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Functions
2.22 Circuit Breaker Failure Protection
Circuit breaker not operational
There may be cases when it is already obvious that the circuit breaker associated with a feeder protection relay
cannot clear a fault, e.g. when the tripping voltage or the tripping energy is not available.
In such a case it is not necessary to wait for the response of the feeder circuit breaker. If provision has been
made for the detection of such a condition (e.g. control voltage monitor or air pressure monitor), the monitor
alarm signal can be fed to the binary input „>CB faulty" of the 7SD5. On occurrence of this alarm and a
trip command by the feeder protection, a separate timer T3-BkrDefective, which is normally set to 0, is
started (Figure 2-195). Thus, the adjacent circuit breakers (bus-bar) are tripped immediately in case the feeder
circuit breaker is not operational.
Figure 2-195
Transfer trip to the remote end circuit breaker
When the local feeder circuit breaker fails, tripping of the circuit breaker at the remote line end is often also
desired. This requires the command to be transmitted.
In case of 7SD5 the corresponding command — usually the trip command which is intended to trip the adjacent
breakers — is assigned to the input function for intertrip of the devices. This can be achieved by external wiring:
The command output is connected to the binary input „>Intertrip 3pol" (No. 3504) (see also Section 2.4).
An easier procedure is to combine the command output with the intertrip input via the user definable logic func-
tions (CFC).
End fault protection
An end fault is defined here as a short–circuit which has occurred at the end of a line or protected object,
between the circuit breaker and the current transformer set.
Figure 2-196 shows the situation. The fault is located — as seen from the current transformer (= measurement
location) — on the busbar side, it will thus not be regarded as a feeder fault by the feeder protection relay. It
can only be detected by either a reverse stage of the feeder protection or by the busbar protection. However,
a trip command given to the feeder circuit breaker does not clear the fault since the opposite end continues to
feed the fault. Thus, the fault current does not stop flowing even though the feeder circuit breaker has properly
responded to the trip command.
406
Circuit breaker not operational
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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