Assessing Small-Signal System Strength Across Modes, Locations and Operating Conditions for Inverter Based Resource Connections

New sub-synchronous oscillations (SSO) have emerged in power systems with rises in connections of Inverter-Based Resources (IBRs). This has prompted a search for better processes for identifying vulnerabilities to SSO during the planning of an IBR connection, recognising the features of IBRs, the modes to which they are vulnerable, and the heightened dependency on the operating point of the system. A metric named Admittance Margin (AM) is described, tailored to this problem, which evaluates small-signal system strength across several modes and locations and is used within a framework to explore operating point dependency. Suites of operating scenarios are developed systematically for an IBR-dominated test system based on historical demand and generation data. A procedure is defined to select the most critical operating points (COPs), to reduce the computational burden of performing system strength studies across all operating scenarios. Visualisation tools are provided to facilitate an effective interpretation of the AM results across multiple case studies so as to highlight where and when problematic conditions may occur.