Effects of Virtual Resistance on Transient Stability of Virtual Synchronous Generators Under Grid Voltage Sag

The virtual synchronous generator (VSG) control of a grid-connected converter is an attractive interfacing solution for high-penetration renewable generation systems. Unfortunately, the synchronous resonance can appear due to the power control loops, which are usually damped by adopting a virtual resistance (VR). However, the effects of VR on the transient stability of the VSG are rarely studied. In this article, a virtual point of common coupling and a virtual power angle concept are proposed to represent the mathematical model of the VSG with VR damping. Based on the model, the transient stability is further analyzed using the phase portrait and the attraction regions of the nonlinear system. It reveals that the VR has negative and different impacts on the transient stability compared with the real grid resistor. In order to keep the VSG with VR to work normally during the grid voltage sag, an enhanced transient stability method, by reducing the active power commands when the grid fault is detected, is introduced. A design-oriented analysis and the parameter design with different VRs are also presented. Finally, the theoretical analysis is verified by the experimental results.