Section 4
1MAC050144-MB C
Protection functions
270
615 series ANSI
Technical Manual
Figure 127:
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
Pickup 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
Trip Tm Freq
, the
OPR_FRQ output is activated if the underfrequency condition still persists. If the
frequency becomes normal before the module trips, 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.
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