Large-scale renewable energy bases in deserts, Gobi areas, and wastelands face challenges in their AC collection grid frameworks, such as lacking conventional power source support and AC main grid connections. Grid-forming converter stations configured for centralized power delivery can provide voltage support to the AC collection grid. Optimizing the placement of these grid-forming converter stations can maximize the voltage strength of the AC grid. To address this, this paper proposes a voltage strength evaluation method for renewable energy collection grids supported by grid-forming voltage-sourced converters （VSCs）. Furthermore, it introduces a multi-objective particle swarm optimization （MOPSO） algorithm to optimize the placement of grid-forming converter stations in such scenarios, thereby enhancing the overall voltage strength of the renewable energy collection grid. Firstly, based on the electrical distance between devices and the voltage fluctuations caused by their grid integration, a voltage strength evaluation method is derived for radial AC collection grids supported by grid-forming converter stations. Secondly, the proposed grid strength evaluation method is integrated into the MOPSO algorithm to construct an optimization model for the placement of grid-forming converter stations, facilitating the planning of renewable energy AC collection grids. Finally, case studies verify the accuracy of the proposed grid voltage strength evaluation method and demonstrate the rationality of optimizing converter station placement for improving overall grid voltage strength.