System Strength Beyond Fault Level

System Strength (SS) is becoming increasingly important while more and more inverter-based resources (IBRs) get connected to weak parts of the grid. There is still, however, a level of ambiguity with respect to formal SS definition. It is to a large extent legacy from conventional systems dominated by synchronous generators, often only simplistically associated with fault level. To overcome these issues and encourage discussion that will lead to more formal SS definition, this paper proposes a new inclusive concept of SS and technical framework to clarify its fundamental technical aspects. Starting from a comprehensive review of existing definitions, assessment metrics, and observed real-life challenges and solutions, the paper identifies the gaps between the technical fundamentals of the SS concept and its legacy perception. This is followed by identification of the key SS components relevant for assessment of both steady-state and dynamic system performance. The paper proposes a new SS classification where the SS "umbrella concept" is broken down into <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">small-signal</i> strength, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">large-signal</i> strength, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">short-circuit</i> strength with relevant SS factors, which may also be used in the context of ancillary services market developments. We indeed highlight how the overall SS can be influenced by a combination of factors, not only by fault level, and recommend that a "voltage-behind-an-impedance" parametrization and model may be more effective in defining SS in many circumstances. Finally, the capabilities of different types of IBRs in providing SS products are discussed along with relevant complexity and limitations.