An intelligent model and simulation of high voltage and medium voltage transmission line protection scheme using time overcurrent relay optimization settings: a case of Rwanda power network

This paper presents an optimized inverse-time overcurrent relay (OCR) setting and coordination methodology for interconnected high-voltage and medium-voltage (HV/MV) transmission systems, validated on a real 110 kV/15 kV utility substation network. Analytical feeder impedance equivalences and phase-to-phase fault current models were developed to evaluate relay performance under multiple fault locations ranging from 0% to 100% of feeder length. Standard inverse (SI), very inverse (VI), and extremely inverse (EI) IEC overcurrent relay characteristics were applied to both incoming and outgoing feeders. The study incorporates practical power system parameters, including short-circuit MVA, positive and negative sequence impedances, X/R ratio, transmission line length, current transformer (CT) ratios, full-load current, line capacity, grid frequency, and real power. Relay pickup currents and a fixed time multiplier setting (TMS = 0.05 s) were carefully selected to ensure safe, reliable, and rapid fault clearance. Fault current levels at each feeder location were computed based on feeder impedance variation, CT ratios, and pickup settings to determine optimal relay operating times and coordination intervals. Simulation results demonstrate that the proposed OCR coordination scheme achieves fast and selective fault isolation, with coordination interval times of 0.062–0.0927 s, 0.0720–0.0949 s, and 0.0661–0.0764 s for extremely inverse, very inverse, and standard inverse relay characteristics, respectively. The extremely inverse characteristic provided the fastest response for remote faults, while standard inverse relays ensured stable operation near the substation. MATLAB/Simulink-based validation confirms improved relay selectivity, reduced coordination delays, and effective mitigation of cascading outages. The results indicate that properly optimized inverse-time OCRs can provide a cost-effective, reliable, and practical protection solution for HV/MV transmission networks, particularly in developing and interconnected power grids where simplicity and coordination speed are critical.