ABB RELION REX640 Technical Manual page 861

1MRS759142 C
REX640
Technical Manual
If a moderate system operates at 50 Hz, an underfrequency should be set for different
steps from 49.2 Hz to 47.5 Hz in steps of 0.3 – 0.4 Hz. The operating time 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 816: 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 large 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 817: 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.
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)
Section 4
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
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