A novel idle cutting-in control strategy based on PLL for DFIG wind turbine

With the disadvantage of indirect control on stator voltage and the dependence on parameters such as stator inductance and flux linkage observer angle, the traditional cutting-in control strategy for doubly-fed induction generator (DFIG) lacks a highly control performance on stator voltage. As a result, the power grid voltage cannot be tracked precisely and a high impact current is generated in the grid-connection process. Therefore, a direct closed-loop voltage control is implemented to control stator voltage precisely. Meanwhile, the control accuracy of rotor exciting current is refined and the linear relationship between rotor exciting current and rotor voltage is obtained when a feed-forward control is undertaken. The performance of stator voltage tracking grid voltage and the disturbance rejection function of stator voltage are improved, which is proved by both theoretical analysis and simulation. On the whole, the stator voltage closed-loop control is an ideal way to realize the grid connection.