The purpose of this dissertation is to study the operating methods of wind turbines with variable speed control. The first chapter introduces an introduction to wind energy as a renewable energy source. In particular, the major advantages and main features of wind energy, as well as the interconnection of wind energy with the electrical grid, are mentioned. The second chapter presents a detailed presentation of the structure and operation of wind systems. Initially, a classification of wind turbines is made on the basis of their construction form and the description of the basic structure of the wind turbines. In addition, aerodynamic analysis of the wind turbines is carried out, which defines the basic aerodynamic sizes for the study of wind turbines. Additionally, wind systems with fixed and variable wind turbines are examined. Finally, the application of induction and modern generators to wind systems, as well as the use of power electronics in wind turbine systems, is evaluated. In the third chapter, wind turbine variable speed control is performed. Initially, the model of the dual-power induction generator is presented, followed by a description of the wind power control system with a dual-power induction generator. Then, the model of the synchronous permanent magnet generator is presented, followed by the description of the control system of the wind systems with a synchronous permanent magnet generator. In the fourth chapter we study the effect of variable speed wind turbines on the electrical grid. First of all, we examine the effect of variable speed wind turbines on voltage control on the electrical power transmission systems, distributed generation power networks and then on the overall electrical grid.