ABB RELION 620 Series Technical Manual page 335

1MAC504801-IB E
620 series ANSI
Technical Manual
GUID-B2B8836B-7D03-4656-902D-D748F164B268 V2 EN
Figure 147: 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.
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
Pickup value df/dt setting.The df/dt detection is activated when the frequency gradient
decreases at a faster rate than the set value of Pickup 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
Section 4
Protection functions
329