ABB RET670 - Setting Guidelines

3.9.1.2 Setting guidelines
All the frequency and voltage magnitude conditions in the system where SAPTUF (81)
performs its functions should be considered. The same also applies to the associated
equipment, its frequency and time characteristic.
There are especially two specific application areas for SAPTUF (81):
1. to protect equipment against damage due to low frequency, such as generators,
transformers, and motors. Overexcitation is also related to low frequency
2. to protect a power system, or a part of a power system, against breakdown, by
shedding load, in generation deficit situations.
The underfrequency PICKUP value is set in Hz. All voltage magnitude related settings
are made as a percentage of a settable base voltage, which normally is set to the
nominal primary voltage level (phase-phase) of the power system or the high voltage
equipment under consideration.
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.
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.
1MRK504116-UUS C Section 3
IED application
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Application manual