Operation and Control of DC Solid State Transformer With Bipolar LVDC Ports

DC Solid State Transformer (DCSST) plays a significant role in medium/low voltage DC distribution grid, which functions as changing voltage level and managing power flow between the MVDC distribution grid and the LVDC micro- grid. The conventional DC transformers can only provide single-pole or pseudo-bipolar LVDC ports. Therefore, this paper proposes a bipolar-type DC transformer topology based on NPC (Neutral Point Clamped)-DAB (Dual Active Bridge) and its asymmetric operation control scheme. This topology has bipolar LVDC ports which can meet the requirements of low- voltage users for multiple voltage levels, power safety and reliable supply. The main work of this paper includes: a DC transformer topology with bipolar LVDC ports based on NPC - DAB are proposed; the steady model based on DC current injection are established; the mechanism of bipolar asymmetric operation principle and the method of eliminating DC bias flux in the transformer are analyzed based on the steady model; the dynamic model based on DC injection are established; the asymmetric operation control strategy and the control parameter design method are proposed based on the dynamic model. Finally, the feasibility and effectiveness of the proposed control strategy are verified by MATLAB/Simulink experiment.