An Aggregate Model for Frequency Response in All-Inverter Power Systems

System frequency models for power systems have been used for decades to estimate the aggregated frequency response of groups of generators. While most existing models have proven to be useful for formulating constraints on frequency response and for providing insight into the frequency dynamics of synchronous generator-dominated power systems, they are unsuitable for capturing the behavior of inverters. This paper presents an aggregate model for the frequency response of coherent inverters in an all-inverter power systems. This reduced-order model offers remarkable computational efficiency and may be applied to all-inverter systems in which all frequency response is provided by droop control grid-forming inverters. The model provides helpful estimates of settling frequency, nadir, and RoCoF for small systems. We demonstrate the limitations of aggregate frequency response models for larger systems and present possible applications of the proposed model.