An Inductive-Filtering Strategy of Submodule Ripple-Power in Triple-Port MMC-Based SST Applied to Hybrid Medium and Low Voltage AC/DC Interface

Abstract

The solid-state transformer based on modular multilevel converter (MMC-SST) is usually composed of MMC-stage and isolated bidirectional dc/dc-stage, which is a preferred solution for realizing flexible interconnection of medium and low voltage hybrid ac/dc networking system. The large size of submodule (SM) capacitor is the key factor that limits the power density of MMC-type systems. This article proposes a four-terminal dual-active-bridge (FDAB), to obtain a new FDAB-based MMC-SST (FM <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C-SST) with high-frequency-link, to realize the inductive-filtering of SMs ripple-power without any control. The synchronous-switching modulation of primary FDAB can make the horizontal three SMs exhibit switched-capacitors characteristics, and based on the three-phase symmetry between ripple-powers, the inductive-filtering on multi-winding transformer can be achieved, which greatly reduces the size of SM capacitors, at the same time eliminating the arms second-order circulating current. In this article, the operating principle, control method and parameters constraints of inductive-filtering process are analyzed. A practical engineering case is used to evaluate the proposed FM <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C-SST, including devices design, volume, cost, power loss and thermal behaviors evaluations, and the operating characteristics under grid unbalance is also discussed. Finally, the correctness and effectiveness of the proposed FM <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C-SST inductive-filtering strategy are verified by simulation and experiment.