A Nonstandard Time-Voltage-Current Characteristic for Overcurrent-Distance Coordination

The protection coordination of overcurrent and distance relays is a highly constrained optimization problem. The task gets more intricated when the power system is meshed which means different fault current directions in one protective zone. The operation time of distance relays in different zones is usually set to constant values. On the other hand, the standard time-current operation characteristic of overcurrent relays does not give a high freedom degree in the optimization process. These two phenomena jointly result in more constraint violation possibility. This paper proposes a relaxed non-standard time-voltage-current characteristic for overcurrent relays that allows a suitable optimization space and lower constraint violation probability. Moreover, a coordination algorithm is devised which reduces the computational burden and gradually increases the number of constraints in the optimization process. In order to evaluate the performance of the proposed method, the IEEE 39-bus test system is put under simulations that compare the operation time and constraint violations in the conventional and proposed coordination procedure.