Discrete Fourier series-based dual-sequence decomposition control of doubly-fed induction generator wind turbine under unbalanced grid conditions

The doubly fed induction generator (DFIG) wind turbines (WTs) are widely installed as distributed generation utilities but their reliability is vulnerable to the grid disturbance, causing the second harmonics and associated low-frequency oscillations. This paper proposes a novel DFS-based dual-sequence decomposition control to mitigate the fatal oscillations and improve the DFIG's low-voltage ride-through (LVRT) capability under unbalanced grid conditions. The controller saturation phenomena under unbalanced conditions are first investigated. To overcome such saturation problem and its resultant oscillations, the positive and negative sequences are extracted by a fast sequence decomposition method using Discrete Fourier Series (DFS), irrespective of the operation being sub- or super-synchronous. A dual-loop control is integrated with the proposed sequence decomposition, using power balance for the minimization of the harmful torque pulsation and DC-link voltage fluctuation. In order to support the grid voltage during LVRT, an advanced reactive power control is further developed to capture more reactive power headroom by abandoning the active power generation with the mandatory absorption of the kinetic energy. Simulation results for a 9MW DFIG WT system validate the effectiveness of the DFS-based dual-sequence decomposition design with the advanced reactive power control, demonstrating its mitigation of oscillations, and improvement of the LVRT capability to support the power grid.