Optimization of modular multilevel converter with half-bridge or full-bridge submodules for energy storage applications

The Double Wye Modular Multilevel Converter (YY-MMC) equipped with energy storage on the DC-link is increasingly used in grid applications, to provide grid support and favor integration of renewable energy sources. This paper presents a comprehensive optimization framework for the Half-Bridge (HB-YY-MMC) and Full-Bridge (FB-YY-MMC) topologies, to determine the optimal converter design by considering key design parameters such as number of submodules (SMs), power losses, semiconductor rating, and SM capacitor size. The YY-MMC design is formulated as a multi-objective optimization problem, where the pole-to-pole DC voltage and the second-order circulating current serve as decision variables. For this purpose, a normalized cost function is developed based on a representative base-case cost distribution, where each cost component scales with its corresponding physical quantity for both topologies. For a given converter power rating, the optimization is performed considering different ratios between the allowed active and reactive powers to determine the most suitable topology for each power level. Finally, experimental results validate the effectiveness of the proposed approach, demonstrating its potential for design optimization of YY-MMC in grid-connected applications.