Dual-Loop Geometric Control of Stator Flux for Improved LVRT Response in DFIG-Based Wind Turbine Systems

Doubly Fed Induction Generator (DFIG) based wind turbines are particularly sensitive to grid disturbances, which has implications for overall power system stability. Grid codes require continual operation of the DFIGs despite certain system disturbances. During a three-phase voltage dip, the decay of a natural flux fixed to the stator of the machine induces large voltages in the rotor windings, leading to saturation of rotor converters and potential damage to the system. A novel LVRT strategy is proposed to provide fast transient response of the stator flux to fault levels, enabling rapid recovery of machine operation. This work presents a dual-loop architecture to solve the transient, allowing the system to regain standard operation and power reference tracking in a fast and effective manner. The proposed control structure implements a unique continuous solution for both steady state and transient operating conditions and does not require any additional hardware, switching circuits, or fault detection mechanisms. The proposed controller is supported by detailed mathematical analysis and validated by simulation results. Experimental results include extended Hardware in the Loop (HIL) validation, with a controller implementation in an industry-standard digital microcontroller.