Hitachi Relion 670 Series Applications Manual page 198

Section 10
Frequency protection
Equipment protection, such as for motors and generators
The setting has to be well below the lowest occurring "normal" frequency and well above the lowest
acceptable frequency for the equipment.
Power system protection, by load shedding
The setting has to be below the lowest occurring "normal" frequency and well above the lowest
acceptable frequency for power stations, or sensitive loads. The setting level, the number of levels
and the distance between two levels (in time and/or in frequency) depends very much on the
characteristics of the power system under consideration. The size of the "largest loss of production"
compared to "the size of the power system" is a critical parameter. In large systems, the load
shedding can be set at a fairly high frequency level, and the time delay is normally not critical. In
smaller systems the frequency start level has to be set at a lower value, and the time delay must be
rather short.
The voltage related time delay is used for load shedding. The settings of SAPTUF could be the same
all over the power system. The load shedding is then performed firstly in areas with low voltage
magnitude, which normally are the most problematic areas, where the load shedding also is most
efficient.
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