ABB REB670 Applications Manual page 120

Section 6
Differential protection
114
protection scheme for this type of stations. In such application the bus section or bus
coupler current transformers shall be wired just to two separate current input of the IED.
Then in the parameter setting tool (PST) for the corresponding bays the parameter ZoneSel
shall be set to FIXEDtoZA in one bay and FIXEDtoZB in another bay. This will insure that
these currents are given to both differential zones.
When live tank circuit breakers are used, owing to the high cost of the HV current
transformer often only one current transformer is available in bus-section or bus-coupler
bay. The suggested solution in such applications is shown in figure 44.
ZA
1 2 8
ANSI01000014 V1 EN
Figure 44: 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 43. A blind spot exists between the current transformer and the circuit breaker in the
bus section or bus-coupler bay as shown in figure 44.
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 the two differential zones
Blind Zone
1MRK 505 337-UUS -
ZB
1 2 8
en01000014_ansi.vsd
Application manual