Operation, analysis and experiments of DC transformers based on modular multilevel converters for HVDC applications

This paper studies the operation, analysis and experiments of multilevel high frequency link transformers (MHFLT) based on modular multilevel converters (MMC) for high voltage DC applications. The multilevel dual active phase shift is proposed to operate the MHFLT using a high switching frequency, which brings about many advantages and makes the operation quite different from that of the traditional DC transformer (DCT) based on a dual active bridge and the fundamental frequency MMC widely used in flexible HVDC transmission. Specifically, MHFLT is suitable for high voltage levels, which is due to its good switching characterization, ability to cut itself off from an external fault, and it can also achieve redundancy operations when a sub-module fault occurs. In this paper, the operation, modulation method, multilevel high frequency link voltage, current, and power characterization, high frequency commutation, and switching characterization are analyzed comprehensively; the pulse rotation and distributed delay control, and fault handling strategies of MHFLT are then proposed. Finally, a prototype is built, and the experimental results verify the correctness and effectiveness of the proposed solution.