Small-Signal Stability Analysis and Enhancement of Virtual Admittance Control based Grid-Forming MMC

Grid-forming modular multilevel converters (GFMMMCs) have emerged as a promising technology for enhancing small-signal stability of power systems, particularly under weak grid conditions. Virtual admittance control has been employed for current limiting in GFM-MMCs. However, a potential challenge is that virtual admittance control could adversely affect its small-signal stability performance under weak grid condition. In this paper, through the utilization of accurate impedance modeling, the control links of the virtual admittance controlbased GFM-MMC are represented as virtual resistors, inductors, and capacitors. This circuit-theoretic perspective offers insights into the negative resistance characteristic that contributes to small-signal instability. Subsequently, the mechanism of smallsignal instability and a method for enhancing stability are investigated using the established circuit model. Finally, the validation of the proposed method is demonstrated through simulations conducted using Matlab/Simulink and the experimental platform of RT-LAB. These results confirm that the proposed method effectively enhances the stability of GFMbased MMC with virtual admittance control under weak grid condition, without compromising their dynamic performance