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Section 9
Frequency protection
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 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 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:
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 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).
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.
224
1MRK 505 370-UEN D
M14971-3 v7
Busbar protection REB670
Application manual
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