'(& )( )'& &) ( &!& ")& &$(( $# Ć %&( $# #)" &&#$
4 - 21
Siemens AG ⋅ May 1998
After initiation of the breaker failure protection (CBF)
by the TRIP command from the feeder protection, the
delay time (T-CBF) ( ) is startet.
If the CBF initiation is still active after the time T-CBF
has elapsed, then the polarity of the feeder current
for determination of the differential current I is
changed ("unbalancing") in the protection algorithm.
If the TRIP criterion and the current sensor criterion
are fulfilled, then a three-phase TRIP command is
issued to all feeders of the bus section with the CBF
initiating bay unit.
The principle of "unbalancing" is depicted in figure
4.16. Unbalancing causes a differential current of
twice the magnitude of the current in the CBF initiaĆ
ting feeder.
% % !'% $%#&&#" (& %)
$&&#" " '"!""
This operation mode also uses the "unbalancing"
described in chapter 4.6.1.3. Before all feeders of the
bus zone with the CBF initiating bay unit are tripped
three-phase, the TRIP command is repeated.
This operation mode lends itself well for circuit breaĆ
kers which have two trip coils.
I
1
I
2
I
5
I
3
I
4
Z<
Starting of CB failure protection
by feeder protection
Differential current before "unbalance"
I =|I
1
+ I
2
+ I
3
+ I
4
+ I
5
|=0
Differential current after "unbalance"
I =|I
1
+ I
2
+ I
3
+ I
4
- I
5
|=2I
5
Digital Input "CBF Lx"
=-I
5
=-I
5
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