Switching Sequence Predictive Control of Modular Multilevel DC Transformer Under Quasi-Square-Wave Modulation

The quasi-square-wave modulation (QSM) of a modular multilevel DC transformer (MMDCT) produces voltage level transitions with short duration, making real-time selection of the switching combinations during level transitions a challenging task. This paper proposes a switching sequence predictive control strategy that mitigates this challenge by predicting the voltages and currents during voltage transitions at the initial point of the nearest highest or lowest levels, calculating the switching combinations, and selecting the switching sequence using conventional model predictive control (MPC). Since the switching sequence actions of the whole voltage level transition period are determined and stored in advance, they can be readily assigned during voltage level changes. Computation time, the balance of submodule capacitor voltages, and the total number of switching actions are all accounted for in the methodology. Furthermore, an MPC optimization is proposed to define the phase displacement between the primary- and secondary-side bridges to manage the power transferred across the galvanic barrier and control the output dc-link voltage according to a target reference. The correctness and effectiveness of the proposed control technique are verified through offline and real-time simulation results obtained from an exemplary N+1-level MMDCT.