Complex Transfer Function-Based Stability Criterion for Grid-Connected Converters

To simplify impedance-based stability analysis of grid-connected converters with asymmetric control structure, this paper proposes a complex transfer function-based stability criterion, which is rigorously derived from Cauchy's Argument Principle by using the complex-valued impedance model. The proposed criterion is a sufficient and necessary condition for the prediction of system stability. Compared to the generalized Nyquist criterion, the proposed one avoids matrix manipulation resulting in a remarkable simplification of the stability analysis. Meanwhile, it only needs one Nyquist curve to predict stability, which, thus, makes it possible to quantify the stability margin. Lastly, the proposed stability criterion is demonstrated by a case study of a grid-connected grid-following converter, where the frequency-domain analyses together with the experimental tests validate the effectiveness of the proposed stability criterion.