Abstract: This paper presented a hybrid power electronic transformer based on modular multi-level converter and its control method.The basic control strategy of power electronic transformer proposed in this paper adopts voltage and current double-loop control.The outer voltage loop adopts PR controller,and the inner current loop adopts P controller.In addition,a low-frequency ripple control method is added to enable the device to operate normally under the output low frequency condition,which overcomes the defect that the device cannot operate normally under the low frequency condition by traditional control method.When a voltage drop or sudden rise occurs in the power grid,the voltage compensation control proposed in this paper can be used to compensate the voltage and improve the power quality of the grid.The feasibility of the proposed multi-functional hybrid power electronic transformer and its control strategy is verified by the simulation experiments.

Key words: hybrid power electronic transformer, modular multilevel converter, voltage and current double-loop control, low-frequency ripple suppression, voltage compensation control method, voltage drop or sudden rise