A novel fault analysis method considering control states for MMC grid-forming system with renewable energy generation

• The control states of the grid-forming and grid-following converters are investigated. The grid-following converter exhibits three control states. While, the grid-forming MMC demonstrates three control states in the positive-sequence network, and two states in the negative-sequence network. • The presented fault calculation method decomposes the states judgment and models update of grid-forming and grid-following converters into internal chain-loop calculations, thereby significantly enhancing the convergence ability of the system-level calculation in MMC grid-forming systems. • Several unique fault characteristics of the MMC grid-forming system are explored, mainly including the magnitude and phase characteristics of the fault current, which have significant effects on the protection performance. This paper analyzes the fault equivalent models of grid-following and grid-forming converters under various fault conditions. A novel fault calculation method for MMC-HVDC grid-forming systems integrated with 100% renewable energy generation is proposed. The proposed method employs chain-loop iteration to address the divergence issues encountered in conventional fault calculation techniques, which typically rely on the numerical solution of the entire system state equations. By decomposing the state assessment and model updates for both grid-forming and grid-following converters into internal iterative calculations, this approach significantly improves convergence performance in fault analysis. The paper also explores the unique fault characteristics of MMC grid-forming systems, providing a theoretical foundation for protection scheme design. Simulation results obtained from PSCAD/EMTDC validate the effectiveness and accuracy of the proposed method in calculating fault responses.