MIL-HDBK-1003/7
Section 20.
LOAD SHEDDING
Objectives of Load Shedding Program. When a power plant or an individual
20.1
power generating unit experiences a gradual increase in load, or a sudden but mild
overload, the unit governors will sense the resulting speed change and increase the
power input to the generator. The additional load is handled by using the spinning
reserve; that is, the unused capacity of the generator. However, if all generators are
operating at maximum capacity, the spinning reserve is zero and the governors may not be
able to relieve the overload.
When severe overloads occur, or when large
sudden load increases are
experienced, the first effect is a slowing down of the
generator. If the overload is
large enough or if the governor cannot accommodate the
sudden load increase, then speed
and frequency will continue to drop until the plant or
generating unit is tripped to
prevent equipment damage, and the load is lost.
Definition. Load shedding is the deliberate and selective dropping of
20.1.1
electrical load in accordance with a preplanned program.
Purpose. The purpose of load shedding is to reduce plant loads so that the
20.1.2
plant will not trip on overload and so that certain preselected loads can be saved, even
though other loads are lost.
Requirements for Load Reduction. A typical scenario in which load shedding
20.2
would be advantageous would be the case of a power plant operating in parallel with a
utility, with a total system load exceeding the capability of the plant alone. Should
the interconnection trip, the plant would experience a sudden overload from which it
might not be able to recover.
Another scenario could involve an isolated plant (not interconnected with a
utility), with several generating units loaded at or near their combined capability.
Should one unit trip, the remaining units would experience a sudden load increase,
possibly leading to loss of the plant.
Stable Operation and Overload Capability. Generating plants are highly
20.2.1
sensitive to frequency drop. There are two major problem areas.
20.2.1.1 Motor speed. Motor-driven auxiliaries, particularly boiler feedwater pumps,
will slow down, reducing generator output. Safety margins in generator-cooling and
bearing lubricating systems will be reduced. The lowest safe plant operating speed will
depend on the safety margins included in the plant design. However, operation below the
56.6 to 57.5 Hz range is generally not advisable.
20.2.1.2 Turbine Blade Fatigue. The last rows of long, low-pressure blades in steam
turbines are tuned to operate free of resonance in a narrow band of frequencies around
60 Hz. When running under heavy load at about 58.5 Hz or below, the steam excitation
frequency approaches blade resonance. Under this condition, the blades may vibrate
severely, producing fatigue stress. On the average, blades should not be subjected to
more than ten minutes of severe vibration totaled over their lifespan; fatigue is
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