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

Abstract

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 multicriteria 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-activebridge (MB-DAB) type submodule is the most pertinent power conversion topology for cascaded modular isolated MVAC-LVDC power conversion application