High-Frequency-Link Modulation Methodology of DC-DC Transformer Based on Modular Multilevel Converter for HVDC Application: Comprehensive Analysis and Experimental Verification

This paper presents a comprehensive theoretical analysis and experimental verification of high-frequency-link (HFL) modulation of dc-dc transformer (abbreviated as MMDCT) based on the modular multilevel converter (MMC). Three typical HFL dual-active phase-shift modulations are analyzed. Particularly, the HFL voltage, current and power characterization, and different kinds of rating current of the MMDCT during different modulations with the same transmission power are compared comprehensively. On this basis, the detailed switching characterization is presented and the control complexity of the MMDCT during different modulations is discussed. Compared with triangular and sine modulations, square modulation has bettern HFL and switching performances, especially the control is simpler, which are different with the conclusion in traditional line-frequency ac application of the MMC. At last, experimental results verify the theoretical analysis.