Prime Mover Characteristics for Parallel operation

In General, For alternators to operate successfully in parallel, The load speed characteristics of the prime movers should be drooping, that is the speed of prime mover should decrease slightly with increase loads. the speed droop, also called governor droop, or inherent speed regulation, is usually expressed as percentage of the full-load speed.

Nnl  = No load speed,     Nfl  = Full load speed

The percentage of droop normally varies from 2 to 4 percent from no load to full load. Usually the speed load characteristic are linear.

The amount of power generated by a machine is determined by its prime mover. the speed of the prime mover is fixed , but its torque can be varied. this is done by adjusting the spring tension of governor mechanism. the effect of changing the governor characteristic is show in fig. The speed (frequency) load characteristic is shifted to a new position parallel to the initial position.

When two alternators are operating in parallel , an increase in governor set poit of one them,

1. Increase the system frequency 
2. increase the power supply by that alternator and reduce the power supplied by the other alternator.

When two alternator are operating in parallel and the field current of the second alternator is increased  

• the system terminal voltage is increased and
• the reactive power Q supplied by that alternator is increased, while the reactive power supplied by the other alternator is decreased.