Direct power control of doubly-fed-induction-generator-based wind turbines under asymmetrical grid voltage dips

The wind power system based on doubly fed induction generator (DFIG) has become the most popular and widely used in recent years. However, the system is very sensitive to the grid voltage dips, so its low voltage ride-through (LVRT) capability required by grid codes is a challenging problem. In this paper, the LVRT problem for asymmetrical voltage dips is studied. The theoretical analysis shows that disturbance at both of fundamental and twice synchronous frequencies is caused by asymmetrical voltage dips. This paper proposes a direct power control (DPC) algorithm to control the stator active and reactive power directly with a respective single loop. The multi-frequency proportional integral resonant (MFPIR) controllers are involved by the proposed algorithm to significantly restrain the disturbance caused by grid voltage dips. With a crowbar protection strategy and a simple demagnetization method, the proposed scheme can achieve the low voltage ride-through ability under both of symmetrical and asymmetrical voltage dips in a unified algorithm. A 7.5kW DFIG-based wind turbine prototype is built up to verify the effectiveness of the proposed method.