7.1.2.6 Parallel line application with mutual coupling GUID-4720AB55-0E59-4531-AE14-6A6EEC72C924 v1General GUID-E2FFF4A4-7D81-4440-87CE-3DCEAE2E42BD v2Introduction of parallel lines in the network is increasing due to difficulties to getnecessary land to build new lines.Parallel lines introduce an error in the measurement due to the mutual couplingbetween the parallel lines. The lines need not be of the same voltage level in orderto experience mutual coupling, and some coupling exists even for lines that areseparated by 100 meters or more. The mutual coupling does influence the zerosequence impedance to the fault point but it does not normally cause voltageinversion.It can be shown from analytical calculations of line impedances that the mutualimpedances for positive and negative sequence are very small (< 1-2%) of the selfimpedance and it is a common practice to neglect them.From an application point of view there exists three types of networkconfigurations (classes) that must be considered when making the settings for theprotection function.The different network configuration classes are:1. Parallel line with common positive and zero sequence network2. Parallel circuits with common positive but isolated zero sequence network3. Parallel circuits with positive and zero sequence sources isolated.One example of class 3 networks could be the mutual coupling between a 400 kVline and rail road overhead lines. This type of mutual coupling is not so commonalthough it exists and is not treated any further in this manual.For each type of network class, there are three different topologies; the parallel linecan be in service, out of service, out of service and earthed in both ends.The reach of the distance protection zone 1 shall be different depending on theoperation condition of the parallel line. This can be handled by the use of differentsetting groups for handling the cases when the parallel line is in operation and outof service and earthed at both ends.The distance protection within the IED can compensate for the influence of a zerosequence mutual coupling on the measurement at single phase-to-earth faults in thefollowing ways, by using:• The possibility of different setting values that influence the earth-returncompensation for different distance zones within the same group of settingparameters.• Different groups of setting parameters for different operating conditions of aprotected multi circuit line.Section 7 1MRK 505 393-UEN BImpedance protection114 Line differential protection RED650 2.2 IECApplication manual
