ABB RELION REX640 Technical Manual page 854

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Protection functions

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Functional module diagram

Underfrequency detection

The underfrequency detection measures the input frequency calculated from the

voltage signal. An underfrequency is detected when the measured frequency drops

below the set value of the Start value Freq setting.

The underfrequency detection module includes a timer with the definite time (DT)

characteristics. Upon detection of underfrequency, operation timer activates the

ST_FRQ output. When the underfrequency timer has reached the value set by Operate

Tm Freq, the OPR_FRQ output is activated if the underfrequency condition still

persists. If the frequency becomes normal before the module operates, the reset timer

is activated. If the reset timer reaches the value set by Reset delay time, the timer resets

and the ST_FRQ output is deactivated.

df/dt detection

The df/dt detection measures the input frequency calculated from the voltage signal

and calculates its gradient. A high df/dt condition is detected by comparing the

gradient to the Start value df/dt setting.The df/dt detection is activated when the

frequency gradient decreases at a faster rate than the set value of Start value df/dt.

The df/dt detection module includes a timer with the DT characteristics. Upon

detection of df/dt, operation timer activates the ST_FRG output. When the timer has

reached the value set by Operate Tm df/dt, the OPR_FRG output is activated if the df/

dt condition still persists. If df/dt becomes normal before the module operates, the

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