Na7
-
MiCOM P741, P742
1.3.3.2.7 Externally Initiated 50BF
FIGURE 13: CB FAIL ELEMENT LOGIC – EXTERNALLY INITIATED
Taking into account the relationship between the busbar protection and the circuit breaker
failure protection certain operators prefer an integrated solution where the breaker failure
may be initiated by external protection but executed in the busbar scheme. Tripping is then
worked out in the section or zone.
On an overhead line for example the external commands may be generated by the distance
protection (21). Generally these commands are on a per phase basis and therefore the
tripping commands must be to. In the diagrams these signals are labelled Tp
A
, Tp
B
, Tp
C
(Tripping pole A, B or C).
The logic is similar to that for internally initiated CB fail protection but utilises tBf3 for re-trip
and tBF4 for back-trip functions.
1.3.3.2.8 Local re-trip after time tBf3
This re-trip command can be applied via either the main or back up trip coil. It is possible to
choose between the 3 following modes:
• Local re-trip activated/deactivated via PSL. The relay used for this function can use the
same fixed logic for the busbar protection or other independent relays.
• A re-trip can be applied after a time tBF3. This is typically set at 50ms when a single
phase trip and re-trip is used. This prevents loss of phase selectivity by allowing the main
protection trip to execute via the main CB trip coil before re-trip command is executed by
the back-up CB trip coil.
• Single or three phase re-trip is possible. If the feeder protection executes single-phase
tripping, the three-phase re-trip must be carried out in time tBF3 and this must be
adjusted to have a value higher than the normal operation time of the circuit breaker.
Typical setting under this condition is 150ms.
1.3.3.2.8.1 General zone trip after time tBF4
When both the local trip and re-trip have failed, the countdown continues with a second timer
adjusted to have a value of tBF4 - tBF3. The end of this time thus corresponds to total time
tBF4, beyond which a persistent circuit breaker failure condition is declared.
Information is then relayed to the Central unit for routing to the other peripheral units, and the
associated circuit breakers, in the adjacent zone(s) for a general three-phase back-trip.
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