Stability analysis of small signals under different ratios of grid-following to grid-forming converters sent via flexible DC delivery

Under the "dual carbon"strategy, frequent wide-band grid oscillations caused by grid-following converters as the interface for renewable energy integration have become common. Existing studies suggest that the addition of grid-forming converters can address this issue, but few explain the reasons from a stability mechanism perspective. Additionally, most studies overlook the interaction between converters and sending-end converter stations. This paper first conducts sequential impedance modeling. Then, based on impedance network analysis, it examines the impact of sending-end converter stations on stability assessment. Finally, by adjusting the grid-following/grid-forming ratio, Nyquist curves under different ratios are generated to analyze the mechanism by which grid-forming converters enhance grid-following small-signal stability. Corresponding simulation models are also developed for validation.