Impedance Analysis and Stabilization of Virtual Synchronous Generators with Different DC-Link Voltage Controllers Under Weak Grid

In recent years, the virtual synchronous generator (VSG) concept has been widely studied to integrate renewable energy sources. However, instability occurs due to the implementation of the dc-link voltage controllers under the weak grid, and its mechanism remains unclear, which is investigated in this article. At first, the wideband dq-frame impedance models of the VSGs with the dc-link voltage controllers for two cases are established. Then, the stability analyses of the VSGs are compared based on these impedance models. It is revealed that the interaction dynamics between the dc-link voltage loop and the active loop lead to the negative resistor behavior of the q-q channel impedances for both VSGs, which induces the emerging oscillations of the system in a weak grid. Besides, as a useful design guideline, the parameter design of the VSGs is proposed to enhance the system stability. Finally, the experimental results obtained from a 100-kW prototype system show good agreement with simulated results, validating the impedance models and theoretical analysis.