New 4-stage LVRT model improves SG-DFIG hybrid grid stability analysis

Traditional power system stability analysis focuses on synchronous generators (SG), but with high renewable penetration, hybrid systems combining SG and doubly fed induction generators (DFIG) require new approaches. The authors model the complete low-voltage ride-through (LVRT) process of DFIG by dividing it into four distinct stages: pre-fault, during-fault, early post-fault, and late post-fault. They show that most faults force the DFIG into LVRT, causing sequential switching behavior. Crucially, they derive a single generalized swing equation (GSE) that applies to stages 1 through 3, analogous to the classic second-order swing equation for SG-only systems. This unified model simplifies stability analysis significantly.

Using the GSE, the team proposes an improved equal area criterion that incorporates two additional effects: frequency jump at fault clearing and nonlinear damping. This provides a clearer physical picture of transient synchronization stability (TSS) mechanisms. Extensive hardware-in-the-loop experiments and simulations confirm the theoretical results. The work bridges the gap between traditional power system stability analysis and modern hybrid systems with renewable energy, enabling more reliable grid protection during faults and voltage events.