A Selective Method for Identifying Single-Phase Ground Faults with Transient Resistance in Isolated Neutral Medium-Voltage Networks

This article presents a new selective method for identifying a faulty feeder in 6–35 kV electrical distribution networks with the isolated neutral configuration during single-phase ground faults (SPGFs) with transient resistance. The method is based on simultaneous comparison of the zero-sequence current (ZSC) angle and magnitude across all outgoing feeders and substation buses. Unlike conventional protection approaches relying solely on current magnitude or direction, the proposed algorithm enhances sensitivity and selectivity by detecting the 180° phase opposition between the faulted feeder's ZSC and the unfaulted feeders’ capacitive ZSC. To implement this strategy, a Centralized Ground Fault Protection Unit (CGFPU) was developed, capable of real-time feeder status monitoring, fault localization, and differentiated alarm or trip signal generation. Extensive modeling of SPGF processes was conducted in Matlab Simulink, considering transient resistance values ranging from 1 Ω to 10,000 Ω. Results demonstrated the nonlinear behavior of ground fault and capacitive currents relative to the transient resistance, with a notable shift in phasor angles approaching 180° for faulty feeders and near 0° for unfaulted feeders during bus faults. The CGFPU algorithm was tested under various scenarios, including feeder and busbar faults with different resistance levels, validating its capability to issue an alarm at high- resistance faults and trip signals for low-resistance bolted faults. The proposed method effectively ensures stable operation over a wide range of transient resistance values, maintains protection sensitivity even under high-impedance fault conditions, and resists nuisance tripping caused by charging or inrush currents. Integration of advanced concepts such as dynamic threshold setting, angle-magnitude multi-criteria decision-making, and principles of transient fault analysis further enhances the system's robustness. The protection system design is adaptable for future smart grid and microgrid applications requiring precise ground fault detection. The article also provides functional diagrams, operation algorithms, and a thorough theoretical analysis supported by simulation results.