ABB Relion 670 Series Applications Manual page 110

Section 6
Differential protection
breaker status. However with modern busbar protection it is possible to disconnect both CTs from the
relevant zones when the bus-section or bus-coupler circuit 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 47.
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ANSI01000014 V1 EN-US
Figure 47:
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 46. A blind spot exists
between the current transformer and the circuit breaker in the bus section or bus-coupler bay as shown
in Figure 47.
As the example application in Figure
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
104
ZA
Blind Zone
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Example of station with just one main CT in the bus-section bay
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47 showed, for an internal fault in the blind spot, the differential
1MRK 505 370-UUS Rev. K
ZB
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en01000014_ansi.vsd
Busbar protection REB670
Application manual