Third harmonic injection control for co-phase traction power supply system using direct AC/AC full-bridge modular multilevel converter

As the full-bridge modular multilevel converter (FB-MMC) enables direct three-phase AC voltage to single-phase AC voltage conversion with good power quality and high voltage level, it is suitable for the co-phase traction power supply system. However, voltage deviation occurs between arms when the direct AC/AC FB-MMC operates in equal or similar frequency conditions. To solve this problem, a voltage balancing control based on third harmonic injection of co-phase power supply system using direct AC/AC FB-MMC is proposed. With the third harmonic injection, voltage balancing between arms is ensured and the capacity of FB-MMC is enhanced. The third harmonic voltage is directly obtained from existing signals and the third harmonic current reference is produced by simple proportional-integral (PI) controller. The effect of the third harmonic currents under different voltage ratios and phase difference is quantitatively analyzed, optimized range of voltage ratio and phase difference are obtained to minimize arm currents increment and reduce overrating of converter currents. In addition, a comparison between the proposed and the conventional control scheme is studied, showing the superior of proposed method in cost and performance. Finally, the effectiveness of the proposed scheme is verified by simulations and experiments.