Method of operation7SV512
V1
25C53000---G1176---C91
4.2 Circuit breaker failure protection
4.2.1 General
The circuit breaker failure protection provides rapid
back-up clearance of fault, in the event that the cir-
cuit breaker fails torespond to a trip command from
the feeder protection.
Whenever e.g. a short-circuit protection relay of a
feeder issues a trip command to the breaker, this is
repeated to the circuit breaker failure protection. A
timer T---BF in the breaker failure protection is
started. The timer runs as long as a tripping com-
mand is present and current continues to flow
through the breaker poles (Figure 4.2).
feeder
protection
I---
BF
&
T---
BF
protection
trip
Trip bus-bar
circuit breaker
failure protection
7SV512
feeder
bus
bar
Figure 4.2 Simplified function diagram of circuit
breaker failure protection
Normally, the breaker will open and interrupt the
fault current. The current monitoring stage quickly
resets (typical 10 ms) and stops the timer.
If the tripping command is not carried out (breaker
failure case), current continues to flow and the timer
runs to its set value. The breaker failure protection
then issues acommand totrip the back-upbreakers
and interrupt the fault current.
Thereset time ofthe feeder protection isofnoimpor-
tance because the breaker failure protection itself
recognizes the interruption of the current.
For protection relays for which the tripping criteria
are not dependant oncurrent (e.g. Buchholz protec-
tion), current flow is nota reliable criterion for proper
operation of the breaker.
Inthe such cases, thecircuit breaker position canbe
read from the auxiliary contacts of the breaker.
Therefore, instead of monitoring the current, the
condition of the auxiliary contacts is monitored. For
that purpose, the outputs from the auxiliary contacts
must be fed to binary inputs on the relay (refer also
Section 4.2.3).
&
7SV512
L
+
feeder
protection
protection
trip
Trip bus-bar
circuit breaker
failure protection
feeder
bus
bar
T---
BF
Figure 4.3 Simplified function diagram of circuit
breaker failure protection controlled
by circuit breaker auxiliary contact
In addition to this basic operation sequence,
7SV512 contains further possibilities which are de-
scribed in detail in the following sections.
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