A Fuzzy Coordination Method for Ground-Fault Protection Settings in LVDC Microgrids Under Grid-Connected and Islanded Operating Modes

To address the limitations of conventional inverse-time protection in low-voltage DC (LVDC) microgrids operating under both grid-connected and islanded modes, where adaptability is insufficient and rapidity and selectivity are difficult to achieve simultaneously, this paper investigates a TN-S grounded monopolar LVDC microgrid. A unified mathematical model for ground faults under both operating modes is established, and analytical expressions for the ground-fault current and the absolute value of its rate of change are derived. Based on the derived fault characteristics, the absolute value of the fault current rate of change (FCRC) is selected as the protection characteristic quantity, and an inverse-time protection-setting method incorporating a fuzzy acceleration factor is proposed. By employing a fuzzy control algorithm, the proposed method adaptively adjusts the protection operating characteristics and realizes unified protection parameter settings for both grid-connected and islanded operating modes. Simulation results demonstrate that the proposed method can rapidly and reliably clear ground faults under different operating modes. Compared with conventional inverse-time protection, it significantly reduces the protection operating time, improves the sensitivity to faults occurring at the remote end of feeder lines, and enhances the adaptability of the protection scheme to variations in operating conditions.