ABB RELION 620 Series Technical Manual page 714

Section 4
Protection functions
4.6.2.4
708
Throughout this document, "high df/dt" is used to mean "a high rate of
change of the frequency in negative direction."
Once the frequency has stabilized, LSHDPFRQ can restore the load that is shed during
the frequency disturbance. The restoration is possible manually or automatically.
The function contains a blocking functionality. It is possible to block function outputs,
timers or the function itself, if desired.
Operation principle
The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of LSHDPFRQ can be described using a module diagram. All the
modules are explained in the next sections.
GUID-17F7A604-487F-4D45-8150-AE041BB939B1 V2 EN
Figure 359: 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
1MRS757644 F
620 series
Technical Manual