The purpose of the work presented in this paper is to develop an accurate impedance calculation algorithm considering the special characteristics of combined transmission lines which have power cables. In order to achieve such an aim, the actual combined transmission line was actually operated in the power systems was selected and modeled to simulate in EMTP and ATP Draw. In particular, the transient behavior of cables was analyzed in detail under actual cable conditions formulated by using cable equipment like grounding wires and CCPUS, as well as structures of cable systems during the faulty accidence. In addition, this paper proposes an algorithm that will discriminate that whether the fault section is in the overhead line or in the power cable using current and voltage measured at the relay point, when the fault occurs in a combined transmission line.