SAPFRCSTART 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 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.
1MRK 505 291-UEN A Section 8
Frequency protection
171
Application manual
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