Overview and comparative analysis of bidirectional cascaded modular isolated medium-voltage AC–low-voltage DC (MVAC-LVDC) power conversion for renewable energy rich microgrids

As power-electronics is playing a major role in modernization of the existing electrical grid towards renewable energy rich smart grid, isolated medium-voltage AC–low-voltage DC (MVAC-LVDC) power conversion has gained popularity for grid-connected applications. In this paper, an initial discussion is provided on existing solutions related to grid-connected MVAC-LVDC conversion, which briefly describes the shortcomings of the line-frequency-transformer (LFT) based conversion solutions and then discusses the various categories of isolated power-electronic conversion solutions based on modularity. A qualitative comparison of the isolated power-electronics based conversion solutions reveals the obvious merits of the cascaded modular conversion strategy over the modular multilevel, semi-modular and single-cell conversion strategies. Subsequently, comprehensive descriptions for each of the available single-phase isolated AC–DC conversion submodules in the literature are provided, along with pertinent illustrations of circuit diagrams and important electrical quantities’ wave-shapes. Finally, a quantitative comparative evaluation of a 22 kV grid-connected 1 MVA power-rated MVAC-LVDC isolated converter’s three-phase submodule possibilities along with comparable LFT based MVAC-LVDC solution is presented, while considering the criteria of efficiency (η), power density (ρ), cost (σ), failure rate (λ), number of components (NoC) and control complexity (C.C.). This multi-criteria comparative evaluation demonstrates the merits of single-stage isolated AC–DC submodules over the conventional two-stage isolated AC–DC submodules, and reveals that the direct matrix-based dual-active-bridge (MB-DAB) type submodule is the most pertinent power conversion topology for cascaded modular isolated MVAC-LVDC power conversion application.