ABB RELION 620 Series Technical Manual page 342

Section 4
Protection functions
336
The value of the setting has to be well below the lowest occurring normal frequency and
well above the lowest acceptable frequency of the system. The setting level, the number
of steps and the distance between two steps (in time or in frequency) depend on the
characteristics of the power system under consideration. The size of the largest loss of
generation compared to the size of the power system is a critical parameter. In large
systems, the load-shedding can be set at a high frequency level and the time delay is
normally not critical. In small systems, the frequency pickup level has to be set at a low
value and the time delay must be short.
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 271: 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.
Pickup value Freq setting
0.984 · Fn (59 Hz)
0.978 · Fn (58.7 Hz)
0.968 · Fn (58.1 Hz)
0.958 · Fn (57.5 Hz)
0.950 · Fn (57 Hz)
1MAC504801-IB E
Trip Tm Freq setting
45000 ms
30000 ms
15000 ms
5000ms
500 ms
620 series ANSI
Technical Manual