breaker is open. This will insure that if internal fault happen, in the overlapping zone, while
breaker is open, only the faulty zone will be tripped while other busbar section will remain in
service. However, due to low probability of such fault happening, while the breaker is open,
such special considerations are typically not included in the busbar protection scheme for this
type of stations. In such applications, the bus section or bus coupler current transformers shall
be wired just to two separate current inputs of the IED. Meanwhile these currents should be
given to both related differential zones. To ensure this, in two-zone busbar protections, it
requires to set the corresponding bays the parameter
ZoneSel
to
FIXEDtoZA
in one bay and
FIXEDtoZB
in another bay in the parameter setting tool (PST).
When live tank circuit breakers are used, often only one current transformer is available in bus-
section or bus-coupler bay. The suggested solution in such applications is shown in Figure
46.
en01000014.vsd
ZA
21 8
ZB
21 8
BS
Blind Zone
IEC01000014 V1 EN-US
Figure 46: Example of station with just one main CT in the bus-section bay
For this type of solution just one main CT is located on only one side of the circuit breaker.
Thus, there is no zone overlapping across the section/coupler circuit breaker as shown in
Figure
45. A blind spot exists between the current transformer and the circuit breaker in the
bus section or bus-coupler bay as shown in Figure 46.
As the example application in Figure
46 showed, for an internal fault in the blind spot, the
differential zone ZA will unnecessarily operate and open the bus section breaker and all other
feeder breakers associated with it. Nevertheless the fault will still exists on other busbar
section, but it is outside the current transformer in the bus section bay and hence outside the
zone ZB ( that is, it is external fault for zone ZB). Similar problem will also exist if section/
coupler circuit breaker was open before the internal fault in the blind zone. Therefore, the
busbar protection scheme does not protect the complete busbar.
In order to improve the busbar protection scheme with this type of station layout, it is often
required to disconnect the bus-section or bus-coupler CT from all affected differential zones
as soon as the bus-section or bus-coupler circuit breaker is opened. This arrangement can be
easily achieved within the IED. In such applications, the bus section or bus coupler current
transformer shall be wired just to one current input of the IED. Meanwhile this current should
be given to both related differential zones. To ensure this, in two-zone busbar protections, it
requires to set the corresponding bay the parameter
ZoneSel
to
FIXEDtoZA&-ZB
in PST. In six-
zone busbar protections, this single CT current should be connected to both CT inputs on the
bus section or bus coupler function block, and then invert one of them in software by using the
parameter setting
OperMode
in PST. This shall be done taking into account the actual location
of the CT star point in this bay.
1MRK 505 370-UEN D Section 6
Differential protection
Busbar protection REB670 103
Application manual
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