Abstract
High-frequency modular multilevel DC transformer (MMDCT) plays an important role in the bus connection, voltage conversion and power transmission in MVDC distribution networks. However, the variation of power flow in distribution network leads to the high-frequency-link (HFL) voltages in MMDCT mismatch the HFL transformer voltage ratio, resulting in the increase of circulating power and current stress, and the decrease of power factor and efficiency in M2DCT. To solve this problem, a HFL voltage matching phase-shift control (VMPC) strategy for MMDCT is proposed in this paper. Different with the traditional phase-shift control (TPC), the proposed VMPC adjusts the HFL voltages by varying the number of inserted sub-modules (SMs) in real-time, ensuring the HFL voltages match the HFL transformer voltage ratio in the whole operating range, resulting in the improvement of efficiency and performance. The detailed operation principle, control architecture and corresponding capacitor voltage balance algorithm of VMPC are investigated. On this basis, the improvements on the current stress, power factor and efficiency of MMDCT under the proposed VMPC are also analyzed. The experiment results verify the correctness and effectiveness of the analysis and proposed strategy.
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