Figure 227: 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
1MRS758755 C
Protection functions
REC615 & RER615
Technical Manual
463
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