A Comprehensive Closed-Loop Voltage Ripple Control Scheme for Modular Multilevel Converter-Based Power Electronic Transformers

The power electronic transformer (PET) based on a cascade connection of modular multilevel converter (MMC) submodules with dual active bridges (DAB) is a promising topology for future hybrid ac/dc distribution grids. The voltage ripple suppression is of great importance for the power density improvement of this topology. However, traditional voltage ripple control strategies suffer from steady-state errors, couplings among different control loops, and considerable extra power loss. This article presents a comprehensive closed-loop voltage ripple control scheme for the MMC-based PET based on ripple power transfer (RPT). First, a decoupled closed-loop voltage ripple control structure in dual rotating reference frames is derived, offering advantages including high steady-state accuracy and no coupling among different control loops. In addition, to minimize the extra power loss of DABs caused by RPT, an optimal operation strategy based on partial RPT and the corresponding control diagram that is convenient for real-time implementation are presented. Experimental results obtained from a scaled-down PET prototype demonstrate that the proposed voltage ripple control scheme can effectively reduce the steady-state error of the voltage ripple control, eliminate the couplings among different control loops, and improve the overall efficiency.