1MAC309294-MB F Section 4
Protection functions
RER620 225
Technical Manual
Figure 102: 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
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