0 Control Design Methodologies for Vibration Mitigation on Wind Turbine Systems Ragnar Eide and Hamid Reza Karimi Department of Engineering, Faculty of Engineering and Science University of Agder Postboks Grimstad Norway 1. Introduction The world’s energy consumption from the beginning of the industrial revolution in the 18 th. century and until today has increased at a tremendous degree. Since a large part of the energy has come from sources like oil and coal have the negative impacts on the environment increased proportionally. Therefore, more sustainable and climate friendly energy production methods are emphasized among researchers and environmentalists throughout the world. This is the reason why renewable energies, and wind power particularly, have now become an essential part of the energy programs for most of governments all over the world. One example is seen by the outcome of the European Conference for Renewable Energy in Berlin in 2007 where EU countries defined ambitious goals when it comes to the increase in use of renewable energy resources. One of the goals was that by 2020, the EU would seek to get 20% of energy consumption from renewable energies. Wind power, in conjunction with other renewable power production methods, has been suggested to play a more and more important role in the future power supply (Waltz, 2008) and (Lee & Kim, 2010). One of the reasons for these expectations is the enormous available potential when it comes to wind resources. One of the most comprehensive study on this topic (Archer & Jacobson, 2005) found the potential of wind power on land and near-shore to be 72TW, which alone could have provided over five times the world’s current energy use in all forms averaged over a year. The World Wind Energy Association (WWEA) estimates the wind power investment worldwide to expand from approximately 160 GW installed capacity at the end of 2010 to 1900 GW installed capacity by 2020. One example is from the USA, where the current contribution of electricity from wind power is merely 1,8% in (2009). However, the U.S Department of Energy is now laying a framework to get as much as 20% contribution by the year 2030. Due to the economical advantages of installing larger wind turbines (WTs), the typical size of utility-scale turbines has grown dramatically over the last three decades. In addition to the increasing turbine-sizes, cost reduction demands imply use of lighter and hence more flexible structures. If the energy-price from WTs in the coming years are to be competitive with other power production methods, an optimal balance must be made between maximum power capture on one side, and load-reduction capability on the other side. To be able to obtain this is a well defined control-design needed to improve energy capture and reduce 11