Consideration of Harmonic Coupling and Outer Loop Control in the Impedance Modeling of Doubly-Fed Induction Generators

Oscillations in large-scale doubly-fed wind farms connected to the grid are frequent, prompting experts to investigate the instability issues in wind power grid integration. The sequence impedance model has gradually become a widely adopted method for oscillation analysis. In DFIG (doubly-fed induction generators), the rotor-side converter and grid-side converter allow harmonics to circulate through the DC link. Current models of sequence impedance for doubly-fed wind turbines do not consider the impact of DC link voltage dynamics on impedance, and also overlook the influence of the outer control loop on the unit's impedance characteristics. This paper first establishes an analytical sequence impedance model for the doubly-fed induction generator based on the multi-harmonic linearization method. Subsequently, based on the frequency characteristics of small-signal components, a coupling model of machine-side to grid-side impedance is established, revealing the two directions of small-signal transmission and each direction's corresponding coupling pathways. It clarifies that the outer loop control couples GSC small signals to RSC, providing a new pathway for harmonic circulation. Finally, using the machine-side to grid-side coupling model, the paper analyzes the impact of coupling pathways on the unit's impedance characteristics, with a detailed analysis of the influence of rotor-side outer loop control and rotor-side converter current loop control characteristics on the DFIG impedance.