ABB RELION RET670, Applications Manual

Page: 644 / 1016

11.1.3 Setting guidelines
M13355-3 v8
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 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 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.
11.2 Overfrequency protection SAPTOF (81)
IP15747-1 v3
Section 11
1MRK 504 163-UUS A
Frequency protection
638 Transformer protection RET670 2.2 ANSI
Application manual