Operation and Control of Extensible Port-DC Solid State Transformer with Multi-Sequence Circulating Current Injection

Modular multilevel converter (MMC) based on solidstate transformer (M-SST) has emerged as a promising solution in hybrid medium voltage ac/dc distribution grids. By contructing a separate low voltage dc (LVdc) bus on each arm of M-SST, an extensible port-dc solid state transformer (EPDC-SST) is introduced in this paper, which not only meets requirements of different voltage levels, but also supports independent busbars, including true bipolar configuration, thereby enhancing power supply reliability. Furthermore, the six LVdc ports can operate independently while also being flexibly combined into LVdc ports with various voltage levels and capacities, building a more flexible grid architecture. This eliminates the need for additional SSTs or voltage balancers to achieve similar functionality. The key issue of EPDC-SST is how to deal with the power imbalance caused by the ports of each bridge arm, which leads to the submodule (SM) capacitor voltage deviation affecting safe operation. To address this issue, a multi-sequence circulating current injection (MCCI) method is proposed to eliminate unbalanced power and its negative effects. Based on the detailed circuit analysis of EPDCSST, a coordinated control strategy MCCI is designed, incorporating positive-sequence and negative-sequence fundamental-frequency circulating currents as well as zerosequence circulating currents. The feasibility and effectiveness of the proposed EPDC-SST has been verified by simulation and experimental results.