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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 269
Technical Manual
4.3.2.2 Function block
Figure 126: Function block
4.3.2.3 Functionality
The load shedding and restoration function 81LSH 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
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.
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 the load shedding and restoration function can be described using a
module diagram. All the modules are explained in the next sections.
Throughout this document, “high df/dt” is used to mean “a high rate of
change of the frequency in negative direction.”