Hybrid railway power conditioner based on chopper cell modular multilevel converter with four legs

A hybrid railway power conditioner (HRPC) is developed based on chopper cell modular multilevel converter (MMC) with four legs. The HRPC is able to compensate unbalance currents and reactive power. Its operational voltage at one phase is much lower than the voltage of the railway supply system. This phase of the MMC is connected to the 27.5kV bus without coupling transformer by using a MMC. The application of the MMC to the HRPC also brings the benefits of lower current distortion, removing extra filtering capacitor as well as better expansion and flexibility. The circulating current control of the MMC is studied in detail since bulky active power is transferred from one phase to the other phase in the back-to-back MMC. Simulation results are provided to show the validity of the HRPC and its control.