Directional inverse time overcurrent relay for meshed distribution systems with distributed generation with additional continuous relay settings

Coordination between inverse time overcurrent relays within meshed systems are achieved by adjusting two relay settings; pick up current and time multiplier settings. The operating time of these relays is also a function of two additional constants; one represents the constant for relay characteristics and the other one represents the inverse time type. For typical relays, each of these two constants has four definite values and choosing between them indicates the selected relay operating curve: either standard inverse, very inverse, extremely inverse or long time standby earth fault time-current relay characteristics. In this paper, a coordination strategy, that takes advantage of the available capabilities in microprocessor based relays, is proposed by considering the two relay characteristic constants as continuous variable settings that can be adjusted in addition to the conventional pickup current and Time Dial Setting (TDS). The protection coordination problem is formulated as a nonlinear programming problem where the main objective is to minimize the overall time of operation of relays taking into account protection coordination constraints. The proposed approach is applied to the IEEE 14 bus system and is compared with the conventional two setting relay.