Impact of Current Limiting Design on the Transient Stability of Grid-Forming Converters

Grid-forming (GFM) converters are designed to provide grid support services such as fast frequency response, voltage support, virtual inertia, and damping during grid disturbances. Due to the low overcurrent capability of power semiconductors, current limiting control needs to be incorporated into GFM converters. However, the use of current limiters makes GFM converters more prone to loss of synchronism with the external grid. This paper investigates the transient stability of GFM converters with different current limiters (circular, d-priority, and q-priority) by deriving a generalised equivalent circuit model that establishes the $P-\delta$ relationships. The voltage support capability, critical active power reference, critical clearing time (CCT), critical clearing angle (CCA), and recovery time are compared under voltage sag disturbances. It is found that GFM converters with the d-priority current limiter have advantages in terms of CCT, CCA, critical active power reference and recovery time, the q-priority current limiter provides the greatest voltage support, and the circular current limiter exhibits balanced performance across all aspects.