Centralized grid-forming energy storage stations are critical pieces of equipment in high-proportion of new energy power systems. They can actively provide voltage support, inertia support, and damping. Common grid-forming control schemes include voltage and current dual-loop control and single-loop voltage magnitude control. The latter has stronger transient voltage support ability, but faces more severe transient overcurrent risks. To mitigate transient overcurrent, virtual impedance control is introduced on top of single voltage-loop amplitude control. Additionally, the dynamic processes of the active power / phase angle and reactive power / voltage are comprehensively considered to construct the system’s Lyapunov energy function. This function is used to analyze transient stability and determine virtual impedance parameters. Compared to the traditional phase plane method, which only considers the active power / phase angle dynamics, the Lyapunov energy function provides a more accurate parameter range. Simulation results verify that, with the virtual impedance control, the grid-forming energy storage power station can effectively respond to voltage dips and phase jumps while keeping the transient current within a reasonable range. Also, the effectiveness of the Lyapunov energy function method for virtual impedance parameter tuning is confirmed.