ABB RER620 Technical Manual page 238

Section 4
Protection functions
232
Table 242: 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 243: 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 x Fn (49.2 Hz)
0.978 x Fn (48.9 Hz)
0.968 x Fn (48.4 Hz)
0.958 x Fn (47.9 Hz)
0.950 x Fn (47.5 Hz)
Start value df/dt
setting
-0.005 xFn /s (-0.25 Hz/s)
-0.010 x Fn /s (-0.50 Hz/s)
-0.015 x Fn /s (-0.75 Hz/s)
-0.020 x Fn /s (-1.00 Hz/s)
-0.025 x Fn /s (-1.25 Hz/s)
1MAC309294-MB F
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
RER620
Technical Manual