Modelling and control of a novel AT-fed co-phase traction power supply system for electrified railway

A large unbalanced current injected into the power gird and the neutral sections at the exits of traction substations have been the outstanding obstacles in conventional single-phase 25 kV AC traction power supply systems (TPSS). In order to meet the requirements of high-speed and heavy haul transportation systems, a novel autotransformer (AT)-fed co-phase TPSS is proposed, which can provide continuous power for the trains with high power quality (PQ). In this research, the electrical configuration of the new TPSS is presented, which is structured with Vx-connection traction transformer (TT) and power flow controller (PFC) adopting the modular multilevel converter (MMC) topology. Furthermore, the mathematic model of the novel system is built. The unbalanced current and the reactive power caused by traction load can be compensated effectively based on power balance principle. Moreover, a hierarchical control strategy is designed including upper optimized PQ compensation algorithm and low-layer PFC control, which can coordinate dynamic characteristics between different working patterns. Finally, the validity of the control method and the performance of the system are demonstrated by case studies. The results indicate that compared to the conventional co-phase TPSS, the proposed co-phase TPSS is able to reduce the compensation capacity by up to 59%.