Analysis of Capacitor Charging Characteristics and Low-Frequency Ripple Mitigation by Two New Voltage-Balancing Strategies for MMC-Based Solid-State Transformers

This article investigates and compares various modulation methods and capacitor voltage-balancing algorithms of a modular multilevel converter for solid-state transformer applications. Characteristics of capacitor charging and discharging are analyzed for the existing single-step alternating voltage balancing and the conventional sorting algorithms with phase-shift (PS) modulation and nearest level control (NLC) methods. Based on the analysis, the single-step alternating algorithm leads to a low-frequency voltage ripple, while the conventional sorting algorithm introduces additional switching actions. To address these issues, two new voltage-balancing algorithms are proposed: a multistep alternating voltage-balancing algorithm, and a currentless sorting algorithm. The performance of the proposed algorithms with the PS and NLC modulation methods is comprehensively investigated and compared based on theoretical analysis and experimental results. The study results demonstrate the capability of the proposed methods to balance capacitor voltages while reducing the low-frequency voltage ripple and avoiding additional switching actions.