Reliability is a very important factor in designing of power network, because in the power system with low reliability, maintenance costs for power system is increased and causes creating damage that it is not compensated. Combined transmission line with cable line has considerable effects on power system reliability indices, because high voltage insulations are weak points in power systems in view of reliability that they are used in design of high voltage cables. On the other hand, in each power system with combined lines, optimum design of insulation system and considering suitable protection for cable lines by surge arrester against over voltage, increase power system reliability. Therefore in this study, reliability analysis of power system with combined line is done using event tree method. Also effect of location and number of surge arresters on the combined line reliability indices is studied and then variations the reliability indices have been analyzed by changing in length and age of cable line in the network. Results show that effect of length and age of cable and number of surge arresters in combined line on power system reliability indices is significant.

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