Triple-Reference-Signal Pulsewidth Modulation Methods for Four-Level Hybrid-Clamped Converter

Four-level hybrid-clamped (4L-HC) converter is a promising topology for medium-voltage and high-power applications. By viewing the 4L-HC converter as a two-level (2L) converter unit and two three-level converter units, this article provides a new and intuitive pathway to solve the capacitor voltage imbalance issue of it. It is a triple-reference-signal pulsewidth modulation (PWM) method, which can be decoupled into three parts: 1) a variable-reference modulation scheme aims at balancing the middle dc-link capacitor voltage; 2) the scheme “Dual-Reference-Signal PWM (DRSPWM) + Zero Sequence Voltage (ZSV) Injection” is introduced to balance the upper and lower capacitor voltages and suppress the low-frequency voltage oscillations on them as well; and 3) appropriate redundant switching state selection is used to balance the flying capacitor voltages. Furthermore, different ZSV injection methods of three-level converters can be referred for the 4L-HC converter. Therefore, a second ZSV injection method is adopted to develop a second voltage balancing method for the 4L-HC converter. Finally, the proposed two voltage balancing methods are proved to be effective with simulation and experimental results of the 4L-HC converter.