Modified Stability Analysis Model for Paralleled Grid-Connected VSIs Based on Coupled Sequence-Admittance

New studies on grid-connected voltage-source inverters (VSIs) show that sequence-admittance-based stability analysis model cannot be simply simplified as decoupled positive-sequence and negative-sequence impedance ratio because of sequence-couplings resulting from the outer asymmetric power control loops, such as the phase locked loop (PLL) and direct voltage control. Additionally, compared with single grid-connected VSI system, parallel-connected VSIs have stronger circuit-couplings via grid impedance. To get accurate stability conclusions in wider frequency range, both couplings are considered in this paper, and this paper firstly derives a sequence-admittance-based stability analysis model to capture resonances in parallel-connected VSIs. The proposed model is described as open-loop model and closed-loop model, to which Nyquist stability criterion is effectively applied instead of the complicated generalized Nyquist stability criterion. Take the PLL-induced low-frequency resonance for example, stability analysis of the studied cases is successfully conducted. Finally, time-domain simulations verify the accuracy and simplicity of the proposed model.