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4 - 24
Siemens AG ⋅ May 1998
4.6.1.6 Initiation by external breaker failure proĆ
tection
In this operation mode, the bus zone to which the
feeder with the faulty circuit breaker is connected, is
instantaneously isolated according to the isolator reĆ
plica registered in the master unit. Detection of a cirĆ
cuit breaker failure is done outside the 7SS52, usually
by means of a current/time measurement (see also
figure 4.19).
7SS52
autonomous circuit
breaker failure protection
>
&
initiation
feeder protection
feeder current
CBF Lx
t
CBF release
(zone-selective)
phase-selective
TRIP command
(feeder-selective)
transfer trip
command
BI suĆ
pervision
(figure 4.32,
chapter
4.7.8.5)
Figure 4.19 Basic scheme "autonomous circuit breaker failure protection"
4.6.2 Circuit-breaker failure protection for
busbar faults
Circuit-breaker failure can occur during a busbar
short-circuit, too. In this case, the current from the
remote end must be interrupted. (refer Fig 4.20).
An inter-trip signal must be transmitted to the remote
end circuit-breaker. If signal transmission is not availĆ
able, then the distance protection at the remote end
must clear the short-circuit current via its second
zone time (usually 0.5 s).
If signal transmission is used then the current can be
interrupted faster, i.e. after the breaker failure time
T-BF (DA 6204/ZE).
In the 7SS52, the duration of the currents is moniĆ
tored by the time T-CBF (DA 6204/ZE) in all feeders
which are to be tripped after a busbar short-circuit. If
the current persists above the feeder-selective set
threshold I>CBF (DA 67yy/ZE) after the breaker faiĆ
lure protection time has elapsed, then the corresponĆ
ding bay units issue transfer trip commands (refer to
figure 4.21).
If the feeder is equipped with line differential protecĆ
tion, this connection (pilot wires or optical fibre conĆ
nections) can be used to disconnect the remote end
either by local unbalancing (polarity inversion) of the
feeder current or by forced tripping.
circuit-breaker
fails!
circuit breaker at the
remote feeder terminal
Figure 4.20 Circuit-breaker failure during busbar short-circuit
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