The Role of the Internal Energy in MMCs Operating in Grid-Forming Mode

With an increasing number of renewable energy resources being integrated into the power system through power electronics, conventional power plants based on large synchronous machines tend to be reduced in the upcoming future. With the modular multilevel converter (MMC) as the state-of-the-art power electronics solution for power systems, it will be necessary to operate some of these converters in grid-forming mode, in order to either preserve the stability and robustness of the ac grid or as the only solution to generate the ac grid in cases with no available synchronous generation. This article aims to understand how the internal energy control of the MMC can be used to effectively mitigate the propagation of ac power disturbances to the dc side, thanks to its buffering capability. Different energy control structures are explored, and a comprehensive discussion and comparison among them are provided, including stability analysis, transient performance, and tuning guidelines. Simplified and linear models are used to address the analysis, whereas detailed nonlinear models implemented in MATLAB Simulink are used to verify the results via time-domain simulations.