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.

This article describes the development and testing of a modified, semi-empirical ONERA dynamic stall model for an airfoil with a trailing edge flap—a “smart airfoil”—pitching at reduced frequencies up to 0.1. The Reynolds number is ${10}^5$. The model reconstructs the load fluctuations associated with the shedding of multiple dynamic stall vortices (DSVs) in a time-marching solution, which makes it suitable for real-time control of a trailing edge flap (TEF). No other model captures the effect of the DSVs on the aerodynamic loads on smart airfoils. The model was refined and tuned for force measurements on a smart NACA ${64}_3$-618 airfoil model that was pitching with an inactive TEF and was validated against the measurements when the TEF was activated. A substantial laminar separation bubble can develop on this airfoil, which is challenging for modelers of the unsteady response. A closed-loop controller was designed offline in SIMULINK, and the output of the controller was applied to the TEF in a wind tunnel. The results indicated that the model has a comparable accuracy for predicting loads with the active TEF compared to inactive TEF loads. In the fully separated flow regime, the controller performed worse when dealing with the development of the laminar separation bubble and DSVs.

The paper presents a novel method which generates hopping gaits for an articulated single leg. Inspired by human running, the flight phase is assumed to be nearly-passive. Consequently, the initial joint velocities of the flight phase can be solved by using a static optimization procedure, provided that the boundary joint angles have been picked in advance. The two hopping phases can then be dynamically optimized, with the velocity jumps of the joints predicted by a simple collision model. The direct single shooting method solves the nonlinear constrained optimization problem. Ground reaction forces and the zero-moment-point serve as constraints. Simulation results show that the proposed approach produces energy-efficient hopping gaits.