The growing penetration of inverter-based resources (IBRs) in modern power systems has raised significant challenges related to system strength and stability. Unlike traditional synchronous generators, which provide inherent inertia and short-circuit system strength, renewable sources such as wind and solar lack these stabilising characteristics, making networks more vulnerable to instability. Addressing system strength is therefore critical, but the exact requirements and methods of assessment remain uncertain.

Conventional electromagnetic transient (EMT) simulations are often employed to study small signal oscillations, but these are computationally intensive and not always necessary. An alternative is impedance-based frequency domain stability analysis, which characterises the interaction between system components by evaluating network impedance. This approach offers valuable insights into stability margins using the Nyquist criterion and is particularly effective for analysing power electronic systems. Although both synchronous machines and grid-forming (GFM) inverters are recognised as contributors to system strength, there is no generally accepted methodology to quantify their impact. This paper presents an impedance-based approach to assess the contribution of System Strength Mitigating Devices (SSMDs), focusing on small-signal stability. Three SSMDs; two GFM Battery Energy Storage Systems (BESS) and a synchronous condenser are compared, highlighting that stability support depends more on frequency response characteristics than nominal MVA short circuit capacity.