ABB RER620 Technical Manual page 237

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RER620
Technical Manual
Figure 105: Operation of the load-shedding function
Power system protection by load shedding
The decision on the amount of load that is required to be shed is taken through the
measurement of frequency and the rate of change of frequency (df/dt). At a single location,
many steps of load shedding can be defined based on different criteria of the frequency and
df/dt. Typically, the load shedding is performed in six or four steps with each shedding
increasing the portion of load from five to twenty-five percent of full load within a few
seconds. After every shedding, the system frequency is read back and further shedding
actions are taken only if necessary. In order to take the effect of any transient, a sufficient
time delay should be set.
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 start 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.
Section 4
Protection functions
231