Experimental Evaluation of Impact of Short-Circuit Ratio (SCR) and X/R Ratio on Stability of Grid-Forming and Grid-Following Inverters

With ever increasing renewable energy sources, such as wind and solar, being interconnected to power systems, the grid strength, as measured by existing short-circuit ratio (SCR) measures, will become weaker. The high penetration of renewable generation has posed new challenges to the stability of power grids, and grid-forming (GFM) inverters have been introduced as an effective solution to improve power system stability under these conditions. However, the impact of grid strength (e.g., SCR and X/R ratio) on the stability of GFM and legacy grid-following (GFL) inverters is still not well studied, especially because no hardware test/evaluation work has been carried out. To fill this gap, this paper conducts a comprehensive hardware test of two commercial inverters (which can operate in either GFM or GFL control) under varying grid strengths (SCR and X/R) to gain a comprehensive understanding of how grid strength can impact the stability of GFM and GFL inverters. This comprehensive evaluation using commercial inverters reveals that both X/R and SCR affect the voltage stability of GFM and GFL inverters, but they exhibit different trends under varying grid impedances. X/R affects the voltage stability more than SCR; reducing X/R has a negative impact on the GFM inverter's stability, but it has a positive impact on the GFL's stability (this indicates that the GFM inverter might have better stability with transmission systems with a higher X/R ratio, and GFL inverters might have better stability with distribution systems with a lower X/R ratio); and under the same X/R with varying SCRs, the GFM inverter's point of interconnection (POI) voltage decreases with a lower SCR, whereas the GFL inverter's POI voltage increases with a lower SCR.