Environmental Engineering Reference
In-Depth Information
Δ
C
Δ
P
P (MW)
Figure 7.5
Typical input-output curve for a thermal generating unit
O
A
O
B
Figure 7.6
Demonstration of the equal incremental cost solution. (Based on fi gure in Neuenswander,
J.R.,
Modern Power Systems
, Intertext Books, 1973)
unit B is placed at a distance from
O
A
equal to the total demand
P
d
=
P
A
+
P
B
. Any vertical
line intersecting the curves will therefore represent the cost of a given operating condition.
As before a sharing of the demand by the two generators is sought such that the cost is
minimum.
Let the vertical line
C
1
represent the operating point at which the slopes of the two curves
are equal. The length of
C
1
represents the total cost per hour of operating both units. Any
other operating points (such as the ones represented by lines
C
2
or
C
3
) result in higher cost.
Hence the most economic operating point is the one at which the slopes of the cost lines (i.e.
the
incremental costs
) of the two units are equal. This result is valid no matter how many
generators are feeding power to the system. The same conclusion can be proven using the