TB MRF3 04.01 E
11
Application example: f
min
:
The df/dt function of the
MRF3
for load shedding is
only active as from an adjustable frequency threshold
value f
e
. If the measured system frequency drops be-
low f
e
, a time counter is started (adjustment value dt in
periods). If the measured system frequency drops be-
low the tripping value f
T
within the time dt, the
MRF3
switches off immediately. The tripping value f
T
results
from the settings df, f
e
and dt:
df
f
f
dt
e
T
= −
⇒
f
T
= fe -- df
⋅
dt
If the tripping value f
T
is not achieved within time dt, no
tripping will take place.
Only when the frequency rises above the threshold
value f
e
will the
MRF3
be reset automatically.
Figure 4.3: Working principle of the df/measuring
This is to be considered as a simplified description.
The following functional description applies for an ex-
act trip mode at load shedding:
For thresholds f
e
and f
T
this frequency timer is con-
nected in series as measuring repetition timer as de-
scribed for frequency elements f1 - f4 (see chapter
5.4.2). Thus Figure 4.3 only applies when compared
to dt the setting value of T is smaller, otherwise an ad-
ditional trip delay has to be used (see Figure 4.4).
Figure 4.4: Working principle of the df measuring
t1
threshold f
e
is fallen short of
t2
measuring repetition delay T for f
e
has elapsed
t3
threshold f
T
is fallen short of
t4
measuring repetition delay for f
T
has elapsed
The frequency value f
e
is not reached at the instant t1a
or t
1b
and the measuring repetition timer T is started. At
the instant t
2a
or t
2b
the time has elapsed and time dt
starts.
The frequency value f
e
is not reached at the instant t
3a
or t
3b
and the measuring repetition timer T is restarted
by a second timer. If its time has run down and time
dt has not elapsed yet (time 4b), tripping is initiated.
There is no tripping if time dt has elapsed before the
time of the second timer has run down (time 4a).
Summary of Contents for MRF3
Page 1: ...MRF3 Frequency Relay...
