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

Doubly Fed Induction Generator (DFIG) based wind turbinesare particularly sensitive to grid disturbances, which has implications for overall power system stability. Grid codes require continual operation of 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. This work presents a novel dual-loop state-plane based architecture to solve the stator flux transient, allowing the system to regain standard operation and power reference tracking in a fast and effective manner. An outer loop geometric controller is proposed as a framework for achieving rapid and controllable transients in the decoupled state space, and various reference geometries are considered. 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. Performance-based tuning is used to determine a controller that optimizes the performance index across a variety of grid fault levels, and the resultant controller is shown to achieve rapid transient response for all fault levels, with power reference tracking achieved in under one line cycle ($\text{20}\,\text{ms}$) for even the most extreme grid faults. The proposed controller is supported by detailed mathematical analysis and validated by simulation results.