sinusoidal currents the current interruption is detected after approximately
1
/
2
AC cycle. With aperiodic DC
current components in the fault current and/or in the current transformer secondary circuit after interruption
(e.g. current transformers with linearized core), or saturation of the current transformers caused by the DC
component in the fault current, it can take one AC cycle before the interruption of the primary current is reli-
ably detected.
Evaluation of the
breaker auxiliary contacts is carried out only when no current flow is detected at the instant
of initiation, i.e. the trip command of a protection function (internal or external) which is to start the breaker
failure protection. In this case the breaker is assumed to be open as soon as the auxiliary contact criterion indi-
cates open breaker.
Once the current flow criterion has picked up before the trip signal from the initiating protection, the circuit
breaker is assumed to be open as soon as the current disappears, even if the associated auxiliary contact does
not (yet) indicate that the circuit breaker has opened. This gives preference to the more reliable current crite-
rion and avoids false operation due to a defect e.g. in the auxiliary contact mechanism or circuit. If the auxil-
iary contacts indicate open breaker even though current is flowing, an alarm is given (FNos 30135 to 30144).
If both positions of the breaker are indicated (NO contact and NC contact via double point indication) the
auxiliary contact criterion is not evaluated if, at the instant of initiation, an intermediate position is indicated,
but only the current criterion. On the other hand, if the breaker failure protection is already started, the
breaker is assumed to have opened as soon as it is no longer indicated as closed, even if it is actually in inter-
mediate position.
Initiation can be blocked via the binary input
>BLOCK BkrFail
“ (No 047.2404) (e.g. during test of the
feeder protection relay).
Delay Time and Breaker Failure Trip
The breaker failure protection can be operated single-stage or two-stage
With
single-stage breaker failure protection, the trip command is routed to the adjacent circuit breakers should
the local feeder breaker fail. The adjacent circuit breakers are all those which must trip in order to interrupt the
fault current, i.e. the breakers which feed the busbar or the busbar section to which the feeder under consid-
eration is connected.
After initiation the timer T2 is started. When this time has elapsed, the indication
BF T2-TRIP(bus)
(Fno
047.2655) appears which is also intended for trip of the adjacent breakers.
With two-stage breaker failure protection the trip command of the initiating protection is repeated in a first
stage of the breaker failure protection T1 on the feeder circuit breaker, usually on a second trip coil. This is
achieved via the output indication
BF T1-TRIP(loc)
(Fno 047.2654). A second time stage T2 monitors the
response to this repeated trip command and is used to trip the adjacent breakers of the busbar or busbar
section if the fault has not yet been cleared after the repeated trip command. The output indication
BF T2-
TRIP(bus)
(Fno 047.2655) is again used for tripping the adjacent breakers.
Functions
2.17 Circuit Breaker Failure Protection
246 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
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