Setting Guidelines - ABB REL650 Applications Manual

1MRK 506 334-UUS A
9.3.3
Application manual

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 specific 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.
Section 9
Frequency protection
273