ABB RELION Series, Техническое руководство

Страница: 455 / 804

ST_REST
50 Hz
Frequency
[Hz]
48.8 Hz
START
OPERATE
Time [s]
RESTORE
Set Restore delay time
Restore
timer
Timer
starts
Timer
continues
Timer
suspended
GUID-8694ACBB-CC73-46E6-A9C9-5DE27F6FC7AF V3 EN
Figure 228: 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.
1MRS758755 A
Section 4
Protection functions
REC615 and RER615 449
Technical Manual