Inter-Sequence Coupling Estimation and Frequency-Compensated PLL of DFIG During Asymmetric Weak Grid

Under asymmetric weak grid conditions, the coupling between the positive and negative sequences in doubly fed induction generator (DFIG), introduced by the phase-locked loop (PLL), can affect the stability of the grid-tied system. However, the mechanism governing the stabilization of inter-sequence coupling in DFIG system remains unclear. To address such an issue, this article first develops a sequence impedance model of the DFIG system under asymmetric grid conditions. Based on the constructed model, the inter-sequence coupling branch is equated to positive and negative subsystems, respectively, to measure the impact degree of inter-sequence coupling components. Then, the influence of grid strength and PLL bandwidth on the inter-sequence coupling stabilization is estimated. To inhibit the inter-sequence coupling of the DFIG system, a frequency-compensated enhanced PLL is proposed. Finally, simulations and experiments are carried out to validate the accuracy of theoretical analysis.