Protection of power transmission has an important role in power systems. To have a better protection it is common to combine difference types of relays, which combination of overcurrent and distance relays is a well-known protection scheme in transmission lines. Another thing, which should be considered is defining a backup protection system that clear fault when main protection clear the fault. To achieve this aim, coordination between primary and backup protection systems should be performed. Speed, selectivity and stability are constraints, which must be satisfied by performing coordination, and sometimes it is so hard by conventional methods. Therefore, considering coordination of relays as an optimization problem and solving this problem by intelligent methods can be effective. Many intelligent methods have been suggested performing coordination between overcurrent and distance relays. In this paper, an optimal coordination of overcurrent and distance relays is proposed, which uses a hybrid Particle Swarm Optimization (PSO) with a new assumption that satisfies all constraints.

Keywords