Design of a Digital Protection Relay Simulation for 11kV Feeders at Transmission Company of Nigeria Substations

This project designs and simulates a digital protection relay system for 11kV distribution feeders at Transmission Company of Nigeria substations, addressing the critical role of feeder protection in limiting fault damage and minimising outage duration. Legacy electromechanical overcurrent relays still in service at many Nigerian substations lack selectivity, self-testing capability, and the communication interfaces needed for modern substation automation. The simulation environment is implemented in MATLAB Simulink with the SimPowerSystems toolbox, modelling an 11kV radial feeder typical of TCN's distribution interconnection substations. The digital relay model incorporates a definite-time overcurrent element, an inverse-time overcurrent element following the IEC 60255-151 standard curve family, an earth fault element, and an auto-reclose logic with configurable reclose shot count and dead time. Fault scenarios simulating single-phase to earth, phase-to-phase, and three-phase bolted faults at varying feeder distances were applied to the Simulink model. The relay model correctly identified all fault types within one cycle of fault inception and issued trip signals within the design operating times across all tested fault locations. Grading margin calculations for the simulated feeder were performed to ensure discrimination with upstream and downstream protection devices. Sensitivity analysis confirms the relay correctly detects the minimum fault current at the far end of the feeder under minimum generation dispatch conditions. The project provides a validated relay model that can serve as a design reference for TCN engineers upgrading legacy protection systems. Keywords: digital protection relay, overcurrent, 11kV feeder, Simulink, TCN.