Current small-signal modeling and analysis of grid-forming (GFM) modular multilevel converter-based high voltage direct current (MMC-HVDC) systems often employ simplified models neglecting MMC’s internal dynamics

potentially leading to stability judgment errors in practical applications. To deeply investigate the small-signal stability of grid-forming MMC-HVDC systems

a detailed 40th-order small-signal model incorporating MMC’s internal dynamics is established to investigate GFM MMC-HVDC small-signal stability. The study systematically investigates the impacts of key system components on small-signal stability and critical modes when considering MMC internal dynamics. Results demonstrate that the critical mode is influenced by AC grid impedance characteristics and DC voltage synchronization control

showing minimal sensitivity to MMC internal dynamics under optimized circulating current suppression control (CCSC) parameters. However

parameter mismatch in CCSC may lead to subsynchronous oscillation instability risks. An equivalent method for main circuit parameters of simplified average models ignoring submodule capacitor dynamics is proposed. The mechanism of stability judgment errors in simplified models is analyzed

and the applicable scope of simplified models in system stability analysis is clarified. The simplified model can meet the requirements of stability analysis accuracy under weak grid conditions and shows potential application prospects in MMC-HVDC projects.