Enhance DFIG Capability for Simultaneous Active and Reactive Currents Injection During LVRT in Weak Grid Based on Coupling Elimination

The latest grid code indicates that, when a voltage drop occurs in the grid, the doubly fed induction generator (DFIG) needs to inject a certain amount of active current on the basis of meeting the reactive current demand to support frequency. However, in weak grid, further injecting active current can easily lead to small-signal instability issues. To address this, firstly, this paper analyzes the variation of the DFIG impedance with respect to changes in the current during low voltage ride through (LVRT). The result shows that, with the injection of active current, the phase of the DFIG output impedance decreases, which further reduces the system phase margin. Furthermore, this paper explains the reason for this change through theoretical deduction. Secondly, a q-axis voltage compensation-based control method is proposed. This method effectively eliminates the coupling of the DFIG impedance with current and the phaselocked loop (PLL), increasing the magnitude and phase of the impedance. Compared with existing methods, this method can significantly improve the operational capability of the DFIG during LVRT in weak grid, where both active and reactive currents are simultaneously injected. Furthermore, even if the system lacks fault detection capabilities, this method can still ensure stable operation of the DFIG under full active current injection condition during LVRT. Finally, the effectiveness and superiority of the proposed method are validated through simulations and experiments