Siemens SIPROTEC 7SS523 V3.2 Manual page 150

Functions
5.4.2 Protection in the "Dead Zone" of the Bus Coupler
5.4.2.1 Circuit Breaker Closed
134
In the SIPROTEC 7SS52 V4 distributed busbar and breaker failure protection one bus
coupler can be assigned either one (1-bay coupler) or two (2-bay coupler) bay units.
This is mainly determined by the number of current transformers contained in the cou-
pler, although in special cases bus couplers with only one current transformer can be
allocated two bay units as well, e.g. if the bus coupler comprises more than 5 isolators.
In that case the current is fed to both bay units.
The treatment of the dead zone of the bus coupler, however, is the same in both ver-
sions.
Figure 5-30 Allocation of the bus coupler currents with only one bus coupler current transform-
er
When considering a short-circuit in the "dead zone" of the bus coupler, a distinction
must be made between cases with closed circuit breaker and cases with open circuit
breaker.
In the event of a short-circuit in the dead zone of the bus coupler with the circuit break-
er closed, bus zone BZ A is tripped because I
zone BZ B remains in operation, which means that the fault continues to be fed (Figure
5-31, page 135 "(1)"). In order to interrupt the short-circuit current, the second bus
zone must be isolated, too. For this purpose, the bay unit monitors the current in the
bus coupler after BZ A has been tripped. If the current in the bus coupler persists after
the time delay of the breaker failure protection set with the parameter T-BF-1P
(XX20/CU) or T-BF-mP (XX21/CU) has elapsed, the current I
system BZ A and for the currently stabilized measuring system BZ B is set to zero (Fig-
ure 5-31, page 135 "(2)"). This results in an unbalancing of the measuring system BZ
B, which issues a TRIP command to all affected circuit breakers. By connecting in ad-
dition the auxiliary contact "CB Open" of the coupler circuit breaker, the second busbar
can be tripped even before the breaker failure protection time T-BF has elapsed
(Chapter 5.4.2.2, page 135).
and I flow towards busbar A. Bus
1 CP
for the measuring
CP
C53000-G1176-C182-1
7SS52 V4 Manual