Transient Stability in Grid-Forming Converters with Current Limitation and DC-Link Voltage Control

Transient stability of grid-forming (GFM) converters with current limitation is a major challenge in converterdominated grids, and has been investigated through several methodologies, including Equal Area Criterion and phase portraits. Nevertheless, the aforementioned techniques in their traditional formulation can not be applied to GFM converters with regulated dc-link voltage and current limitation, such as the ones used in hybrid AC/DC grids (e.g., HVDC systems). To overcome this shortcoming, this paper proposes a novel transient stability analysis method that combines bisection search and polynomial regression to accurately estimate the critical clearing time (CCT). Unlike traditional large-signal stability analysis methods, which can be applied only to 2nd order converter models, the proposed method applies to high-fidelity high-order models, with the possibility of including more complex converter dynamics such as dc-link voltage and current saturation. The proposed method is also used to investigate active power reference adaptation strategies that prioritize reactive power during current saturation, demonstrating their effectiveness in improving transient stability.