With 1½ breaker configurations, normally the line protection is fed from two CTs.Avoiding to add the currents from the two CTs externally before entering the IEDis important as this will enable possible bias current from both CTs to beconsidered in the current differential algorithm, and in that way assuring that thecorrect restrain will be possible, as shown in figure 29.IEC05000428-2-en.vsdxIEDIED IEDProtected zoneIEC05000428 V2 EN-USFigure 29: Line protection with 1½ breaker configurations, fed from two CTs6.1.2.1 Power transformers in the protected zone M12022-47 v6Line differential protection can also be applied with in-line power transformers inthe protected zone. Such an in-line power transformer can be situated in tap, asshown in Figure 30 or in one end of a two-terminal line. Observe that the currentsof the in-line power transformer are measured by the IED on the low voltage side.A one three-winding transformer can be included.IEC05000429-2-en.vsdxIEDIED IEDProtected zoneIEC05000429 V2 EN-USFigure 30: Transformer situated in tapA current differential protection including power transformers must becompensated for transformer turns ratio and phase shift/vector group. In the linedifferential function this compensation is made in the software algorithm, whichSection 6 1MRK 505 393-UEN BDifferential protection78 Line differential protection RED650 2.2 IECApplication manual
