18.104.22.168 Rate of Load Reduction. Load must be shed fast enough to attain the required
load reduction before plant frequency has deteriorated to an unacceptable level. The
rate of frequency change can be estimated from the following formula.
R = pL(f1 - f0) / H(1 - f12/f02
Average rate of frequency change, Hz per second
Power factor of machine
Average per unit overload
(Load-Power Input)/(Power Input)
Initial frequency, Hz
Final frequency, Hz
Inertial constant of machine, MW sec/MVA
For a typical calculation, assume that a 100 MW, 0.85 power factor
machine with an inertial constant of H=4 experiences a sudden overload to 120 MW and we
wish to find the rate at which frequency drops from the initial frequency of fo = 60 Hz
to f1 = 58 Hz. In this case,
L = (120 MW - 100 MW) / (100 MW) = 0.20
and substituting in Equation (15):
[(0.85)(0.20)(60-58)]/[4(1 - (602/582))]
= - 1.21 Hz/second.
In this case, the minus sign indicates that the frequency is decreasing
at a rate of 1.21 Hz per second.
The total time for the frequency to drop 2 Hz, from fo = 60 Hz to f1 =
58 Hz would be
Time = 2/1.21 = 1.65 seconds
This indicates that the load shedding plan must accomplish a load
reduction of at least 20 MW within 1.65 seconds.
For calculations involving an entire plant, composite values should be
used to obtain a rate of frequency drop for the entire plant. The composite inertial
constant for the plant, H Plant, is calculated from the following formula.
Hplant = (H1MVA1 + H2MVA2+...+HNMVAN)/(MVA1 + MVA2 +...+MVAN) (17)
where the subscripts refer to the individual generating units.