A Current Limiting Method for Single-Loop Voltage-Magnitude Controlled Grid-Forming Converters During Symmetrical Faults

Abstract

This article proposes a current limiting method for single-loop voltage-magnitude controlled grid-forming (GFM) converters to avoid overcurrent during symmetrical faults. The proposed method consists of two parts, which are the outer power reference adjustment to limit the steady-state fault current, and a transient virtual resistor, implemented by directly modifying the modulation voltage reference, to limit the transient fault current during fault-inception and -clearing periods. Furthermore, a nonlinear dynamic model for the GFM converter with the proposed control is developed, and the phase portrait is plotted to reveal the influences of power reference adjustment and of transient virtual resistor control on the transient stability. Finally, experimental tests confirm that the proposed method can effectively limit the fault current to an admissible value while simultaneously keeping the GFM mode during fault ride-through.