Impact of Circular Current Limiters on Transient Stability of Grid-Forming Converters

This study investigates the transient stability of a virtual-admittance-controlled grid-forming (GFM) converter with a circular current limiter. Firstly, to facilitate the transient stability analysis, the inner control loops along with the circular current limiter are represented as a voltage source behind adaptive virtual impedance. Next, a time-rescaling technique is utilized to simplify the closed-loop system dynamics. It is found that transient stability is only affected by the 'relative speed' defined as the ratio between the voltage magnitude control gain and the power one. Based on the simplified system model, the phase plain analysis is applied to evaluate the transient stability of the GFM converter. The results reveal that transient stability can be jeopardized when the circular current limiter is applied, while reacquired by increasing the relative speed. Finally, simulation and experimental tests are conducted to verify these findings.