Circuit Breaker Failure Protection; Method Of Operation - Siemens siprotec 7SA6 User Manual

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Circuit Breaker Failure Protection

C53000-G1176-C133-1

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

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 6-111). A timer

T–BF in the breaker failure protection is started. The timer runs as long as a trip

command is present and current continues to flow through the breaker poles.

Feeder protection

(internal or external)

Figure 6-111 Simplified function diagram of circuit breaker failure protection with current flow

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

command 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

protection 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 6-112). For this purpose, the outputs

from the auxiliary contacts must be fed to binary inputs on the relay (refer also Section

Circuit breaker failure protection

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