Characterizing Sub-synchronous Oscillation Propagation in MMC-MTDC System Based on SSO Propagation Coefficients

This paper studies the sub-synchronous oscillation (SSO) propagation characteristics in the modular multilevel converter-based multi-terminal direct current (MMC-MTDC) system. SSO may be triggered when an MMC is connected to a wind farm, and propagates to the connected converters and AC grids along the DC grid. This SSO propagation may cause damage to the local equipment of the AC grids connected by the MTDC system. This study analyzes the unequal SSO power phenomenon in MMCs and proposes the use of SSO propagation coefficients to investigate the impact of SSO propagation within the DC and AC grids. SSO propagation characteristics in the MMC-MTDC system are also analyzed. A four-terminal MMC-MTDC system is adopted to conduct the SSO propagation performance evaluation. The findings of this study help determine the nodes with high SSO observability, which offers a guideline for SSO monitoring.