Flux modulation for multilevel inverters

This paper presents a new flux modulation approach for the closed-loop control of the output voltage of multilevel inverters, using hysteresis with a rectangular bounding box in the synchronous d-q reference frame and a simple set of switching rules. The modulator is constrained to select only from the three active space vectors nearest to the target reference phasor, and consequently achieves a significantly improved harmonic performance compared to other hysteresis modulators. Furthermore, by shifting the reference origin on the vector diagram, the proposed modulator can achieve the same optimal space vector sequence and placement, and therefore the same harmonic performance, as the well-known centered space-vector modulation. Without major hardware modifications, the proposed flux modulation scheme can also be readily modified to achieve reduced-common-mode voltage switching, while retaining the same optimal vector sequence and placement. Also, since the modulator is a closed-loop system, it exhibits excellent robustness to parameter variations and other system disturbances. The performance and practicability of the new modulator have been verified both in simulation and experimentally.