Arm Current Deviation Minimizing Method for Co-Phase Traction Power Supply System Using Direct AC/AC Full-Bridge Modular Multilevel Converter

The direct ac/ac full-bridge modular multilevel converter (FB-MMC) demonstrates a suitable structure for a co-phase traction power supply system. However, it is important to note its inherent current deviation between six arms, which leads to nonuniform temperature distributions and consequently asymmetrical electrical stresses. Furthermore, circulating current distribution and harmonics injection used to ensure sub-module voltage balancing can exacerbate the arm current deviation. Therefore, this article aims to propose an effective method that minimizes inherent arm current deviation. First, the mathematical model of direct ac/ac FB-MMC is built up and root mean square (rms) of each arm current considering harmonics injection and circulating current distribution are derived. By using arm current average difference function, the extent and influencing factors of arm current deviation are revealed. Then, the optimum phase angle of the single-phase side voltage is derived to minimize arm current deviation under both balanced and unbalanced grid voltage conditions. Through purely mathematical manipulations, the voltage phase on the single-phase side can be acquired from that of the three-phase side without additional controllers. Finally, the performance and effectiveness of the control scheme are verified by both simulation and experimental results.