ABB Relion 670 Series Applications Manual page 108

Section 6
Differential protection
Such feeder set-up can be often found in GIS stations where cable CTs are used for busbar protection. If
in such feeder the line disconnector 989 is open and then immediately the grounding switch 189G is
closed before the busbar disconnectors 189G & 289 are open there is a danger to get current unbalance
into the zone differential measurement under following circumstances:
• In case of parallel lines zero sequence mutual coupling can induce zero sequence current into the
grounded line especially during externalground-faults.
• In case of cable feeder the stored energy in the cable will be discharged through the grounding
switch at the moment of its closing.
In order to avoid such problems for busbar protection the status of line disconnector can be monitored by
busbar protection and CT measurement can be disconnected from both differential zones as soon line
disconnector is open. Similar functionality can be obtained by instead monitoring the position of feeder
breaker 152. In such case the breaker closing signal shall be connected to busbar protection as well.
6.1.3.5 CT connection control
The IED offers an extremely effective solution for stations where zone selection (that is, CT switching) is
required. This is possible due to the software facility, which gives full and easy control over all CT inputs
connected to the IED.
In applications where zone selection (that is, CT switching) is required (for exampledouble or multiple
busbar stations) all CTs will be permanently connected to the analogue input module(s), as shown in
Figure 63. Therefore, all necessary switchingof currents will be performed in internal software logic.
In general, there are dedicated binary inputs available for one CT at each bay, to control its current
connection towards differential zones, based on operational status of related disconnectors. In addition,
in two-zone busbar differential protections, a setting parameter is also introduced to determine, together
with the binary inputs, how the bay CT current shall be connected to the differential zones.
The philosophy is to allow every CT input to be individually controlled by a setting parameter. This
parameter called ZoneSel can be individually configured for every CT input. This parameter, for every
bay, can be set to only one of the following five alternatives:
1. FIXEDtoZA
2. FIXEDtoZB
3. FIXEDtoZA&-ZB
4. CtrlIncludes
5. CtrlExcludes
If for a particular CT input setting parameter ZoneSel is set to FIXEDtoZA, then this CT input will be only
included to the differential zone A. This setting is typically used for simple single zone application such
as: single busbar staions, breaker-and-a-half stations or double breaker stations.
If for a particular CT input setting parameter ZoneSel is set to FIXEDtoZB, then this CT input will be only
included to the differential zone B. This setting is typically used for applications such as: breaker-and-a-
half stations or double breaker stations.
If for a particular CT input setting parameter ZoneSel is set to FIXEDtoZA&-ZB, then this CT input will be
included to the differential zone A, but its inverted current value will be as well included to the differential
zone B. This setting is typically used for bus coupler or bus section bays when only one current
transformer is available see Figure 47.
If for a particular CT input setting parameter ZoneSel is set to Ctrl_Includes, then this CT input will be:
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1MRK 505 370-UUS Rev. K
M12108-3 v4
Busbar protection REB670
Application manual