Protection coordination of directional overcurrent relays considering fault current direction

Directional inverse time overcurrent relays are used in protection systems of meshed networks to operate for fault currents in its forward zone of operation. They are used for primary and backup protection and their operation needs to be coordinated to assure selectivity. Different optimization techniques have been used to select their settings to achieve protection coordination, however the optimal protection coordination problem formulation doesn't consider the fault current direction to assure correct operation especially for backup relays. This paper proposes the addition of fault current direction constraint to the formulation of the optimal directional inverse overcurrent relays coordination. The proposed formulation is tested on the distribution portion of the IEEE 30 bus system considering the effect of distributed generation addition. Results show that the proposed formulation gives more accurate optimal relay settings and thus highlighting the importance of inclusion of the proposed direction constraint in the protection coordination problem.