Optimal Control Selection for Grid-Forming MMC-Based Assets: An analysis of interplay between GFM and internal MMC controls

Over the last decade, the modular multilevel converter (MMC) has become a crucial technology for high-voltage applications, offering unparalleled flexibility. At the time of writing, grid-forming (GFM) controls are considered essential for synchronizing power converters with the grid, enhancing stability, promoting the integration of renewable energy sources, and advancing the reliability of transmission networks. This paper explores the control of a grid-forming modular multilevel converter. Our focus centres on comparing two distinct grid-forming controls — one with a current controller and another without. Additionally, we investigate two internal MMC controls — total energy control and per-leg energy control. Through a comprehensive analysis, we assess the performance of various control combinations in both the time and frequency domain. By delving into the intricacies of GFM control strategies, we aim to shed light on their impact on the overall operation of the MMC. The comparison will unveil insights i