Stability Assessment of a High-Voltage DC Transmission System Using MIMO- and SISO-Based Impedances

Grid-connected inverters used in High-Voltage Direct Current (HVDC) transmission systems, which have become increasingly popular worldwide in recent years, employ a variety of control methods for each frequency band, and resonances can occur between multiple control devices within a device or between multiple devices placed in close proximity. Practical impedance-based data-driven methods, rather than linear approximation models, are becoming widely used for these stability analyses. On the other hand, HVDC transmission systems require multiple domain transformations between synchronous and stationary reference frames. Therefore, in this study, both Multiple-Input Multiple-Output (MIMO) analysis using impedances on the direct-quadrature (dq) synchronous frame and Single-Input Single-Output (SISO) analysis using positive and negative sequence impedances on the stationary reference frame were performed. Consequently, the two unstable frequencies identified in the eigenvalue trajectories for the dq impedance were also identified within a single Nyquist plot for the sequence domain impedance, validating both the unstable frequencies observed in the dq domain. These frequencies suggest that weak AC systems on the generation side of the HVDC transmission system may be causing instability in the DC transmission lines and AC systems on the demand side. Additionally, the results of this study also suggest that the introduction of an advanced inverter control method, called the Virtual Synchronous Generator (VSG) control method, in the grid-connected inverter on the receiving side may mitigate the effects of having weak AC systems on the generation side.