Optimum setting and coordination of overcurrent relays considering cable damage curve

Relay coordination of distribution networks is somehow similar to the transmission networks by some differences. One of the differences is the nature of distribution feeders are usually cables. Occurrence of high current faults in distribution networks may cause thermal damages to the cable insulations. The fault current should be cleared by main overcurrent relay. In the case of operation failure of the main relay, backup relays should be tripped after discrimination time. Optimum coordination of overcurrent relays for both distribution and transmission networks have been developed by many authors before. The authors of this paper have developed a new method for overcurrent relay coordination using genetic algorithm (GA). In this paper the objective function (OF) of the existing GA method is improved to incorporate the cable thermal damage curves. The advantages of this method are decreasing the ageing rate of the cable insulations and increasing reliability of the distribution networks. The algorithm is applied to a sample distribution network and results shows the new settings are more realistic than the results of the previous methods.