A Novel Voltage Balancing Strategy for Four-Level Hybrid-Clamped Converters Under Selective Harmonic Elimination PWM

This article presents a novel voltage balancing method with redundant switching angle sets for four-level hybrid-clamped (4L-HC) converters under selective harmonic elimination pulse-width modulation (SHE-PWM). A modification to traditional SHE-PWM formulation is made to achieve additional solution spaces, which reduces the maximum number of harmonics eliminated with a minimum effect on the waveform's harmonic distortion. Facilitated by this modification, three redundant switching angle sets with natural-balancing, overall charge, and overall discharge abilities for central dc-link capacitor can be searched out by incorporating the corresponding capacitor charge equation into the SHE formulation, then applied for active balancing issue. For upper and lower dc-link capacitors, they possess natural balancing abilities with the well-controlled middle dc-link capacitor. Besides, no control is required for flying capacitors, since no current will flow through them with this method. In steady states, the switching angle set with natural-balancing ability is used, while the other two sets are used if the voltage drifts beyond the expected band limits. Compared with the existing balancing strategies for 4L-HC under SHE-PWM, this method will not result in additional switching frequency increase while still possessing good harmonic performances. Simulation and experimental results demonstrate the validity of this method.