cumulative. Operation below about 58.0 to 58.5 Hz should be avoided; the generator
and time limits for turbines should be specified by the turbine manufacturer, and
protective system operation by the plant designer.
Generating Unit Sizes. In order to design an effective load shedding scheme,
the following information is required for each generating unit in the plant.
The lowest safe operating speed (F) in Hz.
The rated capacity (MW) of the machine in megawatts.
The power factor (p) rating of the machine, dimensionless.
The inertial constant (H) of the machine in megawatt seconds per MVA.
Number of Generating Units. The value of plant capability used to design a
load shedding program should take into account the possibility that one or more
generating units may be out of service when the overload incident occurs. This
determination must be based on plant configuration and judgment. However, the following
conditions should be considered.
All units in operation.
All but the largest unit in operation.
All but the two largest units in operation.
Reduced Plant Capability. The reduction in plant capability resulting from
underspeed operation should be known but will generally be unavailable. As an
approximation, assume a capability loss of about four percent capability for each Hz of
Load Reduction Requirements. In order to attain stable plant operation, load
must be shed to such a degree that the total load served does not exceed plant
capability. As already explained, the determination of plant capability must take into
account the possibility that one or more generating units in the plant may be out of
service and also that the remaining plant may experience some loss of capability because
of underfrequency operation.