ABB RELION 670 Series Applications Manual page 110

Section 6
Differential protection
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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 as soon
as the bus-section or bus-coupler circuit breaker is opened. This arrangement can be easily
achieved within the IED. In such application the bus section or bus coupler current transformer
shall be wired just to one current input of the IED. Then in the Parameter Setting tool for the
corresponding bay parameter
current is given to both differential zones. In order to disconnect this current from both zones,
when the coupler/section breaker is open additional logic as shown in figure
the configuration. The following two binary inputs are at least necessary in order to guaranty
proper operation of such logic:
• Normally closed contact of the bus section or bus coupler circuit breaker
• Signal from the bus section or bus coupler circuit breaker closing circuit that somebody wants
to close the breaker
This solution does not depend on contact timing between the main contacts and auxiliary contact
of the circuit breaker. It directly follows the philosophy used for RADSS/REB 103 schemes used for
similar applications before. Principle connection between the bus-coupler CB normally closed
auxiliary contact (b-contact), REB670 and internal configuration logic, as shown in figure
104
ZA
Blind Zone
2 8
ZoneSel shall be set to FIXEDtoZA&-ZB . This will insure that this
ZB
1 2 8
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have to be done in
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Application manual