Contributions to DC fault ride-through in MTDC meshed grids with MMC stations in grid-forming control considering impacts on AC system stability

DC faults in High Voltage Direct Current (HVDC) systems can significantly impact AC grid stability, especially in Multiterminal DC (MTDC) grids. Effective DC Fault Ride-Through (FRT) analysis is crucial for maintaining AC system stability. Initially, a DC fault may manifest as a sudden loss of power input, prompting primary control to restore balance using reserves like FCR and inertia. However, HVDC protection often blocks converters during current peaks, failing to address power imbalances on the AC side. This necessitates coordinated AC-DC grid actions to sustain stability. Grid-Forming (GFM) control in Modular Multilevel Converters (MMCs) enhances system operation by providing inertia, voltage support, and reactive power control. However, the role of GFM in DC FRT is underexplored. This paper evaluates DC faults in MTDC systems with GFM converters, considering AC grid characteristics alongside DC-side impacts like submodule energy control and DC voltage regulation. An outer energy control loop and a shift from GFM to Grid-Following (GFL) control are proposed solutions in this paper to improve DC FRT capability and to assure stable operation against DC faults. The findings provide key recommendations for enhancing DC FRT in MTDC grids with GFM converters, emphasizing control and protection integration.