Final answer:
The parallel operation of alternators is necessary to meet large power demands and ensure reliability. Mechanical torque alterations in one alternator affect its contribution to the total power output in the grid, which necessitates precise control to maintain system stability.
Step-by-step explanation:
The need for parallel operation of alternators is primarily to meet large power demands and ensure reliability in power systems. When two or more alternators are connected in parallel, they share the total load demand, providing flexibility and efficiency in power generation and distribution.
Changing the mechanical torque, which is the prime mover input on alternators, affects their parallel operation significantly. When the mechanical torque on one alternator is increased, it drives the alternator to speed up and increase its electrical output.
If the output frequency of the alternator is synchronized with the grid, the increase in torque will lead to an increase in active power contributed by the alternator. Conversely, if one alternator is delivering less torque, it will contribute less active power to the system.
It is essential to control the torque precisely to maintain the stability of the power system since any imbalance in active power could lead to frequency deviations, which can adversely affect the entire grid.
Alternators must have their voltages, frequencies, and phases matched to operate smoothly in parallel, and any deviation can cause undesirable effects such as circulating currents or system instability.