Small-Signal Stability Analysis and Current Control Reference Optimization Algorithm of DFIG-Based WT During Asymmetric Grid Faults

The stability issues of weak grid-connected doubly fed induction generator (DFIG) based wind turbines during asymmetric grid faults become very acute. In this article, a small-signal state-space model was established to deal with the stability problems of the DFIG system during asymmetric grid faults. Based on the model, the interaction mechanism between the power grid and the DFIG system during asymmetric faults is described and the modal analysis is performed. The analysis results show that the dominant unstable poles of the system are mainly affected by the phase-locked loop (PLL). Then, according to the vector relationships of voltages and currents, the effect of positive-sequence (PS) and negative-sequence (NS) d-axis currents on the PLL output angle is analyzed. It is pointed out that the system stability can be effectively improved by injecting reasonable d-axis components of PS and NS currents into the power grid during faults. Furthermore, considering the current capacity limitation of DFIG, a novel PS and NS current control reference optimization algorithm is proposed, which can suppress the oscillation of the system. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are validated by the simulation and experiments.