MIL-HDBK-1003/7

20.2.5.1 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

EQUATION:

(15)

where:

R

=

Average rate of frequency change, Hz per second

p

=

Power factor of machine

L

=

Average per unit overload

=

(Load-Power Input)/(Power Input)

fo =

Initial frequency, Hz

f1 =

Final frequency, Hz

H =

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,

EQUATION:

L = (120 MW - 100 MW) / (100 MW) = 0.20

(16)

and substituting in Equation (15):

[(0.85)(0.20)(60-58)]/[4(1 - (602/582))]

R=

= - 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.

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.

EQUATION:

Hplant = (H1MVA1 + H2MVA2+...+HNMVAN)/(MVA1 + MVA2 +...+MVAN) (17)

where the subscripts refer to the individual generating units.

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