Basic Operation Principles and Electrical Conversion Systems of Wind Turbines

This paper gives an overview of electrical conversion systems for wind turbines. First, the basics of wind energy conversion with wind turbines are reviewed and requirements with respect to the electric system are considered. Next, the three classical conversion systems are described with their strengths and weaknesses: constant speed, variable speed with doubly-fed induction generator and variable speed with direct-drive generator. The applied power electronic converters are shortly addressed. Finally alternative generator systems and trends are discussed. There is a clear trend towards variable speed systems. Doubly-fed induction generator systems are increasingly equipped with grid fault ride through capabilities. For direct-drive turbines, the radial flux permanent-magnet synchronous generator is cheaper and more efficient than the electrically excited synchronous generator. It is expected that the voltage level of generators will increase up to values in the order of 5 kV.