Flexible Low-Frequency AC (FLFAC) transmission technology has currently emerged as a critical solution for efficient aggregation and delivery of renewable energy. To address the stability challenges posed by high-penetration renewable energy integration, this study focuses on quantifying the operational boundaries of FLFAC systems encompassing renewable energy generation units, transmission corridors, and low-frequency converter stations. A stability boundary analysis method based on impedance modeling is proposed, providing a reference basis for the construction of the renewable energy transmission project using flexible low-frequency AC technology. To overcome the challenges associated with modeling high-order system impedances, a modular frequency-sweep approach is adopted to construct a comprehensive impedance model integrating renewable energy units, multi-level low-frequency transmission lines, and converter stations. First, the topological structure of the system model is outlined. Then detailed impedance models for M3C converters, wind turbine, and low-frequency transmission lines were established, which was validated through SIMULINK simulations. After that system-level impedance is synthesized via impedance model cascading. Last, stability boundary and influencing factor were analyzed, and were verified by SIMULINK simulations. Research results demonstrate that appropriately increasing the reactive power output of new energy sources effectively eliminates oscillation risks. This research provides a theoretical framework complementing stability analysis for FLFAC-based renewable energy transmission systems, offering practical guidance for engineering application.