Development of digital distance relaying algorithm in combined transmission lines with underground power cables

This paper presents a digital distance relaying algorithm in combined transmission lines with underground power cables. When underground power cables are used as part of a transmission line, the design and setting of the protective relay will have to consider the different impedance characteristics because of the configuration characteristics of the cables. A power cable system consists of the conductor, a sheath, several kinds of joints, grounding types of such joints, cable covering protection units and grounding wires with resistance. Simulation for an actual complex transmission line with 154 kV XLPE cables was carried out to develop a correct distance relaying algorithm using modeling which was, in detail, established using EMTP and ATP Draw. This paper describes the accurate calculation algorithm of line impedance with a compensation factor at the relay point as well as a method to discriminate the fault section between both sections when a single line-to-ground fault occurs. Simulation results reveal that the accuracy of the line impedance obtained by the proposed algorithm has been increased compared with the impedance obtained using the general distance relaying algorithm.