Modular multilevel power electronic transformer

In this paper, a modular multilevel power electronic transformer structure for power distribution system is discussed. The proposed structure consists of two completely decoupled power stages. Modular multilevel converter is adopted as the front-end ac-dc stage to convert the medium ac voltage to medium dc voltage, or vice versa. Galvanic isolation and voltage conversion are realized by the cascaded dc-dc stage. Only one high frequency transformer is used in the dc-dc converter, which dramatically simplifies the circuit and galvanic isolation design. A dual phase shift control method is proposed to regulate the output and balance the submodule voltage in the dc-dc stage. To verify the effectiveness of the proposed converter and control method, a down-scaled prototype is constructed. The experimental results comply with the theoretical analysis very well.