ABB Relion 670 Series Applications Manual page 282

Section 11
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.
11.2 Overfrequency protection SAPTOF
11.2.1 Identification
Function description
Overfrequency protection
11.2.2 Application
Overfrequency protection function SAPTOF is applicable in all situations, where reliable
detection of high fundamental power system frequency is needed. The power system
frequency, and rate of change of frequency, is a measure of the unbalance between the actual
generation and the load demand. High fundamental frequency in a power system indicates
that the available generation is too large compared to the power demanded by the load
connected to the power grid. SAPTOF detects such situations and provides an output signal,
suitable for generator shedding, HVDC-set-point change and so on. SAPTOF is very sensitive
and accurate and can also be used to alert operators that frequency has slightly deviated from
the set-point, and that manual actions might be enough.
11.2.3 Setting guidelines
All the frequency and voltage magnitude conditions in the system where SAPTOF performs its
functions must be considered. The same also applies to the associated equipment, its
frequency and time characteristic.
There are two application areas for SAPTOF:
276
IEC 61850
identification
SAPTOF
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IEC 60617 ANSI/IEEE C37.2
identification device number
81
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