Abstract: When a severe voltage sag occurs in the power grid, the wind power generation system should keep connected to the power grid and provide dynamic reactive power support to the power grid. The doubly-fed induction generator（DFIG） based wind turbines using the traditional excitation converter usually require additional devices to meet this requirement, which leads to complicated systems with low reliability and efficiency. Aiming at this problem, this paper analyzes the bottleneck of low voltage ride-through（LVRT） of DFIG-based wind turbines, and proposes the idea of building a forced excitation system. A new type of forced excitation converter based on variable structure quasi-Z-source is designed for DFIG-based wind turbines. The traditional capacitive bus is replaced by a quasi-Z source network. When the power grid is normal, the proposed converter operates in the state of single capacitive bus with adjustable voltage. When the power grid voltage drops deeply, the proposed converter can raise the DC-link voltage quickly, ensuring the rotor side converter（RSC） remain controllable during the fault. The simulation and experimental systems for LVRT of DFIG-based wind turbines are constructed. The simulation and experimental results indicate that the proposed forced excitation converter topology has good steady-state and dynamic performances, and it can effectively control the rotor current when the voltage drops deeply and realize the LVRT and reactive power support of DFIG-based wind turbines.