Exploring an Impedance-Based SCR for Accurate Representation of Grid-Forming Converters

The strength of an electrical network connection point is often characterized by the short circuit ratio. Analysis of the fault current contribution at the node can provide information on system impedance, voltage stability, maximum power transfer and system recovery time. However, the introduction of more power converter connected generation decreases the validity of the current short circuit ratio definition. Mainly, the parameters that determine the strength of a connection point cannot be inferred from the fault level contribution. This article presents a discussion on the pitfalls of quantifying the strength of an electrical network using fault current contribution. The limitations of the method for converter dominated networks are presented and alternative definitions from literature are discussed with drawbacks explored. Further considerations for a new index are described, and the article suggests utilizing system impedances for investigation of different stability components. Varying converter control algorithms are explored in terms of impedance, both grid-following and grid-forming. Grid-forming structures without a current loop were found to provide the greatest improvement in voltage stiffness.