Preview Scheduled Model Predictive Control For Horizontal Axis Wind Turbines

This research investigates the use of model predictive control (MPC) in application to wind turbine operation from start-up to cut-out. The studies conducted are focused on the design of an MPC controller for a 650 KW, three-bladed horizontal axis turbine that is in operation at the National Renewable Energy Laboratory\u27s National Wind Technology Center outside of Golden, Colorado. This turbine is at the small end of utility scale turbines, but it provides advanced instrumentation and control capabilities, and there is a good probability that the approach developed in simulation for this thesis, will be field tested on the actual turbine. MPC is an active area for turbine control research, because wind turbine operation is complicated by multiple factors that are intrinsic to harvesting power from the wind resource: Since the goal of the turbine is to produce as much energy as possible from the available power in the air ow passing through the turbine\u27s rotor plane, either the turbine\u27s blade pitch (used to regulate aerodynamic torque), or the generator load torque (used to regulate rotor speed at the optimal tip-speed-ratio) are routinely set at the limits of their operating range. There is a significant variation in the gain from perturbations in blade pitch to perturbations in bending moments and torque. This variation is dependent on the relative speed between the blade and wind, and the nominal blade pitch. As a result, gain scheduling techniques are found to be necessary in order to obtain adequate speed regulation, and optimal load mitigation. The three individual pitch (IP) commands and the generator load command, along with structural loads that can be in conflict with speed regulation objectives, make the turbine control problem inherently multi-input-multi-output (MIMO) in nature. Advanced measurement technologies like LIDAR (light detection and ranging) make the use of preview control plausible in the near future. Standard formulations of MPC accommodate each of these issues. Also, a common MPC technique provides integral-like control to achieve offset-free operation [9]. At the same time in wind turbine applications, multiple studies [38, 5, 73] have developed \feed-forward\u22 controls based on applying a gain to an estimate of the wind speed changes obtained from an observer incorporating a disturbance model. These approaches are based on a technique that can be referred to as disturbance accommodating control (DAC) [32]. In this thesis, it is shown that offset-free tracking MPC [52] is equivalent to a DAC approach when the disturbance gain is computed to satisfy a regulator equation. Although the MPC literature has recognized that this approach provides \structurally stable\u22 [20] disturbance rejection and tracking, this step is not typically divorced from the MPC computations repeated each sample hit. The DAC formulation is conceptually simpler, and essentially uncouples regulation considerations from MPC related issues. This thesis provides a self contained proof that the DAC formulation (an observer-controller and appropriate disturbance gain) provides structurally stable regulation

A Spectral Model for Evaluating the Effect of Wind Evolution on Wind Turbine Preview Control

Abstract — As wind turbines become larger and more flexible, the potential benefits of load mitigating control systems become more important to reduce fatigue and extend component life. In the last five years, there has been significant research activity exploring the effectiveness of preview control techniques that may be feasible using advanced wind measurement technologies like LIDAR (light detection and ranging). However, most control development tools use Taylor’s frozen turbulence hypothesis. The end result is that preview measurements made up-stream from the rotor can be obtained with unrealistic accuracy, because the same wind velocities eventually arrive at the turbine. In this study, we extend the spectral methods commonly used to generate turbulent wind fields for controls simulation, but in a way that emulates wind evolution. This changes preview measurements made upwind from the rotor, in such a way that the differences – between the preview measurements and speeds arriving at the turbine – increase with distance from the rotor. We then evaluate the degradation in load mitigation performance of a controller that uses preview measurements obtained at various distances in front of the rotor. I

A Tutorial of Wind Turbine Control for Supporting Grid Frequency through Active Power Control

Abstract—As wind energy becomes a larger portion of the world’s energy portfolio and wind turbines become larger and more expensive, wind turbine control systems play an ever more prominent role in the design and deployment of wind turbines. The goals of traditional wind turbine control systems are maximizing energy production while protecting the wind turbine components. As more wind generation is installed there is an increasing interest in wind turbines actively controlling their power output in order to meet power setpoints and to participate in frequency regulation for the utility grid. This capability will be beneficial for grid operators, as it seems possible that wind turbines can be more effective at providing some of these services than traditional power plants. Furthermore, establishing an ancillary market for such regulation can be beneficial for wind plant owner/operators and manufacturers that provide such services. In this tutorial paper we provide an overview of basic wind turbine control systems and highlight recent industry trends and research in wind turbine control systems for grid integration and frequency stability. I

A buried spruce forest provides evidence at the stand and landscape scale for the effects of environment on vegetation at the Pleistocene/Holocene boundary

1. Due to a unique set of circumstances, we were able to excavate an entire spruce (Picea) forest in Michigan\u27s Upper Peninsula, USA, which was buried in the early Holocene (9928 ± 133 uncalibrated 14C years BP). Trees ranged from \u3c 5 cm to \u3e 50 cm in diameter, and dominants were approximately 9 m tall. The stand was multi-aged, with a maximum tree age of 145 years. Well-preserved stem cross-sections (n = 140) were recovered and the entire stand was mapped. 2. Stand reconstruction combined with pollen and sediment analysis revealed a pure spruce forest in the sandy lowlands surrounded by hills dominated by pine, oak and birch. These results are consistent with the idea that topography and substrate played an important role in determining forest composition during Holocene plant migrations. 3. Very rapid climate fluctuations were occurring 10 000 BP. The extensive spruce forests that once dominated much of interior North America were being replaced by subboreal and temperate conifer and hardwood species migrating northward. During this dynamic period in the Earth\u27s history, patterns of plant community change are usually inferred from the study of pollen assemblages, and lack of detailed stand level information has prevented the direct comparison of the composition, age, structure and growth rates of Holocene forests with those that exist today. 4. Our findings that the age-class distribution, size, spatial distribution of trees and radial growth rates of white spruce (Picea glauca) can be remarkably similar between the two time periods suggest that forest population and growth processes responsible for today\u27s advancing tree-lines appear to have been at work 10 000 years ago