Grid-Forming Converters under Current Limitation Considering Emerging Guidelines and Implications for Grid Services

Grid-forming inverters (GFIs) have emerged as key enablers for establishing and stabilizing power grid voltage in inverter-dominated grids. International activities increasingly emphasize verification-oriented specifications for GFIs to support secure integration. The recent FNN note on grid-forming inverters provides structured requirements and test considerations to define and prove grid-forming behavior. However, while core properties and current limiting are addressed, it offers limited guidance for transition process between current-limiting and normal operation. This work investigates a representative GFI employing a voltage phasor-based limiting mechanism. Based on the FNN note on grid-forming behavior, we performed further experimental tests transitioning the GFI between current-limiting and normal grid-forming operation. The test framework was extended to include fault ride-through (FRT) combined with islanding, which reveals the voltage source characteristics of the GFI during limiting conditions. The employed current-limiting approach demonstrates adequate transition performance across these test scenarios. Even though, further investigations and finetuning have been identified as necessary to achieve optimised release from current limiting mode.