Alternators on Infinite Busbars
Alternators on Infinite Busbars:
In practice, generating stations do not operate as isolated units but are interconnected by the national grid. The result is that a very large number of alternators operate in parallel.
An alternator connected to such a network is said to be operating on infinite busbars. The behaviour of alternators connected to an infinite busbars is as under:
(i) Any change made in the operating conditions of one alternator will not change the terminal voltage or frequency of the system. In other words, terminal voltage (busbars voltage) and frequency are not affected by changing the operating conditions of one alternator. It is because of large size and inertia of the system.
(ii) The kW output supplied by an alternator depends solely on the mechanical power supplied to the prime mover of the alternator. An increase in mechanical power to the prime mover increases the kW output of the alternator and not the kVAR. A decrease in the mechanical power to the prime mover decreases the kW output of the alternator and not the kVAR.
(iii) If the mechanical power to the prime mover of an alternator is kept constant, then change in excitation will change the power factor at which the machine supplies changed current. In other words, change of excitation controls the kVAR and not kW.
The change of driving torque controls the kW output and not kVAR of an alternator. The change of excitation controls the kVAR and not the kW output of an alternator.
An infinite busbars system has constant terminal voltage and constant busbars frequency because of its large size and inertia. However, the busbars voltage can be raised or lowered by increasing or decreasing simultaneously the field excitation of a large number of alternators. Likewise, system frequency can be raised or lowered by increasing or decreasing the speed of prime movers of a large number of alternators.