Grid-following (GFL) and grid-forming (GFM) hybrid systems combine the advantages of both GFL and GFM converters, making them an ideal configuration for future renewable energy power plants. However, transient instability remains a critical concern for hybrid systems and requires further analysis and resolution. Therefore, this paper analyzes the transient stability of hybrid systems by using the Lyapunov direct method. First, a transient large-signal model of the hybrid systems is derived. Subsequently, the energy function of hybrid systems is derived and then used to estimate the system's critical clearing time (CCT), thereby analyzing the system's transient stability. Simulation results demonstrate the accuracy and effectiveness of the proposed analytical method.

GFL; GFM; Hybrid System; Lyapunov's Direct Method; Transient Stability.