Overcurrent Limiting in Grid-Forming Inverters: A Comprehensive Review and Discussion

Grid-forming (GFM) inverters are increasingly recognized as a solution to facilitate massive grid integration of inverter-based resources and enable 100% power-electronics-based power systems. However, the overcurrent characteristics of GFM inverters exhibit major differences from those of conventional synchronous machines. Accordingly, an in-depth characterization of GFM current-limiting strategies is needed to ascertain their performance during off-nominal conditions. Although GFM current-limiting controls are primarily necessary to protect the inverter power stage, they determine the inverter behavior during and after an off-nominal system disturbance. As a result, they can profoundly impact device-level stability, transient system stability, power system protection, and fault recovery. This paper offers a comprehensive review of state-of-the-art current-limiting techniques for GFM inverters and outlines open challenges where innovative solutions are needed. One key contribution of this paper is the use of graphical methods that allow for intuitive understanding and visually-aided comparisons of current-limiting methods. With this approach, we evaluate various performance criteria for different limiting methods, such as fault current contribution, voltage support, stability, and post-fault recovery. We also discuss the latest standards and trends as they require inverter dynamics under off-nominal conditions and outline pathways for future developments.