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1MRK 505 370-UEN D
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 earthed line especially during external earth-faults.
•
In case of cable feeder the stored energy in the cable will be discharged through the
earthing 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 QA1. 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 62. 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
parameter, for every bay, can be set to only one of the following five alternatives:
FIXEDtoZA
1.
FIXEDtoZB
2.
FIXEDtoZA&-ZB
3.
4.
CtrlIncludes
CtrlExcludes
5.
If for a particular CT input setting parameter
will be only included to the differential zone A. This setting is typically used for simple single
zone application such as: single busbar staions, one-and-a-half breaker stations or double
breaker stations.
If for a particular CT input setting parameter
will be only included to the differential zone B. This setting is typically used for applications
such as: one-and-a-half breaker stations or double breaker stations.
If for a particular CT input setting parameter
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 46.
If for a particular CT input setting parameter
will be:
Busbar protection REB670
Application manual
ZoneSel can be individually configured for every CT input. This
ZoneSel is set to FIXEDtoZA , then this CT input
ZoneSel is set to FIXEDtoZB , then this CT input
ZoneSel is set to FIXEDtoZA&-ZB , then this CT
ZoneSel is set to Ctrl_Includes , then this CT input
Section 6
Differential protection
M12108-3 v4
101
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