DC-link voltage balancing method for a hybrid asymmetric multilevel converter

The paper presents the dc-link voltage balancing method for a five-level hybrid asymmetric multilevel converter based on a neutral point clamped (NPC) converter with H-bridge (HB) converters. The considered converter, which is referred to as NPC-HB, is treated in this paper as a modification of the classic three-level NPC converter. It improves the harmonic performance of output voltage compared to the NPC converter. This improvement is made by increasing the number of levels in the converter output voltage. Nevertheless, due to the use of floating H-bridge converters inside the NPC-HB topology, the dc voltage balancing method is implemented. Basing on the converter ac side current and HB converter dc link voltage, the proposed method modifies the PWM modulation scheme in both NPC and HB converters. This modification is applied to each switching period while the converter output voltage reference is kept unchanged. Such modulation scheme is characterized as robust and guaranteeing the low harmonic content in the output voltage. Other dc voltage balancing methods of the hybrid NPC-HB converter are compared in this paper. The proposed method is experimentally verified.