Grid strength impedance metric: an alternative to SCR for evaluating system strength in converter dominated systems

Callum Henderson, Agusti Egea-Alvarez, Thyge Knueppel, Guangya Yang, Lie Xu

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)
281 Downloads (Pure)


Short circuit ratio is an outdated measure of system strength in converter dominated systems as the useful behaviour of inverter-based resources is not represented. While still useful for determining fault current provision, converter connected generation contributes to short circuit level and system strength in a different way to traditional synchronous generation. Additionally, traditional measures only account for system strength at the fundamental frequency while converters possess complex interactions across a frequency range. A novel method of determining system strength across this frequency range from the network impedances is proposed in this paper. The approach known as the Grid Strength Impedance Metric, calculates the system strength independently from short circuit level. The unique contribution of grid-following and grid-forming converters is represented by including the multiple-input multiple-output converter output impedances in the calculation. Small-signal impedance models are employed, and the metric is largely concerned with short-term voltage stability, but some inferences can be made for longer timescales. The approach is validated using time domain simulations to check for any identified areas of weakness in the frequency range. The Grid Strength Impedance Metric successfully represents the useful behaviour of grid-forming converters, indicating an enhancement in voltage stiffness in locations of deployment.
Original languageEnglish
Pages (from-to)1-10
Number of pages10
JournalIEEE Transactions on Power Delivery
Early online date9 Jan 2023
Publication statusE-pub ahead of print - 9 Jan 2023


  • short circuit ratio (SCR)
  • grid stiffness
  • converter impedance
  • power system stability
  • grid forming converter


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