Mechanism analysis of subsynchronous control interactions between full-converter-interfaced generation and a series-compensated network

Recent studies reveal that unstable subsynchronous control interactions (SSCIs) between full-converter-interfaced generation (FCIG) and the series-compensated transmission line may occur when high-bandwidth phase-locked loops (PLLs) and high series compensation level (SCL) are implemented. This paper proposes a damping contribution analysis (DCA) method to investigate the SSCI mechanisms involving an FCIG-based power plant connected to a series-compensated network. Analytical expressions of the damping contribution coefficients from the series-compensated network to the PLL and q-axis current control oscillation loops are first derived. These coefficients offer a clear physical explanation of how and why high PLL bandwidth and SCL will deteriorate the damping of the dominant oscillation mode, thus leading to system instability. Additionally, a new SSCI phenomenon is identified, where dynamic interactions between the series compensation and the inner q-axis current control loop with slow dynamics also pose SSCI risks. Furthermore, the damping contribution coefficient facilitates evaluating the impacts of changes in system operating parameters on the modal shift of the concerned oscillation mode. The theoretical findings and conclusions obtained through the DCA are validated using modal analysis and electromagnetic transient simulations.