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 672: 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 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 673: 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 674: Setting for a five-step restoring operation
Load-shedding steps
Restoring 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
4 0.990 · Fn (49.5 Hz) 50000 ms
5 0.990 · Fn (49.5 Hz) 10000 ms
1MRS757644 H Protection functions
620 series
Technical Manual
717
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