Modular Multilevel Converter With Multitasking Magnetics for AC/AC Power Conversion

This article presents an AC/AC MMC topology with integrated transformer that provides an overall voltage boosting effect beyond that enabled by the transformer turns ratio alone. The proposed three-phase topology also has a simple structure characterized by only six arms and can achieve a low total semiconductor effort. It is thus well suited for applications where higher AC voltage step ratios are needed. These salient features are enabled by a novel power transfer mechanism where the converter-side transformer windings are multitasked to carry current with multiple frequency components, while the core is rated only for a single AC frequency flux similar to conventional power systems transformers. A control strategy based on the dq reference frame is developed that exploits decoupled control of differential mode and common mode currents in the phase arms. The converter's performance is studied under different scenarios using simulations, including fixed frequency and variable frequency operation. A scaled-down 208/104 V, 1 kW three-phase laboratory prototype is employed to further validate the converter operating principle through a series of experiments.