ABB RELION Series Technical Manual page 450

Section 4
Protection functions
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Figure 225: 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
reset timer is activated. If the reset timer reaches the value of the Reset delay time
setting, the timer resets and the ST_FRG output is deactivated.
1MRS758755 A
REC615 and RER615
Technical Manual