Siemens siprotec 7SA6, Manual

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Functions
6-248
7SA6 Manual
C53000-G1176-C156-2
6.18 Circuit Breaker Failure Protection (optional)
6.18.1 Method of Operation
General The circuit breaker failure protection provides rapid back-up fault clearance, in the
event that the circuit breaker fails to respond to a trip command from a feeder protec-
tion.
Whenever e.g. a short-circuit protection relay of a feeder issues a trip command to the
circuit breaker, this is repeated to the breaker failure protection (Figure 6-120). A timer
T–BF in the breaker failure protection is started. The timer runs as long as a trip com-
mand is present and current continues to flow through the breaker poles.
Figure 6-120 Simplified function diagram of circuit breaker failure protection with current flow
monitoring
Normally, the breaker will open and interrupt the fault current. The current monitoring
stage quickly resets (typical 10 ms) and stops the timer T–BF.
If the trip command is not carried out (breaker failure case), current continues to flow
and the timer runs to its set limit. The breaker failure protection then issues a com-
mand to trip the back-up breakers and interrupt the fault current.
The reset time of the feeder protection is not relevant because the breaker failure pro-
tection itself recognizes the interruption of the current.
For protection functions where the tripping criteria is not dependent on current (e.g.
Buchholz protection), current flow is not a reliable criterion for proper operation of the
breaker. In such cases, the circuit breaker position can be derived from the auxiliary
contacts of the breaker. Therefore, instead of monitoring the current, the condition of
the auxiliary contacts is monitored (see Figure 6-121). For this purpose, the outputs
from the auxiliary contacts must be fed to binary inputs on the relay (refer also Section
6.22.2).
I > BF
Feeder protection
&
T–BF 0
Circuit breaker failure protection
Trip
bus-bar
(internal or external)
Bus-bar
Protection
Feeder
trip