Sequence Impedance Modeling and Stability Comparative Analysis of Voltage-Controlled VSGs and Current-Controlled VSGs

The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency area and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

I. Introduction

With the increasing shortage of fossil fuels and environmental pollution, the renewable energy generation has been developed rapidly. The power system integrated a large share of renewable energy generation that causes a reduction of the system rotational inertia, thus resulting in big variations of frequency and voltage during large disturbances [1]. The virtual synchronous generator (VSG), which emulates the rotational inertia, droop characteristics, and damping characteristics of the synchronous generator, can provide necessary frequency and voltage support for the weak grid, and has been becoming a research hotspot for large-scale friendly grid-connected renewable energy generation [2]–[14].