Impact of double circuit lines operated in parallel on distance protection

Double circuit lines, operated in parallel, have become popular in the Portuguese transmission network. When a new line is constructed between two substations, the towers are initially built to encompass two circuit lines even if only one bay is constructed at each substation line ends. The two circuits are operated in parallel having the advantage of postpone the cost of constructing new bays in the substations to the time where the power transfer demand requires. The presentation describes the impact of such topology on the selectivity performance of distance protection used on transmission lines. An analytical model is presented to evaluate the impedance seen by the distance protection for several types of faults along the line revealing the underreach effect. A numerical simulation using ATP/EMTP is conducted to validate the analytical model results. At the end, some solutions and recommendations are proposed to mitigate the problems encountered.