Na7
-
MiCOM P741, P742
1.3.3.2.6.1 Description of the Logic for Internally Initiated CBF
Local o
ve
r
current element 87BB confirmation
I<
Dead pole detection threshold
I>
(note 2)
Local overcurrent element CBF confirmation
&
&
&
tBF1
tBF2-tBF1
&
BBx
Local
Retrip
Local
Circuit
Breaker
T
pABC
: T ripping signal from 87BB
Note 1: Signal to CU for back -trip (including adjacent zone(s) if failed CB is bus section or bus coupler circuit breaker
Back trip
(Note 1)
I>BB
(note 2)
Trip signal
from CU
Main busbar protection trip signal
Note 2: I>BB and I> could be enabled or disabled (scheme shown is with the 2 functions enhanced)
P3771ENa
FIGURE 10: CB FAIL ELEMENT LOGIC – INTERNALLY INITIATED
1.3.3.2.6.2 Initial Trip
A trip signal is issued by the central unit and then confirmed by the local peripheral unit. If
the (optional) threshold for the local Overcurrent protection setting for busbar protection
(I>BB) is exceeded then the local circuit breaker trip coil is energised and subsequently the
local circuit breaker is tripped.
1.3.3.2.6.3 Re-Trip after time tBF1
The peripheral unit’s dead pole detection threshold (I<) and external protection initiation (I>)
trigger the first breaker failure timer (tBF1). This signal in turn is passed through an AND
gate with the signal from the local Overcurrent protection for busbar protection (I>BB) (if a
circuit breaker failure condition has evolved this will still be present) and a re-trip command is
issued. Re-trip output contacts should be assigned using the PSL editor (including in default
PSL settings).
1.3.3.2.6.4 Back-Trip after time tBF2
A signal from the first circuit breaker timer triggers the second breaker failure timer (tBF2).
This in turn is passed through an AND gate with the signal from the local overcurrent
protection for busbar protection (I>BB), if a circuit breaker failure condition has persisted this
will still be present, and a general bus-zone back-trip signal issued via the central unit.
In summary tBF1 is used for re-trip and tBF2 for general bus zone back-trip
Because the busbar protection scheme uses the substation topology, during circuit breaker
failure conditions, circuit breaker operations are executed according to on the current state of
the system. It is therefore of paramount importance that should an internally initiated scheme
be implemented, the circuit breaker tripping order, must be thoroughly defined within the
scheme topology to guarantee correct scheme operation.
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