In the PV-hydro complementary modular multilevel converter-based high voltage direct current (MMC-HVDC) transmission system

the share of hydropower output under weak grid conditions affects the transmission capacity of the DC power. Integrating grid-forming renewable energy sources can improve adaptability to weak grids

but the impact of grid-forming inverters on the power transmission capacity of the system remains uncertain. Therefore

this paper first establishes the steady-state mathematical model and the small-signal model for the photovoltaic power stationwith grid-following and grid-forming heterogeneous inverters and the hydropower complementary MMC-HVDC transmission system. Then

taking into account both the steady-state electrical parameters

such as system operational limits and busbar voltage

and the small-signal stability

a calculation method for the power transmission capacity of the HVDC transmission system is proposed. Furthermore

the DC power transmission capacity is quantitatively evaluated under different PV-hydro ratios in a fully grid-following photovoltaic scenario

so that the boundary conditions for configuring grid-forming inverters are obtained. The mechanism underlying the limited power transmission capacity of the system under low hydropower output share is further analyzed through root trajectories and participation factor analyses. Subsequently

within the boundary conditions

the feasible proportion range of grid-forming photovoltaic to maintain the rated DC transmission power under low hydropower share is quantified. The maximum transmission power of the MMC-HVDC under different grid-forming photovoltaic outputs is determined

and the key influencing factors limiting DC transmission capacity are analyzed. Finally

the effectiveness of the proposed calculation method is validated on the PSCAD/EMTDC platform.