Page 67 - Handbook of Electrical Engineering

46 HANDBOOK OF ELECTRICAL ENGINEERING

and
i=n
1 G i
b = is also a constant (2.68)
f o D i
i=1
(2.65) and (2.66) represent the overall droop characteristic of the power system.

The application of (2.63) and (2.64) can be demonstrated graphically for a system in which
two generators are sharing a common load.
Consider two gas turbine generators, called Gen.1 and Gen.2, of the same size are sharing a
common load. Assume Gen.1 takes 60% and Gen.2 the remaining 40%. Let the system frequency be
60 Hz at full load and the droop of each machine be 4%.
The speed (frequency) versus load sharing situations can be shown graphically as in Figure 2.14
where point ‘A’ is the initial situation.

Now, supposing it is necessary to equalise the load shared by the two machines, then one
or both of the speed settings will need to be adjusted depending upon the final common speed
(frequency) required by the machines. It can be seen that unless the speed settings are changed, the
load taken by each machine cannot change. There are several methods by which this may be done,
by changing the speed setting of Gen.1 or Gen.2 or both.
Method 1. Change the speed setting of Gen.1 only:
The droop characteristic line 1A-A must be lowered to the new position ID-D so that
it crosses the line 2A-D of Gen.2 at point ‘D’ for 50% sharing of load. Thus the speed

Figure 2.14 Frequency droop governing and load sharing of two gas turbines.