A Multiport Power Electronic Transformer Based on Three-Phase Four-Arm Full-Bridge Modular Multilevel Converter

This article presents a multiport power electronic transformer (PET) based on a three-phase four-arm full-bridge modular multilevel converter (MMC), which is suitable for the hybrid ac–dc distribution grid because it can provide four ports, including medium-voltage ac and dc ports, low voltage dc and ac ports. Compared with the conventional MMC-based PET, which adopts three-phase six arms MMC with full-bridge submodules (FBSMs), the proposed PET uses three-phase four arms MMC with FBSMs, which can reduce the number of submodules. Meanwhile, the modulation index can be increased and the dc-bus voltage utilization can be improved. In addition, the concentrated capacitor on the medium-voltage dc port is eliminated in the proposed PET, so high transient discharging current from the concentrated capacitor can be avoided under the dc-bus short-circuit condition. The operating principle, applied control strategy and PWM scheme are presented in detail. The proposed PET is investigated and simulated in a hybrid 10 kV ac/±10 kV dc distribution grid. A scale down experimental test is also performed. Simulation and experimental results confirm the feasibility and validity of the proposed PET and control method.