In the scenario of large-scale new energy and thermal power bundling system via line commutated converter-based high voltage direct current (LCC-HVDC) transmission

the variation characteristics of sub-synchronous torsional interaction (SSTI) damping under different power supply ratios are still unclear

and the proportion of grid-forming (GFM) converters on SSTI damping is unknown. This paper establishes the state space model and the motion equation model for photovoltaic (PV) with grid-following (GFL)-based and GFM-based converters and thermal power bundling system via LCC-HVDC transmission. Multiple paths reflecting the SSTI damping characteristics are decomposed. Subsequently

in the case of pure GFL-based PV

the damping variation characteristics of the SSTI mode under different thermal-PV ratios are investigated. The minimum proportion of thermal power to ensure the stable transmission of rated DC power is obtained

and the key path dominating the damping changing trend of the SSTI mode is identified. Furthermore

the influence characteristics of the GFM-based PV power proportion on SSTI mode damping are analyzed. The minimum proportion of GFM-based converters to stabilize the system with low thermal power proportion is calculated

and its influence mechanism is revealed according to the damping characteristics of different paths. The results indicate that

when the proportion of new energy power output is too high

the supporting capacity of GFL-based converters for the weak AC grid is insufficient

leading to the weakening of SSTI damping

when GFM-based converters undertake higher proportion of power output instead of GFL-based converters

the negative impact on AC grid caused by the insufficient support capacity of GFL-based converters can be weakened

i.e. the inclusion of GFM-based converter in the PV power plant can improve the SSTI damping.