Analysis and Control of a Four-Port Megawatt-Level High-Frequency-Bus-Based Power Electronic Transformer

Aiming for mutual isolation between ports, this article proposes a four-port high-frequency-bus-based power electronic transformer (HFB-PET), the core of which is the modular multiactive bridge (MMAB). In addition to port isolation, it also has the advantages of modularity, scalability, and reduced power conversion stages. First, the configuration of the proposed PET is introduced and compared with the existing solutions. Then, based on the mathematical model, a power coordinated control scheme is presented, including the power decoupling control for the cascaded H-bridge converter of the medium-voltage ac port to handle unbalanced grid voltages and active loads, the cell-power balance control for the medium-voltage dc port to achieve dc-link voltage equalization, and the power cross-coupling suppression control employed to improve the dynamic performance of the MMAB. On top of that, a hierarchical control framework with the global synchronous clock is constructed. Besides, a soft startup scheme considering the power balance between cells is introduced. Finally, the effectiveness of the proposed control scheme under the bidirectional power flow for the dc ports and the four-quadrant power flow for the ac ports is verified by experiments conducted on the laboratory scale-down prototype and the 10 kV/2 MVA industrial prototype.