A Method for Impedance Modeling of Modular Multilevel Converter-based DC Transformer Using Switching Functions

This paper proposes an impedance modeling method for Modular Multilevel Converter-based DC Transformers (MMC-DCT). The study begins with a systematic analysis of the topological configuration and operational principles of MMC-DCT. Building upon the equivalent circuit diagram, separate impedance models for the MMC module and the uncontrolled rectifier unit are developed. Specifically, based on the problem of discontinuous conduction of MMC-DCT uncontrolled rectifier bridge arm diodes, the harmonic state-space (HSS) model of the rectifier is derived through the construction of current switching functions and voltage switching functions. Finally, frequency-sweep simulations are conducted to verify the theoretical model obtained, proving the accuracy of the established model.