Scott connection transformer can be used as traction transformer, which is an important part of electrified railway traction power supply system. The application of Scott connection transformer can solve current unbalance problem of three phase power source and realize bilateral power supply of the contact network. Scott transformer and back-to-back converter can be joined together to form the railway co-phase power supply system, which will not only provide the traction load with all needed active power, but also compensate negative sequence components, harmonics and reactive power. This provides a new idea for the development of traction power supply. A simulation with Matlab/Simulink is performed to validate the feasibility of this proposed control strategy.