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1MRS 755310
4.1.4.2.
Feeder Protection Relay
Technical Reference Manual
Overcurrent protection
The non-directional overcurrent protection detects overcurrent caused by
phase-to-phase and phase-to-earth short circuits.
When one or several phase currents exceed the set start value of the low-set stage,
I>, the stage will generate a start signal after a ~ 55 ms' start time. When the set
operate time at definite-time characteristic or the calculated operate time at IDMT
characteristic elapses, the stage will generate a trip signal.
Stage I> has a settable resetting time (both at definite-time and IDMT
characteristics), t
, for reset coordination with existing electromechanical relays or
r
for reducing fault clearance times of recurring, transient faults. If stage I> has started
and the phase currents fall below the set start value of the stage, the start of the stage
will remain active for the set resetting time. If the phase currents exceed the set start
value again, while the timer is being reset, the start of the stage will remain active.
Consequently, the set resetting time ensures that when the stage starts because of
current spikes, it will not be immediately reset. However, if stage I> has already
tripped, the stage will be reset in 50 ms after all three phase currents have fallen
below 0.5 times the set start value of the stage.
The inverse-time function of stage I> can be set to be inhibited when stage I>> and/
or I>>> starts. In this case, the operate time will be determined by stage I>> and/or
I>>>. The selection is made in SGF4.
It is possible to block the tripping of the low-set overcurrent stage by applying a
digital input signal to the relay.
When one or several phase currents exceed the set start value of the high-set stage,
I>>, the stage will generate a start signal after a ~ 30 ms' start time. When the set
operate time at definite-time characteristic elapses, the stage will generate a trip
signal. Stage I>> can be given an instantaneous characteristic by setting the operate
time to the minimum, i.e. 0.04 s.
The set start value of stage I>> can be set to be automatically doubled in a start
situation, i.e. when the object to be protected is being connected to a network.
Consequently, a set start value below the connection inrush current level can be
selected for stage I>>. A start situation is defined as a situation where the maximum
phase current rises from a value below 0.12 x I> to a value above 1.5 x I> within less
than 60 ms. The start situation ends when all phase currents fall below 1.25 x I> and
remain below for at least 200 ms. The selection is made in SGF4.
It is possible to block the tripping of the high-set overcurrent stage by applying a
digital input signal to the relay.
Stage I>> can be set out of operation in SGF3. This state will be indicated by dashes
on the LCD and by "999" when the set start value is read via serial communication.
When one or several phase currents exceed the set start value of the instantaneous
stage, I>>>, the stage will generate a start signal after a ~ 30 ms' start time. When
the set operate time at definite-time characteristic elapses, the stage will generate a
trip signal. Stage I>>> can be given an instantaneous characteristic by setting the
operate time to the minimum, i.e. 0.04 s.
REF 610
19
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