Analysis of Maximum Load Unbalancing Limits of Cascaded H-Bridge Rectifier Under Unity Power Factor

In a cascaded H-bridge rectifier (CHBR), the dc capacitor voltages need to be controlled to balance by the voltage balance controller when the loads are unbalanced. However, the voltage balance controller will fail to balance the dc capacitor voltages when the load unbalancing range exceeds a certain limit under unity power factor; that limit is constrained by the AC voltage and transmission power of each H-bridge. First, the relationships in CHBR were analyzed. Then, the expression between AC voltage and the power of each H-bridge was derived. Then the load unbalancing limits to maintain the dc capacitor voltage balance of each H-bridge were further deduced. In addition, the influence of ripple power and total load power on the load unbalancing limits was investigated. Finally, an experimental prototype of a single-phase three-cell cascaded H-bridge rectifier was built and the correctness of the proposed load unbalancing limits was verified by the experiment. The proposed load unbalancing limits are universal, which provides theoretical support for the stable operation and power transmission of CHBR.