Comparison of small-signal dynamics in MMC and two-level VSC HVDC transmission schemes

Small-signal stability studies of power systems including Voltage Source Converter (VSC) HVDC transmission schemes are commonly based on state-space models representing two-level converters and their related control loops. However, an increasing share of HVDC links are based on Modular Multilevel Converters (MMCs), which present additional dynamics and control loops associated with their internal capacitive energy storage. MMC-based HVDC systems do not depend on a large dc-side capacitor as two- or three-level VSCs, leading to differences in the dynamic interactions between the converter terminals and HVDC cables. This paper identifies and analyzes the differences in small-signal dynamics between two-level VSC- and MMC-terminals in HVDC transmission schemes by eigenvalue analysis. In particular, the differences in interaction patterns within a point-to-point HVDC system are investigated by participation factor analysis. The validity of the models used to generate the presented results is verified by time-domain simulations.