Modal parameters and fault transients in double-circuit transmission lines : on the effects of conductor transposition, shield wires, and lossy soil modeling

Abstract

This work analyzes the impact of conductor transposition, shield wires, and soil modeling on modal parameters of a double-circuit transmission line. Computational modeling routines are developed to represent transmission line parameters span by span, capturing parameter variation along the right-of-way. The circuit model is constructed using EMTP-type software, considering impedances and admittances calculated to account for conductive and displacement current effects on lossy earth. Transient studies are performed on a real 500 kV double-circuit transmission line, considering various scenarios of soil resistivity, transposition and shield wire modeling. Faults are applied at every 1% of the power line, which represents 100 faults per scenario. Results show that a uniform soil model calculated from the average of deep-layer soil model resistivities along the right-of-way accurately represents heterogeneous resistivity distribution along the line, with less than 1% discrepancy. Transposition schemes rotating phases in opposite directions result in lower short-circuit currents, improving safety. The use of Kron reduction, as traditionally performed in line parameters calculations, yields conservative results compared to the realistic model, overestimating fault branch currents and short-circuit contribution currents by 1.1 p.u. (600 A).