ABB RELION REF615R, Technical Manual

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stepwise in such a way that it does not lead the system back to the emergency condition.
This is done through an operator intervention or in case of remote location through an
automatic load restoration function. The load restoration function also detects the system
frequency and restores the load if the system frequency remains above the value of the set
restoration frequency for a predefined duration.
ST_REST
60 Hz
Frequency
[Hz]
58.5 Hz
Start of restore delay counter
RESTORE output activated
Restore delay counter suspended
Restore delay counter continues
PICKUP
TRIP
Time [s]
GUID-F9B2F5DC-9395-4C60-B49F-E794A3C70E60 V2 EN
Figure 131: 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.
Section 4
1MRS240050-IB C
Protection functions
278 REF615R
Technical Manual