Damage mitigating control for wind turbines
In the last few decades the wind industry has made great strides in reducing the cost of energy of utility scale wind turbines. In an attempt to reduce infrastructure costs and improve efficiency, the trend has been to develop larger variations of existing designs.
In the past, the wind turbine controller was used primarily for rotor speed control and prevention of catastrophic damage from extreme wind conditions or component failures. The recent trend of wind turbine growing in size has resulted in wind turbines becoming much more flexible, and now the emphasis of wind turbine controls research focuses on how to damp resonances and avoid dangerous excitations that may lead to structural failure.
Control of the fatigue loads on the wind turbine structure addresses neglects the fatigue mechanism of the material. The conversion of loads into stresses and those stresses into fatigue damage is a highly nonlinear process and is based on the so-called "cycle-counting" methods. Since the cycle counting methodology is difficult to convert into the time or frequency domains, these components have been generally avoided in controls research.
Without modeling the damage dynamics, the wind turbine controller cannot efficiently reduce the fatigue of the structural components. The result is that only small decreases of fatigue damage are realized by current load reduction strategies at the expense of excessive control actuation.
This dissertation introduces the concept of Damage Mitigating Control (DMC) as it applies to utility scale Horizontal Axis Wind Turbines (HAWTs). The work presented extends earlier work in damage mitigating and life extending control in several ways and then applies then applies this control strategy to reduce the fatigue damage suffered by wind turbines during operation. By modeling fatigue damage dynamics within the wind turbine controller, the life of the turbine can be extended significantly without sacrificing performance.