Grounding Fault Protection Scheme for 0.4 kV Switchgear in Metro Substations

［Objective］ Single-phase grounding faults account for over 80% of the total distribution line faults, with most inter-phase faults originating from single-phase grounding faults. In metro power distribution systems, failure to promptly isolate single-phase grounding faults in 0.4 kV switchgear may increase the risk of electric shock to personnel and lead to arcs and sparks at the fault site, potentially causing fires and explosions. Therefore, it is essential to thoroughly analyze single-phase grounding fault protection for 0.4 kV switchgear and establish appropriate protective measures. ［Method］ Based on the characteristics of metro systems and relevant regulatory guidelines, requirements of single-phase grounding fault protection configurations, principle setting, sensitivity coefficient check, and time sequence coordination for 0.4 kV main busbar and 0.4 kV distribution lines are proposed. The protection scheme for grounding faults in 0.4 kV incoming panels is clarified. The typical short-time delay settings for 0.4 kV feeder circuits are clarified. Simplified calculations are performed to determine the maximum cable length for using overcurrent protection as dual-purpose grounding fault protection for 0.4 kV feeders. Principles for grounding fault protection principles and setting criteria are analyzed. ［Result & Conclusion］ Based on the characteristics of metro systems, the following protection schemes are proposed: using overcurrent protection on the 35 kV high-voltage side to serve as single-phase grounding fault protection for the 0.4 kV main busbar; for single-phase grounding fault protection in 0.4 kV incoming circuits, it is recommended to operate in alarm mode; in the 0.4 kV feeder circuits, prioritize using overcurrent protection as dual-purpose grounding fault protection, and if dual-purpose requirements are not met, zero-sequence overcurrent grounding fault protection is recommended.