Load
shedding
control
ST_FRQ
OPR_FRQ
Restore
detection
Under
frequency
detection
df/dt
detection
ST_FRG
OPR_FRG
ST_REST
START
U3P
BLOCK
BLK_REST
OPERATE
RESTORE
MAN_RESTORE
Blocking
logic
dF/dt
F
Figure 516: 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 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.
Protection functions
1MRS759142 F
930
REX640
Technical Manual