Wind turbines are getting larger to increase power capacity. Their longer blades sample a larger area of the spatially and temporally varying turbulent wind field, leading to increased periodic blade load and fatigue damage over time. Individual pitch control (IPC) has proven effective in alleviating these loads by pitching the blades. Conventional IPC fully attenuates the periodic blade loads, which requires excessive pitching, leading to additional stresses on the pitch system. To balance pitch actuation and load alleviation, bounds can be set on the pitch signal (input-constrained IPC), or on the load (output-constrained IPC). While input-constrained IPC has been abundantly researched, little research has focused on output-constrained IPC and on the trade-off when operating between full IPC and no IPC. Therefore, we propose an output-constrained IPC method using an adaptive leaky integrator. The natural frequency of the leaky integrator is adapted on the error between the reference and resultant blade moment. This allows the control scheme to attain every load alleviation level between full and no IPC. Furthermore, in realistic turbulent wind conditions, operating close to full IPC leads to diminishing returns, showing that the proposed controller achieves a superior trade-off between load reduction and actuator effort.

As wind turbine sizes and their rated power capacities increase, the spatial and temporal load imbalances over the rotor surface increase due to larger wind asymmetries, aerodynamic imbalances, and calibration offsets. The multiblade coordinate (MBC) transform-based individual pitch control (IPC) has garnered significant attention in the literature, and it considers loads in a non-rotating reference frame. This leads to increased pitch actuation for all wind turbine blades when subject to large load imbalances. On the contrary, the single-blade control (SBC) IPC strategy is well suited for handling such load imbalances as it involves equipping each blade with a localized control system, thereby ensuring an independent operation in a rotating reference frame. However, unlike MBC transform-based IPC, the effects of system phase lag on SBC performance have not been investigated. This article investigates the effects of such phase lags and multivariable coupling on SBC performance, and proposes a novel framework for phase compensation in SBC as a convenient method for constructing and calibrating a lead compensator. Using midfidelity OpenFAST simulations, it is demonstrated that phase compensation in SBC improves load mitigation at the targeted frequencies and reduces actuation effort. In contrast, the absence of phase compensation can lead to load amplifications, especially for larger wind turbines.

MegAWES is a reference design and simulation framework for ground-generation, fixed-wing airborne wind energy systems with a nominal power output of 3 MW. The winch size of MegAWES is based on a smaller system and needs to be scaled up because the current size leads to unrealistically fast dynamics, which require saturation. However, there is no available method to select an appropriate size for the winch. Additionally, while it has been hypothesized that the size of the winch has a significant effect on the dynamics of the overall system for ground-generation concepts, this effect has not been quantified. In this work, we first analyze the effects of the winch size on the system dynamics using a linearized model. Second, we present a method to find the upper bound for the size of the winch based on a selected maximum tether force overshoot during nominal operation. Third, we apply this method to find an upper bound for the winch size for the MegAWES reference design. Using the nonlinear MegAWES simulation framework, we validated this upper bound. At the upper bound, the system accurately tracked the reference tether force without overshoot and when exceeding our upper bound, the tether force response was oscillatory and overshot its ideal value.