Circuit Dynamics Analysis and Control of the Full-Bridge Five-Branch Modular Multilevel Converter for Comprehensive Power Quality Management of Cophase Railway Power System

This article presents an in-depth study on the circuit dynamics and control strategy of the recently proposed full-bridge five-branch modular multilevel converter (FB5B-MMC) to qualify its application potential for comprehensive power quality management of cophase railway power system (RPS) with detailed consideration of nonideal operating conditions. In particular, the circuit dynamics coupling induced by branch inductor parameter discrepancy, and the effects on branch voltages, branch currents, capacitor voltage stability, and fluctuations brought by compensating nonlinear traction loads are thoroughly investigated. With the presence of nonlinear traction loads and branch inductance discrepancy, current controllers based on the repetitive control theory are proposed to achieve high-performance suppression of the grid-side rich-content harmonic currents along with other power quality management goals. In addition, branch energy controllers are well designed to ensure capacitor voltage stability against disturbances. Therefore, the power quality management performance and converter operating stability can be effectively enhanced. The effectiveness of the FB5B-MMC and the proposed control strategy for cophase RPS power quality management are verified by means of simulations and experiments.