An improved control scheme for Doubly Fed Induction Generator during grid fault ride-through

The increasing wind power penetration brings new challenge for wind turbine to provide Low Voltage Ride-Through (LVRT) capability. Doubly Fed Induction Generator (DFIG) has been widely used in wind turbines; due to its cost-effective partially power rated of the converter and Variable-Speed Constant-Frequency (VSCF) operation. However, the DFIG system has two poorly damped poles and is sensitive to the grid disturbance. For the stator is directly connected to the grid, the symmetrical voltage dip results in a dc component in the stator-flux which reflects a serious oscillation in the synchronous reference frame. This transient stator-flux induces overvoltage and overcurrent in the rotor circuit. The paper analyzes the characteristic of the transient stator-flux under voltage dip. Based on the eigenvalues of the system matrix, the damping rates of the transient stator-flux are investigated. An improved control scheme of the Rotor Side Converter (RSC) is presented to accelerate the transient stator-flux damping rate. The proposed control improves the rotor current control performance and relieves the fluctuation of electromagnetic torque, dc-link voltage, stator current and rotor current. It is also beneficial to the fast reactive power support to the grid during voltage dip. The simulation results of a 1.5-MW DFIG system and experimental tests on a 90-kW DFIG rig validate the theoretical analysis and the feasibility of the proposed control scheme.