Circuit Breaker Failure Protection; Functional Description - Siemens siprotec 7SD5 User Manual

2 Functions
2.20

Circuit Breaker Failure Protection

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 protective func-
tion of the local circuit breaker.

2.20.1 Functional Description

General 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 2-143). 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.
Figure 2-143
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 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 2-144). For this purpose, the outputs
from the auxiliary contacts must be fed to binary inputs on the relay (refer also to
Section 2.23.1).
358
Simplified function diagram of circuit breaker failure protection with current flow
monitoring
7SD5 Manual
C53000-G1176-C169-1