Handbook of Electrical Engineering

50 HANDBOOK OF ELECTRICAL ENGINEERING

Similarly for generators Nos. 2 and 3,

fz 2 =61.411 Hz andfz 3 =61.951 Hz

Step 7. Find the new set points that will recover the frequency to 60 Hz whilst maintaining equally
loaded generators.

Let the desired frequency of 60 Hz be denoted atfd. In order to reach this frequency all the
set points need to be increased by the difference betweenfdandf,whichis,

f=fd−f= 60. 0 − 59. 25101 = 0 .749 Hz

Therefore,

fz 1 = 60. 871 + 0. 749 = 61 .62 Hz

fz 2 = 61. 411 + 0. 749 = 62 .16 Hz

and
fz 3 = 61. 951 + 0. 749 = 62 .70 Hz

Check thatf has now the correct value, by using (2.62),

f=fz 1 −

D 1 P 1 fo

G 1

= 61. 62 −

0. 03 × 18. 0 × 60

20

= 61. 62 − 1. 62 = 60 .0Hz

f=fz 2 −

D 2 P 2 fo

G 2

= 62. 16 −

0. 04 × 13. 5 × 60

15

= 62. 16 − 2. 16 = 60 .0Hz

and,

f=fz 3 −

D 3 P 3 fo

G 3

= 62. 70 −

0. 05 × 9. 0 × 60

10

= 62. 70 − 2. 70 = 60 .0Hz

2.5.5 Load sharing controllers

The above worked example illustrates the combination of droop governing with an overall isochronous
control function. In a practical control scheme the following variables can be easily measured by
suitable transducers,

f =the system frequency.
Pi=the electrical power at the terminals of the generator (the generator losses and
gearbox losses can be ignored).
fzi=the governor set point within the controller that drives the fuel valve. A suitable
potentiometer can be used to derive the signal.