Faults of the traction power system threaten the reliability and safety seriously in the urban rail transit. This paper establishes the state space average (SSA) model of the traction power system to explore the effects caused by the short-circuit fault and open-circuit fault. The SSA model solutions prove that the short-circuit fault affects train operations obviously. While the open-circuit fault affects little owing to the unilateral power supply mode. Moreover, based on the SSA model, traction network current distributions of metro lines can be predicted for protections, which is conductive to improve the reliability of traction power system.