ABB RELION REF615R, Technical Manual

Page: 279 / 664

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
value set by Trip Tm df/dt , the
OPR_FRG
output is activated if the df/dt condition still
persists. If df/dt becomes normal before the module trips, 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
PICKUP
and
TRIP
output
signals are activated.
When the
PICKUP
output is active, the percentage of the elapsed delay time can be
monitored through
PICKUP_DUR
which is available as monitored data.
1MRS240050-IB C
Section 4
Protection functions
REF615R 273
Technical Manual