ABB RELION REX640, Техническое руководство, Страница: 937 / 1960

for the underfrequency can be set from a few seconds to a few fractions of a second
stepwise from a higher frequency value to a lower frequency value.
Table 893: Setting for a five-step underfrequency operation
Load-shedding steps Start value Freq setting Operate Tm Freq setting
1 0.984 · Fn (49.2 Hz) 45000 ms
2 0.978 · Fn (48.9 Hz) 30000 ms
3 0.968 · Fn (48.4 Hz) 15000 ms
4 0.958 · Fn (47.9 Hz) 5000ms
5 0.950 · Fn (47.5 Hz) 500 ms
The rate of change of frequency function is not instantaneous since the function
needs time to supply a stable value. It is recommended to have a time delay long
enough to take care of the signal noise.
Small industrial systems can experience the rate of change of frequency as high
as 5 Hz/s due to a single event. Even large power systems can form small islands
with a large imbalance between the load and generation when severe faults or
combinations of faults are cleared. Up to 3 Hz/s has been experienced when a small
island becomes isolated from a large system. For normal severe disturbances in
large power systems, the rate of change of the frequency is much less, often just a
fraction of 1.0 Hz/s.
Similarly, the setting for df/dt can be from 0.1 Hz/s to 1.2 Hz/s in steps of 0.1 Hz/s
to 0.3 Hz/s for large distributed power networks, with the operating time varying
from a few seconds to a few fractions of a second. Here, the operating time should
be kept in minimum for the higher df/dt setting.
Table 894: Setting for a five-step df/dt< operation
Load-shedding steps Start value df/dt setting Operate Tm df/dt setting
1 -0.005 · Fn /s (-0.25 Hz/s) 8000 ms
2 -0.010 · Fn /s (-0.50 Hz/s) 2000 ms
3 -0.015 · Fn /s (-0.75 Hz/s) 1000 ms
4 -0.020 · Fn /s (-1.00 Hz/s) 500 ms
5 -0.025 · Fn /s (-1.25 Hz/s) 250 ms
Once the frequency has stabilized, the shed load can be restored. The restoring
operation should be done stepwise, taking care that it does not lead the system
back to the emergency condition.
Table 895: Setting for a five-step restoring operation
Load-shedding steps Restore start Val setting Restore delay time setting
1 0.990 · Fn (49.5 Hz) 200000 ms
2 0.990 · Fn (49.5 Hz) 160000 ms
3 0.990 · Fn (49.5 Hz) 100000 ms
Table continues on the next page
1MRS759142 F Protection functions
REX640
Technical Manual
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