contain current sample values, time information, trip signals, block signals, alarmsignals and 8 binary signals, which can be used for any purpose. Each IED canhave a maximum of two communication channels.On a two terminal line there is a need for one communication channel provided thatthere is only one CT at each line end (see Figure 34).IEDProtected zoneIEC05000437-2-en.vsdxIEDIEC05000437 V2 EN-USFigure 34: Two-terminal lineIn case of 1½ breaker arrangements or ring buses, a line end has two CTs (seeFigure 35).IEDProtected zoneIEDIEC05000438-2-en.vsdxIEC05000438 V2 EN-USFigure 35: Two-terminal line with 1½ breakerIn Figure 35, both local CTs on the left side are treated as separateends by the line differential protection function.Current values from two CTs in double breakers, ring main or 1½ breaker systemends with dual breaker arrangements need to be sent to the remote end. As a 64kbit/s LDCM only has capacity for one three-phase current (duplex), this impliesthat two communication channels are needed at both ends, and this is also thenormal solution.To reduce the number of communication channels needed, it is also possible to sumup the two local currents before sending by using a software in the IED. However,this is not recommended as it reduces information about bias currents. Bias currentis considered the greatest phase current at any line end, and it is common for allthree phases. When sending full information from both local CTs to the remoteend, the principle works, but when the two local currents are summed up before1MRK 505 393-UEN B Section 6Differential protectionLine differential protection RED650 2.2 IEC 83Application manual
