Such results are mainly due to the complicated power cables. Fault characteristics of cables are very different compared with overhead lines. The general power cables in Korea consist of a conductor, an insulator, a sheath, several kinds of joints, cable covering protection units(CCPUs), and grounding wires connected at all joints. Accordingly, as faults occur in such combined transmission lines, fault current passes through various points to the ground. This results in a large difference between the apparent impedance calculated at the relay point and the actual impedance up to the fault point. Therefore, to cover such a difference of impedance between both, a compensation method is requireed in combined transmission lines in order for then to be tripped accurately.