Quasi-Square-Wave Modulation of Modular Multilevel High-Frequency DC Converter for Medium-Voltage DC Distribution Application

In a direct current (dc) distribution network, the modular multilevel high-frequency dc converter (MDCC) can achieve electrical isolation, voltage conversion and power transmission between the low- and medium-voltage dc buses. In this paper, quasi-square-wave (QSW) modulation, which avoids the $dv/ dt$ stress problem and maintains a high dc voltage utilization, is applied in MDCC, and the mathematical model of QSW modulation with a dual-phase-shift (DPS) scheme is established. Based on the model, the influences of staircase rising and falling process in QSW modulation on MDCC performance, including the power characteristic, current stress, switching characteristic, and efficiency characteristic, are analyzed. The influences of QSW modulation under DPS scheme on MDCC performance are also investigated. Moreover, to achieve a comprehensive optimization of MDCC performance, a multiobjective optimized control strategy based on QSW modulation under DPS scheme is proposed. Finally, a MDCC prototype is set up and the experimental results verify the correctness and effectiveness of the analysis and proposed strategy.