Control of Doubly Fed Induction Generator to Ride Through Recurring Grid Faults

The wind turbine system (WTS) is required to ride-through recurring grid faults by the new grid codes. Under single grid faults, the fault ride-through (FRT) strategy with rotor-side crowbar is normally used for the doubly-fed induction generator (DFIG) WTS. However, under recurring faults, larger transient current and voltage may be produced, and the DFIG may fail to ride-through the second fault even with the rotor-side crowbar. The crowbar can be active again during the voltage recovery, but large electromagnetic torque (EM-torque) fluctuations will be introduced. The reliability of the mechanical system will be influenced. In this paper, an FRT strategy for the DFIG WTS to ride-through recurring symmetrical grid faults is investigated. An improved control strategy is introduced and it is applied during the voltage recovery of the grid faults. The decay of the stator natural flux can be accelerated so the transient current and voltage under recurring fault will be no larger than that under single fault. At the same time, the EM-torque fluctuations can be suppressed, the mechanical stress can be reduced, and higher reliability can be expected to the WTS. It is verified by simulations on a 1.5-MW DFIG model and by experiments on a 30-kW DFIG test system.