Adaptive Modulation Strategy for Modular Multilevel High-Frequency DC Transformer in DC Distribution Networks

The modular multilevel high-frequency DC transformer (MDCT) plays an important role in the bus connection, voltage conversion, power transmission and electrical isolation in the DC distribution networks. During practical operation, the mismatch between high-frequency-link (HFL) voltages and HFL transformer voltage ratio caused by the variation of DC bus voltages occurs, resulting in the increased reactive power, larger current stress and decreased efficiency in MDCT. To solve this problem, an adaptive modulation strategy (AMS) is proposed in this paper. The proposed AMS adjusts HFL voltages by varying the number of inserted sub-modules adaptively, ensuring the HFL voltages match HFL transformer voltage ratio. Compared with the conventional phase-shift strategies, the proposed strategy obtains higher transmission power capacity and HFL power factor, and it maintains the ideal efficiency of MDCT under light load situation. Besides, the proposed strategy avoids the optimal phase-shift angle calculation containing numerous parameters, reducing computing burden and improving practicality for MDCT. Moreover, the hybrid-wave modulation in proposed strategy makes HFL voltages compose of quasi-square-wave and square-wave, addressing the $\text{d}v/\text{d}t$ stress problem and ensuring the higher DC voltage utilization and power transmission capability simultaneously. The mathematical model, operation principle and control architecture of proposed AMS are investigated, and the experimental results in a MDCT prototype verify the correctness and effectiveness of the analysis and proposed strategy.