Abstract: Multi-port SST is the key equipment of multi-voltage level AC/DC hybrid distribution network. MMC-SST has medium voltage AC, medium voltage DC and low voltage DC ports, which can be connected to the medium voltage DC distribution network and form flexible interconnection of the multi-area AC distribution network. For low-voltage side, the bipolar DC bus with unbalanced operation ability can meet the demands of users for multi-voltage level, safe and reliable power supply, while the conventional MMC-SST can only provide unipolar low-voltage DC port. Therefore, this paper proposed a bipolar MMC-SST topology and an optimal operation scheme of voltage balance based on active circulating current injection. The main work included: analyzing topology and the operation principle of MMC-SST; establishing its dynamic model, revealing the coupling relationship between the unbalanced power, capacitance voltage and circulating current; establishing and analyzing the steady-state model; proposing a novel voltage balance control strategy and optimizing the amplitude and phase of circulating current. Finally, the feasibility and effectiveness of the proposed topology and control scheme were verified by simulation model.