As a crucial component of power systems, AC transmission lines play a pivotal role in the energy exchange process during system oscillations, especially in prevalent scenarios where the time-scale dynamics of internal potentials in new energy generation equipment are comparable to those of transmission lines. Under such conditions, the dynamic characteristics of AC transmission lines will profoundly participate in the energy exchange process of system oscillations. Therefore, investigating the energy storage mechanism of AC transmission lines and their fundamental modes of involvement in the energy exchange of system oscillations hold foundational significance for clarifying the physical mechanisms behind these oscillations. However, existing researches have predominantly focused on mathematical characterizations of AC transmission lines based on instantaneous voltage and current values or harmonic relationships, lacking a physical understanding of how transmission lines participate in the energy exchange process of system oscillations. Consequently, the oscillation mechanisms in new energy power systems remain unclear. Firstly, a typical converter-interfaced AC system is taken as an example, the basic operational principles of AC transmission lines is elucidated under the objective of stable power transmission. Secondly, the analytical relationships characterizing the dynamics of AC transmission lines is established. Thirdly, their dynamic characteristics and energy storage implications are analyzed. Finally, a detailed examination of the energy exchange behavior in closed-loop system dynamics is conducted, deepening the understanding of how AC transmission lines intrinsically participate in system dynamic processes and laying a vital cognitive foundation for revealing the general energy exchange mechanisms underlying current system oscillations.