According to the requirements for the low voltage ride through(LVRT) in the standard of State Grid Corporation for uninterrupted operation of wind turbine unit, the effects of asymmetrical grid fault on the double-fed wind turbine unit are analyzed, and a mathematical model for the doublefed induction generator is established. Based on the mathematical model, the through plan by combing the rotor Crowbar circuit and DC Chopper circuit is designed and the corresponding control strategies are analyzed. Finally, the correctness and the feasibility of the circuit are verified.