ABB RED615 Technical Manual page 516

Section 4
Protection functions
510
Frequency
[Hz]
50 Hz
48.8 Hz
START
OPERATE
ST_REST
RESTORE
Set Restore delay time
Restore
timer
GUID-8694ACBB-CC73-46E6-A9C9-5DE27F6FC7AF V3 EN
Figure 266: 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.
Timer Timer
Timer
starts
suspended continues
1YHT530004D05 D
Time [s]
615 series
Technical Manual