Example 5.5

Consider an interconnected 50-Hz power system that contains four turbine-generator units rated 750 MW, 500 MW, 220 MW and 110 MW. The regulating constant of each unit is 0.05 per unit based on its own rating. Each unit is operating on 75% of its own rating when the load is suddenly dropped by 250 MW. We shall choose a common base of 500 MW and calculate the rise in frequency and drop in the mechanical power output of each unit.

The first step in the process is to convert the regulating constant, which is given in per unit in the base of each generator, to a common base. This is given as

(5.26)

We can therefore write

Therefore

per unit

We can therefore calculate the total change in the frequency from (5.25) while assuming ΔP_ref = 0, i.e., for no change in the reference setting. Since the per unit change in load - 250/500 = - 0.5 with the negative sign accounting for load reduction, the change in frequency is given by

Then the change in the mechanical power of each unit is calculated from (5.22) as

It is to be noted that once ΔP_m2 is calculated to be - 79.11 MW, we can also calculate the changes in the mechanical power of the other turbine-generators units as

This implies that each turbine-generator unit shares the load change in accordance with its own rating.