ABB RELION REX640 Technical Manual page 937

1MRS759142 F
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
1
2
3
4
5
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
1
2
3
4
5
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
1
2
3
Table continues on the next page
REX640
Technical Manual
Start value Freq setting
0.984 · Fn (49.2 Hz)
0.978 · Fn (48.9 Hz)
0.968 · Fn (48.4 Hz)
0.958 · Fn (47.9 Hz)
0.950 · Fn (47.5 Hz)
Start value df/dt setting
-0.005 · Fn /s (-0.25 Hz/s)
-0.010 · Fn /s (-0.50 Hz/s)
-0.015 · Fn /s (-0.75 Hz/s)
-0.020 · Fn /s (-1.00 Hz/s)
-0.025 · Fn /s (-1.25 Hz/s)
Restore start Val setting
0.990 · Fn (49.5 Hz)
0.990 · Fn (49.5 Hz)
0.990 · Fn (49.5 Hz)
Protection functions
Operate Tm Freq setting
45000 ms
30000 ms
15000 ms
5000ms
500 ms
Operate Tm df/dt setting
8000 ms
2000 ms
1000 ms
500 ms
250 ms
Restore delay time setting
200000 ms
160000 ms
100000 ms
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