Research On Cascaded H-Bridge Rectifier Stage And Balance Control For DC-link Capacitor Voltages

This paper takes the cascaded H-Bridges rectifier (CHBR) stage of cascaded multi-level converter without line-frequency transformer as research subject. The control methods and mathematical models of its single-phase and three-phase system are thorough discussed. The problem of DC-link capacitor voltage balance is analysised. The modulation strategies of single-phase CHBR stage with two cells are researched in depth. A novel modulation stragtegy are proposed for balancing DC-link capacitor voltage perfectly based on two-dimensional modulation. A special operation point in the stable operating area of CHBR stage is proposed for extending unbalanced loads range and increasing voltage balance speed when the system works in the unity power factor. A novel extension method is proposed for cascading any number of H-Bridges for a single-phase system using proposed modulation strategy. The extension method of three-phase system is discussed, and a zero-sequence voltage is injected to the three-phase system for the clustered balancing control. A small power experiment platform is developed, and experiment results verify these theories proposed by this paper.