P54x/EN AP/La4 Application Notes
(AP) 6-72
MiCOM P543, P544, P545 & P546
AP
4. APPLICATION OF NON PROTECTION FUNCTIONS
4.1 Single and three phase auto-reclosing
4.1.1 Time delayed and high speed auto-reclosing
An analysi
s of faults on any overhead line network has shown that 80-90% are transient in
nature.
In the majority of fault incidents, if the faulty line is immediately tripped out, and time is
allowed for the fault arc to de-ionize, reclosure of the circuit breakers will result in the line
being successfully re-energized. Auto-reclose schemes are employed to automatically
reclose a switching device a set time after it has been opened due to operation of protection,
where transient and semi-permanent faults are prevalent.
The principal benefit gained by the application of auto-reclosing to overhead line feeders is
improved supply continuity and possibly reduced costs since fewer personnel may be
required. On some systems the application of high speed auto-reclose may permit a higher
level of power transfer while retaining transient stability for most faults which are likely to
occur. High speed single phase auto-reclosure can offer increased benefits over high speed
three phase auto-reclosure in terms of a higher power transfer limit and reduced stress on
reclosing.
4.1.2 Auto-reclose logic operating sequence
The Mi
COM P543 and P545 have a standard auto-reclose scheme configured to permit
control of one circuit breaker only.
The MiCOM P544 and P546 can be used in applications such as breaker-and-a-half, or ring
bus topologies, where two circuit breakers feed each line and both need to be controlled by
the auto-reclose logic.
For high speed auto-reclose only the instantaneous protection would normally be set to
initiate auto-reclose. This is because for best results when applying high speed auto-reclose
to improve a system stability limit, it is important that the fault should be cleared as quickly as
possible from both line ends.
The auto-reclose scheme in the P544 and P546 provides auto-reclosing of a feeder terminal
switched by two circuit breakers. The two circuit breakers are normally arranged to reclose
sequentially with one designated leader circuit breaker reclosing after a set dead time
followed, if the leader circuit breaker remains closed, by the second circuit breaker after a
further delay, the follower time.
4.1.3 Setting guidelines
4.1.3.1 Circuit breaker healthy
The P54x monitors the st
ate of the auxiliary contacts (52A, 52B) of the controlled circuit
breaker(s) to determine healthy circuit breaker status before allowing auto-reclose.
Monitoring of the auxiliary contacts is recommended, but this check can be disabled by not
allocating opto inputs to this function, and deliberately applying logic 1 onto the
corresponding DDB signals within the PSL.
4.1.3.2 Number of shots
An important consideration is the ability of the circuit breaker to
perform several trip close
operations in quick succession and the effect of these operations on the maintenance period.
The fact that 80 - 90% of faults are transient highlights the advantage of single shot
schemes. If statistical information for the power system shows that a moderate percentage
of faults are semi-permanent, further DAR shots may be used provided that system stability
is not threatened.
Note: DAR shots will always be three pole.
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