Assessing Grid Strength of 100% Inverter-Based Power Systems

 The increasing integration of renewable resources via power electronic inverters is shifting a modern power system toward a 100% inverter-based power system (IBPS). To maintain the stable operation of a 100% IBPS, it is important to identify the small-signal stability issues resulting from the interaction between the power network and inverter-based apparatuses. Due to the complexity of the interaction in a large-scale power system with many inverter-based apparatuses, grid strength assessment is a useful tool for fast identifying the stability issues. However, existing methods are not applicable to the 100% IBPS composed of grid-following (GFL) and grid-forming (GFM) inverters. To fill this gap, the paper proposes a method for fast assessing grid strength in terms of small-signal stability in the 100% IBPS. First, we formulate a multi-inverter system modeling for the small-signal stability analysis of the 100% IBPS. According to the analysis results, a measure and its threshold are then defined to characterize the gird strength of such a system. Also, an analytical expression is derived to determine the threshold and differentiate the impacts of GFL and GFM inverters on the stability boundary. Based on the defined measure and its threshold, our method is proposed and then validated on the modified IEEE 39-bus system and a realistic power grid with high wind penetration.