A Mode-Adaptive Power-Angle Control Method for Transient Stability Enhancement of Virtual Synchronous Generators

The virtual synchronous generator (VSG) is emerging as an attractive solution for interconnecting converter-based resources with the power grid. However, due to the nonlinear power-angle relationship, VSGs are, similarly to synchronous generators, prone to the loss of synchronization (LOS) under large grid disturbances. This paper thus analyzes the large-signal synchronization stability, i.e., the transient stability of grid-connected VSGs, and then proposes a mode-adaptive power-angle control method for enhancing the transient stability. In this approach, the positive-feedback mode of the power-angle control of the VSG is detected and adaptively switched to the negative-feedback mode after large disturbances. Thus, the LOS of the VSG can be avoided when there are equilibrium points after the disturbance. Moreover, during severe grid faults without any equilibrium points, a bounded dynamic response of the power angle can be obtained with the mode-adaptive control, and the VSG can still be stabilized even if the fault clearing time is beyond the critical clearing time. These superior features prevent the VSG-based system from collapsing due to the delayed fault clearance or the malfunction of protective relays. Finally, time-domain simulations and experimental tests are performed to verify the effectiveness of the control method.