The Hybrid Passivity-Based Control Strategy of a Novel Unidirectional Five-Level Rectifier Under the Unbalanced Load

The unidirectional five-level rectifiers proposed recently show great potential in high power medium-voltage industrial applications. However, their static and dynamic performance under conventional control strategies is far from satisfactory. The main drawbacks include complex control structures, coupling effect between the flying capacitor voltage and the dc-link capacitor voltage, significant dc-link midpoint voltage fluctuations, and unideal large-scale stability with unsatisfied dynamic performance, particularly under the unbalanced load. To address these issues, a hybrid passivity-based control (PBC) strategy comprising a nonlinear PI controller for voltage outer-loop and a PBC controller for current inner-loop is proposed. To improve the application of PBC theory in this article, a traditional equivalent model of the rectifier is replaced by a refined model. Based on the refined Euler-Lagrange model, a PBC law is derived to achieve decoupling control between the flying capacitor voltage and the dc-link capacitor voltage. In order to enhance the balancing speed of capacitor voltages and mitigate dc-link midpoint voltage fluctuations, a space vector pulsewidth modulation (SVPWM) algorithm for this rectifier is further introduced. A downscaled prototype is built to validate the feasibility and effectiveness of the proposed control strategy and SVPWM algorithm.