ABB RELION 620 Series Technical Manual page 715

1MRS757644 F
620 series
Technical Manual
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.
Load-shedding control
The way of load-shedding, that is, whether to operate based on underfrequency or high
df/dt or both, is defined with the Load shed mode user setting. The valid operation
modes for the Load shed mode settings are "Freq<", "Freq< AND df/dt" and "Freq<
OR df/dt".
Once the selected operation mode conditions are satisfied, the START and OPERATE
output signals are activated.
When the START output is active, the percentage of the elapsed delay time can be
monitored through START_DUR which is available as monitored data.
Section 4
Protection functions
709