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 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.
Protection functions
1MRS758755 C
458 REC615 & RER615
Technical Manual
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