Optimized Operation of the Full-Bridge Five-Branch Modular Multilevel Converter for Power Quality Enhancement of Cophase Railway Power System

This article investigates the recently proposed full-bridge five-branch modular multilevel converter (FB5B-MMC) for power quality enhancement of cophase railway power system (RPS) with high-level penetration of nonlinear inductive traction loads. Since the circuit topology of the FB5B-MMC is asymmetric with respect to its two ports, two different system configurations exist when connecting the FB5B-MMC into the cophase RPS. First, an optimized system configuration and the corresponding power quality compensation scheme are proposed. The optimized operation scheme features uniform distribution of branch current stresses among the FB5B-MMC’s two phase-units and lower total branch capacity requirement, which are beneficial for branch power loss reduction and better rating utilization of semiconductor devices for cost-saving. Since the traction loads are time-varying and highly distorted, repetitive controllers are applied to further enhance the harmonic compensation performance with efficient parameter tuning and robustness. The proposed optimized operation scheme and the repetitive control strategy are validated by simulations and experiments.