Application of Dual-Grid-Forming MMC for Seamless Transition between Grid Connected and Islanded Operation in Microgrids

This paper presents a control strategy based on dual-grid-forming applied to modular multilevel converters. The goal is to ensure seamless transitions between grid-connected and island operations in ac microgrids. This control strategy is tested in a case where a back-to-back multilevel converter interconnects a microgrid to a system with a low short-circuit ratio. While conventional approaches rely on islanding detection algorithms and control mode shifts, the proposed technique eliminates the need for such mechanisms, averting potential instabilities. The dual-grid-forming approach synchronizes the grid frequency with the converter’s internal energy while enabling concurrent dc voltage regulation. This paper explores the design, implementation, and performance of such a control strategy showcasing its effectiveness in enhancing grid stability and ensuring reliable operation at several grid strengths.