Type-Z and Type-Y Representation of Grid-Forming Converters: a Preliminary Analysis

Grid-forming converters (GFM) are a key enabling solution for the large-scale integration of renewable energy sources in a power-electronics-based grid. However, the complex dynamics introduced by their closed-loop nested controls requires the application of adequate modeling for reliable small-signal stability assessment. In this paper, the stability properties of the Type-Z and Type-Y representations of a virtual-admittance (VA) GFM control scheme are evaluated. The Bode plots of the terminal impedance and admittance of the system are evaluated and compared for different configurations of controller parameters. Preliminary numerical results suggest that the VA GFM control scheme may call for a natural type-Y representation instead of the commonly used Type-Z representation.