following are the advantages of operating alternators in parallel:
- Efficiency: The load on the power system varies with time as shown below.
Since alternators operate most efficiently when delivering full load, units can be added or put off depending upon the load requirement. This permits the efficient operation of the power system.