Issues and Solutions for Optimum Overcurrent Relays Co-Ordination in Medium Voltage Radial Distribution System

In medium voltage distribution systems, generally, electrical power is supplied through the radial feeder and ring main distribution systems. Further to protect such feeder’s overcurrent relay coordination is preferred. The overcurrent relay protection is very much efficient for these feeders. Still, there are some issues related to the overcurrent relay coordination scheme. These issues are the maloperation of relays under normal operating conditions, backup relays operation before primary response, speed of primary and backup relays, underreach and overreach problems, and coordination of overcurrent relays with fuses. In this paper, two main issues are discussed. These are backup relays responding without failure of primary relay and speed of primary and backup relays. The solution to such issues is presented. Further, one new constraint in the overcurrent relay coordination scheme has been considered along with one objective function reported in the literature. The reported objective function, after essential modification, is considered for problem formulation of over-current relay coordination. Considering the new objective function and constraint, the optimum relay setting of two systems (radial and ring main distribution systems) is achieved. Results show the effectiveness of the modified objective function and new constraint over the conventional method. For optimum overcurrent relay coordination, a genetic algorithm is used.