Utility Direct Interfaced Charger/Discharger Employing Unified Voltage Balance Control for Cascaded H-Bridge Units and Decentralized Control for CF-DAB Modules

A bidirectional high-frequency isolated ac-dc converter for utility direct interfaced electric vehicle charger with vehicle-to-grid (V2G) capability is presented in this paper. This type of charger can achieve the direct utility interface with medium voltage input. It is composed of multilevel cascaded H-bridge ac-dc converter as the first stage and modular current-fed dual active bridge dc-dc converters as the second stage. For the cascaded H-bridge converter, a unified control to balance the cascaded H-bridge dc voltages is proposed for both charging and discharging modes. For the second dc-dc isolation stage, the current-fed dual active bridge converter is used in facing of the wide conversion gain in battery charging application and also to reduce the charging current ripple. Meanwhile, to improve the system flexibility and reliability, decentralized control is utilized for individual current-fed dual active bridge converters in view of the battery charging profile. The proposed method along with the presented topology can achieve voltage balance in spite of the power flow direction, simplification of the controller for the dc-dc modules and can also bear unbalanced power flow. The simulation and experimental results have been presented to show the effectiveness of the proposed control.