Dual-Loop Control Strategy for DFIG-Based Wind Turbines Under Grid Voltage Disturbances

For a multimegawatts doubly-fed induction generator (DFIG), the grid voltage disturbances may affect the stator flux and induce the transient stator flux, due to the direct connection of the stator and the grid. The accumulation of the transient stator flux caused by the variations of the stator voltage may introduce harmful power and torque oscillations to the DFIG, and even lead to rotor overcurrent. For the conventional field-oriented vector control strategy, the design of the controller is based on the steady-state model of the DFIG, which neglects the dynamic of the stator flux, and, therefore, it cannot work well during the transient state to decay the transient flux and to suppress the flux accumulation. In this paper, a dual-loop control strategy, which includes the conventional current loop and an additional flux loop, is proposed to not only control the active and reactive power, but also decay the stator transient flux, and avoid the accumulation of the stator transient flux. Moreover, the proposed strategy can obtain nearly constant stator active power and electromagnetic torque, which may prolong the lifetime of the drive train. A case study on a typical 2-MW DFIG-based wind turbine demonstrating the effectiveness of the proposed control methods is verified with simulations in MATLAB/Simulink. The proposed control methods are also experimentally validated using a scaled-down 7.5-kW DFIG. The simulation and experimental results clearly validate the effectiveness and feasibility of the proposed strategy, and show the improved dynamic performances of the DFIG.

I. Introduction

For wind power plants, the doubly-fed induction-generator (DFIG)-based wind turbine (WT) is an economical choice due to the back-to-back voltage source converter rated at only 30%–35% of the generator power [1], [2]. The typical topology of the DFIG-based wind energy conversion system (WECS) is shown in Fig. 1. Compared to the WECS with full-scale power converters, the DFIG has no dc link to isolate the generator and the grid, and its stator windings are directly connected to the grid through a setup transformer. Therefore, the DFIG is more sensitive to grid disturbances. Fig. 1.

DFIG-based WECS.