ABB RELION Series, Technical Manual

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4.5.2.3 Functionality
The load-shedding and restoration function LSHDPFRQ is capable of performing
load-shedding based on underfrequency and the rate of change of the frequency. The
load that is shed during the frequency disturbance can be restored once the frequency
has stabilized to the normal level.
The measured system frequency is compared to the set value to detect the
underfrequency condition. The measured rate of change of frequency (df/dt) is
compared to the set value to detect a high frequency reduction rate. The combination
of the detected underfrequency and the high df/dt is used for the activation of the load-
shedding. There is a definite time delay between the detection of the underfrequency
and high df/dt and the activation of LSHDPFRQ. This time delay can be set and it is
used to prevent unwanted load-shedding actions when the system frequency recovers
to the normal level.
Throughout this document, “high df/dt” is used to mean “a high rate of
change of the frequency in negative direction.”
Once the frequency has stabilized, LSHDPFRQ can restore the load that is shed during
the frequency disturbance. The restoration is possible manually or automatically.
The function contains a blocking functionality. It is possible to block function outputs,
timers or the function itself, if desired.
4.5.2.4 Operation principle
The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of LSHDPFRQ can be described using a module diagram. All the
modules are explained in the next sections.
1MRS758755 A
Section 4
Protection functions
REC615 and RER615 443
Technical Manual