Sequence-Impedance-Based Stability Comparison Between VSGs and Traditional Grid-Connected Inverters

Traditional grid-connected inverters (TGCI) could suffer from small-signal instability owing to the dynamic interactions among inverters and a weak grid. In this letter, the small-signal sequence impedance model of the virtual synchronous generator (VSG) is built, and the sequence impedance characteristics of the VSG and the TGCI are compared and analyzed. The sequence impedance of the TGCI is mainly capacitive in the middle-frequency area, and the impedance amplitude is quite high. By contrast, the sequence impedance of the VSG, being consistent with the grid impedance characteristics, is generally inductive, and the impedance amplitude is quite low. Based on the sequence impedance model and the Nyquist stability criterion, the influence of the grid stiffness, the number of paralleled inverters, and the phase-locked loop (PLL) bandwidth on the stability of the VSG and the TGCI systems is analyzed. The stability analysis results show that the TGCI loses stability easily, whereas the VSG still works well without PLL restrictions under an ultraweak grid or with a large number of inverters connected to the grid. Therefore, the VSG is more suitable than the TGCI for achieving high penetration of renewable energy generation in an ultraweak grid from a system stability viewpoint. Finally, the experimental results validate the sequence impedance model and the stability analysis.