Optimal coordination of directional earth fault overcurrent relays 67N in interconnected electric power systems

In this paper, the classic optimization model applied to coordinate 67 directional overcurrent relays in interconnected systems is applied to coordinate 67N neutral directional overcurrent relays. To do so, single-phase to neutral short-circuit currents are determined using the multiphase-multiground (4-wire) network model developed by EPRI for the OpenDSS platform. The OpenDSS network model is based on primitive impedances. This model overcomes the limitations of symmetrical components by including the effect of unbalanced loads, systems with different X/R ratios, neutral grounding through earth resistances and fault impedances on short-circuit currents passing through all relays of the system. As a key contribution, we investigate the impact of high-impedance single-phase to neutral/earth faults in the optimal clearing times and selectivity of the protection system. The results show the optimal operation time decreases as the fault impedance increases until the calculated time dial setting stagnates at the minimum value. Beyond this point, the clearing times reach a minimum and deteriorate as far the fault impedance increases.