A Capacitor Voltage-Balancing Method for Modular Multilevel Converter with Three-level SMs in Variable-speed Drives

The three-level submodule (3L SM) has emerged as a viable alternative to the conventional half-bridge submodule in modular multilevel converters (MMC), primarily due to its reduced footprint. Despite this, there has been no in-depth exploration of MMC topologies utilizing 3L SMs, particularly in the context of motor drive applications. The 3L SM features two capacitors, and the MMC relies on a cascade connection of multiple 3L SMs per phase, necessitating voltage-balancing control for these capacitors. This paper introduces a voltage-balancing control strategy for multiple capacitors, incorporating high-frequency voltage and circulating current injections to ensure optimal performance in low-speed motor drive operations. The voltage commands are normalized and compared with phase-shifted carrier (PSC) PWM triangular waveforms to generate gating signals. The proposed voltage-balancing control method has been validated through a 4160-V/1-MW simulation model using a hardware-in-the-loop (HIL) system.