ABB RET650 Applications Manual page 173

1MRK 504 169-UEN A
Transformer protection RET650 2.2 IEC
Application manual
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).
The start value for SAPFRC 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.
Section 9
Frequency protection
167