Phase Capacitor Voltage Balancing Control for Co-phase Traction Power Supply System Using Direct AC/AC Ful1-bridge Modular Multilevel Converter

Full-bridge modular multilevel converter (FB-MMC) is an appropriate configuration for co-phase traction power supply systems. When unbalanced grid occurs, symmetrical grid current control is commonly employed to prevent over-currents. However, this control method leads to voltage deviation between different phases and consequently affect device life. To address this issue, a phase voltage balancing method using circulating current distribution is suggested. Above all, math model of direct AC/AC FB-MMC under asymmetric grid voltage condition is built up and instantaneous power expressions are derived, showing that power deviation affects voltage balancing between different phases. After that, a circulating currents distribution scheme is proposed to balance phase voltage. Based on distribution functions, the effects of the circulating currents distribution on arm currents are analyzed and optimal phase difference are obtained to achieve minimized electrical stress. Furthermore, the suggested scheme does not impact the symmetrical grid currents control and traction network. The accuracy and efficiency of the suggested method are ultimately verified by both simulations and experiments.