Flexible low-frequency AC transmission (FLFAC) technology has the advantages of strong transmission capacity and flexible control capability and can be applied in large-scale renewable energy wide area collection and transmission scenarios. The overhead line transmission capacity can directly reflect the available voltage level and range of the low-frequency system. Firstly, function with X/R as the variable was established and the voltage and current characteristics are analyzed under different voltages and frequencies, as well as the static voltage stability. The main influencing factors of the low-frequency system's capability were proposed. Then, a transmission capacity analysis method for the for renewable energy transmission was proposed, and the transmission capacity of 10kV-1000kV systems was carried out. The transmission capacity of low-frequency systems can be increased by more than 2.5 times and the power supply radius is about three times larger than the power frequency. Finally, low-frequency transmission system rated at 220kV/1000MW with 260km overhead lines was built in PSCAD/EMTDC to verify the stability of the low-frequency system in long-distance high-capacity transmission scenarios.