Two Bidirectional Switches‐Based Three‐Phase Hybrid Modular Multilevel Converter for Medium‐Voltage AC Drives Applications

Abstract

ABSTRACT Recently, modular multilevel converters (MMCs) have been increasingly employed in medium‐voltage drive applications with a growing role in enhancing system performance. Nevertheless, MMCs for AC drive applications encounter challenges, particularly during low‐/zero‐frequency operations. The voltage ripple of MMC capacitors is inversely proportional to the frequency, resulting in high capacitor voltage ripple during low‐frequency operation which impacts output voltage quality. Various techniques have been proposed in the literature to ensure that MMC‐based AC drives maintain minimal capacitor voltage ripple during low‐/zero‐frequency operations. This paper proposes a new hybrid MMC, which integrates a conventional half‐bridge submodule (HBSM)‐based MMC with two bidirectional switches. The first switch connects the midpoints of the first and second MMC legs, while the second switch connects the mid‐points of the second and third legs. These switches are controlled, ensuring equalization of capacitor voltages during low‐/zero‐frequency operation. The proposed converter has a low number of semiconductor devices compared to the other existing hardware‐based solutions. A detailed illustration of the proposed architecture, analysis, and design is presented. Simulation results for a three‐phase medium‐voltage converter are presented, where a capacitor voltage ripple of less than 1% is achieved at zero‐frequency operation. Finally, experimentation results for a three‐phase scaled‐down prototype are presented to demonstrate the viability of the suggested approach.