Comparison study of the fundamental switching frequency modulation for Dc-Dc modular multilevel converter

Recently, dc-dc modular multilevel converter (MMC) with medium or high frequency transformer has attracted more and more attentions for its many advantages such as flexible dc power flow control, good voltage matching and inherent dc current limitation. So far, the arm-voltage of dc-dc MMC has been modulated in many forms such as sinusoidal, trapezoid or even rectangular to gain different benefits. However, the power-flow and capacitor-voltage fluctuation analysis only considering the dc and fundamental component for all kinds of modulations, leads to the deviation between analysis and practice. To bridge this gap, this paper proposes a general Fourier-Series-Expansion-based analysis approach for dc-dc MMC. Such a general approach suits for any modulations, and the power-flow, arm current and dc-capacitor voltage fluctuation in the Sub-modules (SMs) under different modulations can be calculated with accepted tolerance. The concept, analysis and example comparison are given in this paper, and the theoretical analysis are verified by both the simulation and experimental results.