Dynamic and Frequency-Domain Assessment of Power System Strength in Grids Dominated by Inverter-Based Resources

As synchronous generators are progressively phased out and replaced by IBRs, the traditional understanding of system strength—mainly based on the fault current contribution of synchronous machines and quantified through indicators such as the Short Circuit Ratio

(SCR)—is no longer sufficient. Converter control schemes have been developed in the past relying on the infinite bus behaviour of the grid, and some of the internal loops, that prove to be effective in getting the best performances of the converters (such as the PLL and the grid voltage feed-forward loop), reveal to introduce possible source of unstable operation when the grid grows weaker. The increasing complexity of interactions between multiple IBRs and their control systems further requires enhanced tools to assess system stability and robustness, especially considering the potential emergence of oscillatory phenomena at sub- and supersynchronous frequencies.

The analyses presented in this paper build upon previous studies, aiming to further develop methodologies for system strength assessment by combining traditional static indicators with advanced dynamic analysis techniques.

A key innovation introduced in this work is the use of the Impedance Margin Ratio (IMR), a frequency-domain metric that evaluates the ratio between the allowable impedance variation and the original inverter impedance at the point of interconnection. Unlike SCR, IMR can identify weak points in the network that are vulnerable to small-signal stability issues, such as sub- and super-synchronous oscillations, by effectively capturing the dynamic interactions of inverter control loops and system impedance characteristics.

The analyses carried out in this paper aim to complement and extend previous assessments of network robustness by integrating existing indicators with the enhanced performance offered by the IMR metric, as previously described. Results obtained by applying the IMR are compared with time domain and stability analysis on a simple system before applying this combined approach to a forecasted portion of the Italian power system, considering medium to long-term scenarios characterized by significant growth of renewable generation and IBR penetration.

Results demonstrate that converter control strategies play a crucial role in enhancing system stability and resilience. Furthermore, thanks to the IMR metric, the contributions of converter control architectures and control loops to system stability can be effectively identified and assessed, allowing for a more accurate evaluation of stability margins in future grids.

In conclusion, for effective planning and operation of the power system in a future where inverter-based resources will dominate generation, it is increasingly essential to account for the detailed dynamic behaviour of converter control systems. This is fundamental to ensure appropriate stability margins, secure system performance, and enable a reliable integration of renewable energy sources in compliance with the evolving energy transition objectives.