Setting Guidelines - ABB Relion 670 Series Applications Manual

Section 9
Frequency protection
can be used both for increasing frequency and for decreasing frequency. SAPFRC (81) provides an
output signal, suitable for load shedding or generator shedding, generator boosting, HVDC-set-
point change, gas turbine start up and so on. Very often SAPFRC (81) is used in combination with a
low frequency signal, especially in smaller power systems, where loss of a fairly large generator will
require quick remedial actions to secure the power system integrity. In such situations load
shedding actions are required at a rather high frequency level, but in combination with a large
negative rate-of-change of frequency the underfrequency protection can be used at a rather high
setting.
9.3.3

Setting guidelines

The parameters for Rate-of-change frequency protection SAPFRC (81) are set via the local HMI or
or through the Protection and Control Manager (PCM600).
All the frequency and voltage magnitude conditions in the system where SAPFRC (81) performs its
functions should be considered. The same also applies to the associated equipment, its frequency
and time characteristic.
There are two application areas for SAPFRC (81):
1. to protect equipment against damage due to high or too low frequency, such as generators,
transformers, and motors
2. to protect a power system, or a part of a power system, against breakdown by shedding load
or generation, in situations where load and generation are not in balance.
SAPFRC (81) is normally used together with an overfrequency or underfrequency function, in small
power systems, where a single event can cause a large imbalance between load and generation. In
such situations load or generation shedding has to take place very quickly, and there might not be
enough time to wait until the frequency signal has reached an abnormal value. Actions are
therefore taken at a frequency level closer to the primary nominal level, if the rate-of-change
frequency is large (with respect to sign).
SAPFRC (81)PICKUP value is set in Hz/s. All voltage magnitude related settings are made as a
percentage of a settable base voltage, which normally is set to the primary nominal voltage level
(phase-phase) of the power system or the high voltage equipment under consideration.
SAPFRC (81) is not instantaneous, since the function needs some time to supply a stable value. It is
recommended to have a time delay long enough to take care of signal noise. However, the time,
rate-of-change frequency and frequency steps between different actions might be critical, and
sometimes a rather short operation time is required, for example, down to 70 ms.
Smaller industrial systems might experience rate-of-change frequency as large as 5 Hz/s, due to a
single event. Even large power systems may form small islands with a large imbalance between
load and generation, when severe faults (or combinations of faults) are cleared - up to 3 Hz/s has
been experienced when a small island was isolated from a large system. For more "normal" severe
disturbances in large power systems, rate-of-change of frequency is much less, most often just a
fraction of 1.0 Hz/s.
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Application manual