Small-Signal AC-DC Coupling Analysis of Grid-Following and Grid-Forming MMCs

AC-DC coupling in MMC-based HVDC terminals tends to play an important role in the small-signal stability and oscillation propagation of hybrid AC/DC grids. Efforts have been contributed to mitigate such coupling, e.g. by adding internal energy control, applying compensated modulation and exchanging the embedded current control loops of the power and energy control. However, their effects in reducing MMC small-signal AC-DC coupling over a wide frequency range have rarely been reported, particularly when considering grid-forming (GFM) control, as required by German grids codes for future HVDC applications. This paper thus utilises MMC multiport admittance model to investigate the impacts of GFL and GFM control, MMC operation modes and decoupling measures on the small-signal AC-DC coupling. Analysis results highlight the importance of considering AC-DC coupling in dynamic interaction studies and oscillation propagation analysis, particularly for GFM and DC voltage controlled MMCs. Time-domain simulation in MATLAB/Simulink verifies the frequency-domain analysis.