A Strategy for Voltage Balancing of Three-Phase Cascaded H-Bridge Rectifier

The power electronic transformer (PET) is a module-cascaded converter with high-frequency-link technology to realize high-power conversion. A typical structure of PET contains a three-phase cascaded H-bridge rectifier in the first stage and several dual active full-bridge (DAB) dc-dc converters in the second stage. Due to dc-link voltage and power unbalancing in the cascaded modules in the first stage, the system may have overvoltage or overcurrent issues. In this paper, based on star-connected cascaded H-bridge (CHB), a zero-sequence voltage injection method is proposed based on the three-phase d-q model to balance dc-link voltage in mutual phase while a hybrid modulation method is proposed to realize dc-link voltage balancing in inner phase. Besides the function of the dc-link voltage balancing, the proposed method is also capable of suppressing the negative sequence and zero sequence components in the system and reducing switching losses.