Feasible-Region-Based Limit Analysis and Adaptive LVRT Control of Grid-Forming VSGs in Weak Grids

This paper proposes an adaptive low-voltage ride-through (LVRT) control framework for grid-forming virtual synchronous generators (VSGs) in weak and ultra-weak grids based on feasible-region and fault ride-through limit-boundary analysis. The proposed method achieves coordinated active–reactive power regulation during fault conditions and enhances the fault ride-through capability and synchronization stability of the system. First, an equivalent voltage-vector decomposition is used to establish the fault-stage operating model of the VSG, based on which the feasible active–reactive power region is characterized under current, line-reactance, and apparent-power constraints. Then, the maximum active power transfer capability, maximum reactive power support capability, and critical voltage-sag boundary are derived by considering both current limitation and power-angle stability. Furthermore, unlike existing feasible-domain-based methods that mainly focus on voltage-command limitation, a unified power-circle–capability-cone constraint model is developed to directly generate feasible active–reactive power references within the original VSG framework. A voltage-dependent adaptive droop coefficient is introduced to dynamically coordinate active and reactive power allocation, thereby enlarging the feasible LVRT region and improving the stability margin. Finally, a distributed consensus mechanism is designed to coordinate active and reactive power references among multiple VSGs within their feasible regions and suppress fault-induced power oscillations. Simulation results verify that the proposed method enhances voltage support, expands the feasible LVRT operating region, suppresses power and power-angle oscillations, and improves transient stability under weak-grid conditions.