The increasing penetration of inverter-based resources (IBRs) in modern power systems has led to significant challenges related to system strength and grid stability. Traditional synchronous generators provide inherent inertia and short-circuit strength, which are crucial for maintaining grid stability.

However, with the replacement of these conventional generators by renewable energy sources such as wind and solar power, power systems are becoming more susceptible to instability. Addressing system strength issues is essential; however, the extent and nature of this requirements remain unclear. Full-scale electromagnetic (EMT) simulations are commonly used to detect small-signal oscillations, although they are computationally intensive and often unnecessary for this task.

An alternative approach is to utilize impedance-based frequency domain techniques. Impedance-based frequency domain analysis examines the electrical characteristics of power system passive and active components by assessing their influence on the overall network impedance. Unlike traditional time-domain stability analysis, impedance-based methods provide valuable frequency-domain insights into how System Strength Mitigating Technologies (SSMT) affect system strength.

The impedance-based analysis is particularly useful in power electronics, especially when considering the interactions between different components such as sources, loads, and controllers. By analysing the impedance characteristics, one can determine whether a system will behave in a stable manner or oscillate when subject to disturbances. The general approach involves examining the open-loop impedance of the system and evaluating its stability phase and gain margins via Nyquist stability criterium.

While it is widely accepted in the power systems industry that Grid-Forming (GFM) inverters and synchronous machines contribute to system strength, there is no established methodology to quantify their exact contribution. This paper proposes a power system analysis approach to evaluate the system strength contribution of SSMT devices, with a focus on small-signal stability in the frequency domain.

Three different SSMTs—two Grid-Forming (GFM) Battery Energy Storage Systems (BESS) and one synchronous condenser— are evaluated regarding their respective capability in supporting Grid-Following (GFL) generators. A comparative analysis is provided to draw conclusions on effectiveness of each SSMT.

The presented approach offers deeper insights into determining system strength solutions, emphasizing that the quality of the solution (e.g., frequency response) can sometimes be more critical than the nominal 50 Hz capacity (e.g., MVA rating). Additionally, the application of frequency analysis has significantly streamlined the design process.