Over the past 30 years the cost of wind energy has significantly decreased and already the cost of energy of that of onshore wind production is competitive to that of conventional fossil fuels. However, further reduction are necessary such that offshore wind production becomes competitive without the need of subsidies.
One method of reducing the cost of energy can be attained by reducing the fatigue loads on the wind turbine such that the material costs can be reduced. In this thesis the use of a free yaw configuration is explored which offers the possibility of strongly reducing the fatigue loads in the yaw direction due to the added compliance when compared to fixed yaw, in which case the yaw movements are locked through brakes. The main problem in free yaw however is that the wind turbine is continuously yawing which causes large gyroscopic forces on the blades.
This leads to the problem definition "Can a large horizontal axis wind turbine having a free yaw configuration reduce the loads compared to a fixed yaw configuration by using active control methods?"
In order to apply active control to reduce the loads, first the yaw behavior of the wind turbine is captured into a control-oriented model based on first principles. It was found that in order to achieve the desired load reductions the control-oriented model needed to include the flexibility of the blades. Based on this model a controller was designed that is able to control the yaw position and reduce the gyroscopic forces through the use of individual pitch control.
The developed controllers are tested on the 5MW NREL wind turbine in the aeroelastic simulation tool FAST. Load calculations are performed according to the IEC 61400-3. Results show that active control for the free yaw configuration reduces all relevant fatigue loads compared to a fixed yaw configuration. The addition of a yaw damper finally shows a reduction of the required pitch activity such that it was comparable to the pitch activity in a fixed yaw configuration. This makes the explored free yaw configuration viable and possibly more cost-effective then a fixed yaw configuration.