Impact of Grid-Forming-Based PV on SSTI Stability of PV-Thermal-Bundled System Transmitted by LCC-HVDC

In the renewable energy farms (REFs) and thermal power bundling transmission system via the line commutated converter based high voltage direct current (LCC-HVDC), there may exist the risk of sub-synchronous torsional interaction (SSTI). This issue can be potentially addressed by adding grid-forming-based (GFM-based) converters to the REFs. However, it is still unclear how the GFM-based converter proportion affects the SSTI damping. This article develops the small-signal models for the photovoltaic (PV)-thermal-bundled system transmitted by LCC-HVDC. Then the eigenvalue analysis is adopted to investigate the impact of active power proportion of GFM-based PV on SSTI damping characteristics. It is found out that the GFM-based PV can improve the SSTI damping to some extent at a relatively low thermal power proportion, while it weakens the SSTI damping at a relatively high thermal power proportion. Subsequently, the damping paths contributing to the system SSTI stability, including one self-stabilizing path and four en-stabilizing paths, are decomposed, and the dominant damping path is identified to elaborate the inherent mechanism behind the above phenomenon. On this basis, considering the damping of all SSTI modes, an approach to determine the preferred GFM-based PV proportion is proposed to enhance the SSTI stability at a particular thermal power proportion, by maximizing the stability margin. The simulation results verify the effectiveness of the approach and the preferred GFM-based PV proportion for enhancing the SSTI stability.