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.

Automated summary

This article investigates the impact of virtual resistance (VR) on the transient stability of virtual synchronous generator (VSG) control systems. By introducing the concepts of virtual point of common coupling and virtual power angle, a mathematical model of VSG with VR damping is established, and the stability analysis is conducted using phase portraits and attraction regions of the nonlinear system. The results reveal that the effects of VR on transient stability are different from those of real grid resistors. To ensure the normal operation of VSG with VR during grid voltage sag, an enhanced transient stability method is proposed. Theoretical analysis is validated by experimental results.