A Three-Phase Modular Multilevel DC-DC Converter for Power Electronic Transformer Applications

A three-phase modular multilevel dc-dc converter is proposed and analyzed for power electronic transformer (PET) applications. Similar to a dual active bridge converter, the proposed converter comprises two three-phase inverters/rectifiers, coupled via a medium frequency (MF) transformer. The modular multilevel converter structure is used in the medium-voltage side to meet the high-voltage requirements and to reduce the dv/dt stress on the MF transformer. The frequency of the voltage through MF transformer ( isolation frequency) is the same as the switching frequency of the power semiconductor devices, and zero-voltage switching-ON can be achieved for all the devices. With only one three-phase MF transformer, the proposed topology can greatly simplify the transformer design compared with the existed input-series output-parallel dc-dc converter structures for PET applications. A dual-phase-shift method is presented to control the output power and to balance the submodule capacitor voltages. Simulations and experimental results are provided to validate the theoretical analysis.