Abstract: Nine-switch converters（NSC） can provide two sets of independent three-phase outputs by using the time-shared modulation, and thus can be utilized to replace the traditional back-to-back converters in the doubly-fed induction generation（DFIG） wind power system. This paper proposed an optimal control strategy design for the NSC-based DFIG system by introducing the real-time dynamic dc-link voltage assignment, with which one set of control strategies can be effective under both normal grid-voltage and grid-dip conditions. With the proposed design, high dc-link voltage of NSC in normal grid condition was automatically assigned to the grid-side to match the high grid voltage; and high dc-link voltage when the grid dips was automatically assigned to the rotor-side to match the rotor’s overshoot voltage. Such an automatic assignment makes the NSC-based DFIG system can achieve low-voltage-ride-through（LVRT） without extra auxiliary crowbar circuits, and thus fully use the advantages of NSC; such an automatic assignment can also avoid the mode-switching between different control strategies and the detection of grid-dip moment, and thus can greatly reduce the system complication.