The grid-forming converter which employs virtual synchronous control exhibits dynamic coupling between active and reactive power. Since the energy function considering only the dynamics of the active power loop cannot accurately characterize the frequency-voltage coupling characteristics of the converter under large disturbances

we propose an energy function modeling method for grid-forming converters based on a spring-loaded pendulum dynamical model to precisely analyze its transient synchronization stability. Firstly

a third-order nonlinear model of the grid-forming converter considering the dynamic coupling of active/reactive power and a third-order dynamic model of the spring-pendulum considering the spring-direction/perpendicular-spring-direction motions were established

then the energy function of the converter through the mapping relationship between the convert and spring-pendulum was obtained. On this basis

the optimal design method of the converter parameters was given from the perspective of spring-pendulum stability enhancement

and the range of attraction domain of the converter and the corresponding transient stability margin were estimated by the Lyapunov direct method. Compared with the traditional energy function model

the proposed energy function model can effectively reflect the voltage dynamics under large disturbances. Finally

the effectiveness of the proposed theoretical analysis and parameter optimization design method was verified by simulation and experiments.