MIL-HDBK-1003/7
20.2.5.2 Total Load Reduction. The load shedding plan must accomplish a total load
reduction sufficient to relieve the plant overload and also to provide a slight
underload so that the plant will have reserve capability to reaccelerate to the normal
operating frequency. One must also take into account the loss of capability that
results from underfrequency operation.
In the previous example (a 100 MW generator with a 120 MW load) assume that
the plant frequency dropped to 58 Hz before load shedding was initiated and that the
plant capability loss was four percent for each Hz of frequency drop. In this case, the
plant capability would be only 92 percent (or 92 MW) at 58 Hz. If 30 MW of load were
shed (reducing the total load from 120 MW to 90 MW) then the generator would have a 2 MW
margin (92 MW capability less 90 MW load) to reaccelerate back to the normal operating
frequency of 60 Hz. The time to reaccelerate may be computed from the same formulas
already given.
20.2.5.3 Minimum Power Supply. It is reasonable to design a load shedding plan to drop
nearly all or perhaps even all of the plant load. The rationale is that even if all
load is dropped but the plant is saved, load restoration would be faster and easier than
if the plant were lost also.
Methods of Load Shedding. There are many methods of load shedding, both
20.3
automatic and manual. The automatic methods include underfrequency relaying and various
transfer-trip arrangements. All of these methods have relative advantages and
disadvantages and the choice of the most advantageous method should be based on the
specific conditions that prevail. However, load shedding by underfrequency relaying is
the most common and generally the preferred method.
Underfrequency Relaying. The principal advantage of under frequency load
20.3.1
shedding is that the underfrequency relays respond to the underfrequency condition
resulting from generator overload. They do not respond to the overload directly, but to
the deleterious conditions resulting from overload. Therefore, they respond only if the
generators are suffering the adverse consequences of overload.
20.3.1.1 Automatic Operation, Load shedding with under-frequency relays is generally
performed automatically; when frequency drops to a preset level, generally performed
automatically.
Automatic operation is generally advantageous because, with severe
overloads, system frequency may drop faster than the plant operator can respond.
Critical Frequencies. In general, underfrequency relays are applied to shed
20.3.1.2
load in two to five steps, with three-step load shedding being the most common. Each
underfrequency relay is set to trip a block of load at progressively lower frequencies;
for instance, at 59.5 Hz, 59.0 Hz, and 58.5 Hz. These critical frequencies should be
computed as described previously.
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