DC Voltage Control Strategy of Three-Terminal Medium-Voltage Power Electronic Transformer-Based Soft Normally Open Points

The power electronic transformer (PET) or solid-state transformer (SST) is a key technique in the future distribution grid. One of PET's future possible application is the soft normally open points (SNOP) to handle the power and voltage challenge brought by the renewable energy sources. In this paper, a PET-based three-terminal medium-voltage SNOP topology is proposed. The topology consists of three cascade H-bridge (CHB) stages and a multiactive bridge (MAB) stage. The dc voltage control strategy for the proposed PET-SNOP is studied. Two strategies - the CHB+CHB+CHB strategy and CHB+MAB strategies are developed from the existing strategies but they both have some drawbacks. In order to overcome the drawbacks, a novel dc voltage control strategy-CHBx3+MAB strategy is proposed, which combines the advantages and avoids the drawbacks of these two strategies, moreover, it also brings more flexibility and robustness to the dc voltage control. The effectiveness of the proposed PET-SNOP topology as well as the proposed CHBx3+MAB dc voltage control strategy is verified by both simulation and experiment results.