VSC Input-Admittance Modeling and Analysis Above the Nyquist Frequency for Passivity-Based Stability Assessment

The interconnection stability of a grid-connected voltage-source converter can be assessed via the dissipative properties of its input admittance. In this paper, the modeling of the current control loop is revisited with the aim to improve the accuracy of the input-admittance model above the Nyquist frequency of the converter control system. Particular care is taken in the modeling of the pulsewidth-modulation and sampling processes. It is shown that, in order to eliminate the risk for destabilization of poorly damped grid resonances, dissipativeness of the input admittance is desirable not only for frequencies below, but also above, the Nyquist frequency. Validation is made by simulations and experiments.