Abstract: Due to the special characteristics of the structure and load of the traction power supply system，

serious imbalances can occur in the three-phase power supply system，which restricts the development of highspeed，heavy-haul railways. The structure of a single-three-phase combined co-phase power supply system was

introduced in detail. The system can not only radically improve power quality，but also eliminates the electrical

splitting link at the exit of the traction substation. Firstly，the balance transformation principle of co-phase power

supply equipment based on two“back-to-back”single-phase converters was analyzed. Secondly，the protection

requirements of AC-DC-AC converter，high voltage matching transformer and traction matching transformer were

analyzed，the relay protection scheme of co-phase power supply equipment was studied，and the setting calculation principle was briefly discussed. Finally，Matlab/Simulink was used to establish the simulation model of the combined co-phase power supply system. The proposed single-three-phase combined power supply system has been put into trial operation at the Sha Bath substation to verify the feasibility of the system.

      Key words: single-three-phase combined co-phase power supply system；co-phase supply equipment；relay

protection scheme；simulation analysis