Environmental Engineering Reference
In-Depth Information
3.3.3 Parallel Operation of Two Generators
When two generators are connected in parallel and are jointly supplying the demand in a
small power system, the load is shared according to the set points of their governors. The
best way to illustrate this sharing mechanism and the resulting system frequency is by means
of an example.
In the small power system of Figure 3.5(a) two generators A and B rated at 50 and 100 MW
respectively supply a load of 100 MW. Both generators are fi tted with governors having
a droop of 4% and a no-load set point of 52 Hz. Lines aa and bb in Figure 3.5(b) show the
frequency - power (
f - P
) characteristics of the two generators.
The division of load between the generators when they are supplying 100 MW will now
be determined. Let line cc located
Δ
f
below 52 Hz represent the frequency at which this
sharing takes place.
(a)
Using trigonometry
P
A
/
f
= 100/2. Therefore
P
A
/
P
B
= 1/2, but as
P
A
+
P
B
= 100 MW, then
P
A
= 33.3 MW and
P
B
= 66.6 MW In this case the demand is
shared in proportion to the generator ratings
Δ
f
= 50/2 and
P
B
/
Δ
(b)
To determine the system frequency, from
P
A
/
Δ
f
= 50/2
Δ
f
= 33.3
×
2/50 = 1.33 and
hence the frequency at which line cc intercepts the
f
axis is
f
= (52
−
1.33) = 50.67 Hz
(c) Suppose that generator A is more modern and therefore more effi cient in terms of fuel
used per generated kW h. The set point adjustment of the governors could be used to
arrive at a new more economic load-sharing condition that maximizes the contribution
from generator A.
One way to implement this is to shift the set point of generator B so that its characteristic
is displaced from bb to BB, with the result that each generator is supplying 50 MW. The
system frequency is now 50 Hz and the new set point of generator B can be found from
δ
f
/50 = 2/100. Hence
δ
f
= 1 and the new set point is 51 Hz
f(Hz)
50 MW
100 MW
52
A
B
Δ
f
B
c
c
δ
f
100 MW
50
P
A
50
P
B
100
P(MW)
(a)
B
(b)
Figure 3.5
Parallel generators. (Reproduced with permission of McGraw - Hill)
