ABB Relion 670 Series Applications Manual page 238

Section 10
Frequency protection
The under frequency pickup value is set in Hz. All voltage magnitude related settings are made as a
percentage of a global base voltage parameter. The UBase value should be set as a primary phase-
to-phase value.
Some applications and related setting guidelines for the frequency level are given below:
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 pickup 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 (81) 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.
10.2 Overfrequency protection SAPTOF (81)
10.2.1 Identification
Function description
Overfrequency protection
10.2.2 Application
Overfrequency protection function SAPTOF (81) 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 (81) detects such situations and provides an output signal, suitable for generator
shedding, HVDC-set-point change and so on. SAPTOF (81) 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.
232
IEC 61850
identification
SAPTOF
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IEC 60617 ANSI/IEEE C37.2
identification device number
81
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