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Flexible traction power supply system (FTPSS) can eliminate neutral sections of all catenaries and improve the poor power quality of railway substations. The static power converter (SPC) is the key equipment of FTPSS and its internal faults must be quickly identified and cleared with minimal impact on catenaries to ensure the safety and reliability. This article investigates the two-stage fault characteristics in FTPSS. Before blocking, overcurrent and voltage distortion may result in a wide range of abnormal power supply. After blocking, capacitor overvoltage becomes the primary problem threatening the safety of faulty SPC because of the energy feeding from other SPCs. Equivalent circuits are established and applied to various fault conditions. The most serious fault case can be found through the comparison, so the complicated large-scale simulation scanning can be avoided. A T-shaped grounding method is proposed to solve the problem of fault current and capacitor overvoltage. Based on the calculation and simulation results of several typical fault cases, the dimensioning criterion of grounding circuit is provided for reference. Moreover, a comprehensive scheme of grounding protection is designed to detect and isolate serious faults. The theoretical fault analysis and the proposed grounding method are verified by simulations and experiments.