504 HYDROELECTRIC POWER DEVELOPMENT
Fig. 12.7 Flow–power duration curve (power scale multiplying factor
gH)
plant may be operated on the peak load only. The surplus or secondary
power is all the available power in excess of the primary power, and is
given by the area under the power duration curve between the firm power
line and the total installed capacity of the power plant.
(c) Mass curve
The mass curve (Fig. 12.24) is a plot of cumulative flow against time
throughout the period of record. It is used to estimate storage require-
ments and usable flow for power production (Linsley and Franzini, 1979;
Twort, Ratnayaka and Brandt, 2000). The slope of the curve at any point
indicates the rate of inflow at that particular time.
The gradient of the line OA represents the average rate of inflow
over the total period for which the mass curve has been plotted. If the rate
of withdrawal (draw-off or demand) of water for power production is
uniform, and is given by the gradient of OA, the gradient of the mass
curve at any instant indicates either the reservoir being filled or emptied
according to whether the slope of the mass curve is greater or smaller than
that of the demand line OA. Thus, in order to permit the release of water
at this uniform rate for the entire period, the reservoir should have a
minimum capacity given by the sum of the vertical intercepts BC and
DE. Assuming the reservoir to contain an initial storage of water equal
to DE, it would be full at C and empty at D. Mass curves may also be
used to determine the spill of water from the reservoir of a given capacity
from which water is drawn off to meet the power demand.
