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
81LSH. 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, 81LSH 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.
4.3.2.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are “Enable” and “Disable”.
The operation of 81LSH can be described using a module diagram. All the modules are
explained in the next sections.
GUID-B2B8836B-7D03-4656-902D-D748F164B268 V2 EN
Figure 128:
Functional module diagram
Section 4
1MRS240050-IB C
Protection functions
272
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