Extending the operating range of cascaded H-bridge based multilevel rectifier under unbalanced load conditions

This paper presents a novel control strategy for cascaded H-bridge based multilevel rectifiers (CHBBR) to operate under unbalanced load and dc-bus voltage conditions. The proposed control strategy exploits the power factor as a degree of freedom to stabilize the rectifier operation. The controller is designed for single-phase seven-level rectifiers and its main feature is that it allows the multilevel rectifier to operate under heavy unbalanced load conditions while absorbing current with low harmonic content. It also eliminates the heavy and bulky line side filter and transformer in addition to extra dc-dc converters needed to obtain different voltage levels. Performance of cascaded H-bridge multilevel rectifier and the corresponding controllers are evaluated based on digital time-domain simulation studies in the PSCAD/EMTDC environment. The studies demonstrate the capability of the proposed control strategy to maintain the stability of the rectifier under unbalanced load conditions.