ABB RELION REX640 Technical Manual page 930

Protection and control

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Table of Contents

Protection functions

Figure 516: 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

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

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

the frequency gradient decreases at a faster rate than the set value of

Start value Freq setting.

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

1MRS759142 F

Reset delay time ,

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