6.1.2.5 Parallel line application with mutual couplingGeneralIntroduction of parallel lines in the network is increasing due to difficulties to getnecessary area for new lines.Parallel lines introduce an error in the measurement due to the mutual coupling betweenthe parallel lines. The lines need not be of the same voltage in order to experience mutualcoupling, and some coupling exists even for lines that are separated by 100 meters ormore. The mutual coupling does influence the zero sequence impedance to the fault pointbut it does not normally cause voltage inversion.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 practice to neglect them.From an application point of view there exists three types of network configurations(classes) that must be considered when making the settings for the protection 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 400kV line andrail road overhead lines. This type of mutual coupling is not so common although it existsand is not treated any further in this manual.For each type of network class, there are three different topologies; the parallel line can bein service, out of service, out of service and grounded in both ends.The reach of the distance protection zone 1 will be different depending on the operationcondition of the parallel line. This can be handled by the use of different setting groups forhandling the cases when the parallel line is in operation and out of service and grounded atboth ends.The distance protection within the IED can compensate for the influence of a zerosequence mutual coupling on the measurement at single phase-to-ground faults in thefollowing ways, by using:• The possibility of different setting values that influence the ground-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 6 1MRK 506 334-UUS AImpedance protection120Application manual
