Setting distance relays for multi-coupled lines using permissive under-reach schemes

Environmental and financial pressures force electricity supply organizations to make maximum use of corridors available for transmission lines. This results in extensive use of double circuit lines and even several lines in close proximity. It is well known that zero sequence mutual coupling between such lines distorts the accuracy of distance relay measurements during earth faults. The setting of distance relays for lines that have multiple couplings and are terminated at different busbars, is a complex task. The authors detail some of the related problems and suggest a method for setting distance relays which form a permissive underreach scheme