Transient Stability Enhancement Strategy for Grid-Following Inverter Based on Improved Phase-Locked Loop and Energy Dissipation

In a phase-locked loop (PLL) synchronized inverter grid-connected system, its equivalent damping coefficient is nonlinearly coupled with the operation power angle. Therefore, under a large disturbance, the indefinite damping increases the risk of transient instability in the system. To address this issue, firstly, a structurally modified IPLL is designed by removing the proportional coefficient branch of the traditional PLL and introducing a positive damping feedback branch. This design eliminates the coupling between the equivalent damping coefficient and the power angle, ensuring that the equivalent damping remains consistently positive. Secondly, based on the principle of energy dissipation via positive damping, a damping coefficient switching control strategy is developed. This strategy adaptively adjusts the damping during faults to rapidly dissipate excess kinetic energy, ensuring that the system returns to stability after fault clearance. Notably, the damping coefficient is pre-designed offline without relying on real-time grid parameters or operating data, enhancing the engineering practicability. Lastly, hardware-in-the-loop (HIL) experiments validate the strategy under extreme conditions.