ABB RER620 Technical Manual page 232

Section 4
Protection functions
226
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.
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RER620
Technical Manual