A Comprehensive Review of Capacitor Voltage Balancing Strategies for Multilevel Converters Under Selective Harmonic Elimination PWM

In recent decades, applications of selective harmonic elimination pulsewidth modulation (SHE-PWM) have been extended from two-level to multilevel converters (MLCs). For most MLC topologies, one of the main challenges of using SHE-PWM lies on capacitor voltage balancing, especially with very low switching frequency in high-power applications. Due to the broad variety of MLCs, it is of great difficulty to develop a generalized capacitor voltage balancing method under SHE-PWM that is suitable for all topologies. In order to further facilitate the industrial applications of SHE-PWM on MLCs, this article provides a comprehensive review of the state-of-the-art capacitor voltage balancing strategies for MLCs under SHE-PWM. The voltage balancing methods in this article include self-balancing control, charge amount regulation, zero-sequence harmonic adjustment, redundant switching angle sets adjustment, angle modification, selective harmonic elimination model predictive control, space voltage vectors adjustment, and redundant states adjustment. Detailed comparisons of the eight voltage balancing strategies under SHE-PWM are presented to facilitate the selections of the most suitable method for specific applications. Moreover, discussions on open questions and future trends of this topic are also presented, to motivate future research and explore new alternatives.