Most multi circuit lines have two parallel operating circuits.Parallel line applications GUID-905ACED8-CD00-4651-90F1-96D1885BA856 v1This type of networks is defined as those networks where the parallel transmissionlines terminate at common nodes at both ends.The three most common operation modes are:1. Parallel line in service.2. Parallel line out of service and earthed.3. Parallel line out of service and not earthed.Parallel line in service GUID-22E1A46C-92BD-4EE4-9A2D-9EFE2DB7B4D8 v3This type of application is very common and applies to all normal sub-transmissionand transmission networks.Let us analyze what happens when a fault occurs on the parallel line see figure 55.From symmetrical components, we can derive the impedance Z at the relay pointfor normal lines without mutual coupling according to equation 36.Z UI I Z ZZUI I Kphphphph N=+ ⋅ −⋅= + ⋅3 3300 110IECEQUATION1275 V2 EN-US (Equation 36)Where:Uph is phase to earth voltage at the relay pointIph is phase current in the faulty phase3I0 is earth fault currentZ1 is positive sequence impedanceZ0 is zero sequence impedanceZ0mA BZ< Z< IEC09000250_1_en.vsdIEC09000250 V1 EN-USFigure 55: Class 1, parallel line in serviceThe equivalent circuit of the lines can be simplified, see figure 56.1MRK 505 393-UEN B Section 7Impedance protectionLine differential protection RED650 2.2 IEC 115Application manual
