Application of Impedance-Based Frequency-Dependent System Strength Assessment for Inverter-Dominated Power Systems

The increasing penetration of inverter-based resources in Great Britain's power system has resulted in a decline in system strength, which has given rise to sub-synchronous oscillations. Traditional methods of assessing system strength are inadequate for system operators to mitigate these oscillations, and therefore, new system strength assessment methods are needed. This paper presents the design of an end-to-end small-signal system strength monitoring process using online impedance measurements, developed through collaborative efforts of a consortium of industry and research partners. The process is based on the evaluation of frequency-dependent impedances from grid measurement data, followed by the computation of a new small-signal system strength metric, Impedance Margin Ratio. Considerations on the feasibility of a practical implementation of the process in a real-world system subject to measurement uncertainty, along with technical aspects such as the injection configuration, are investigated. The process is validated in electromagnetic transient simulations in PSCAD, on a power system model that resembles a high-inverter-dominated region in Great Britain. The results demonstrate that system strength is highly dependent on operating conditions highlighting the importance of evaluating it across multiple system operating scenarios.