Abstract
In the field of marine renewable energies, the extraction of marine currents by the use of tidal current turbines has led to many studies. In contrast to offshore wind turbines, the mass minimization is not necessarily the most important criterion. In that case, Direct-Drive Electrically Excited Synchronous Generators (EESG) can be an interesting solution in a context where the permanent magnet market is more and more stressed. In the particular case of a tidal turbine, the electric generator operates at variable torque and speed all the time. Its sizing must therefore take into account the control strategy and check that all the constraints are respected during the working cycle, particularly the thermal one because its permanent regime is never reached. There is no solution today that can completely solve such a sizing problem. The paper presents a specific solution. In particular, we will see that the method presented allows an avoidance of an oversizing of the generator compared to conventional methods while finding the optimal control strategy. Thus, the design optimization of an EESG is conducted considering the variable torque and speed profiles related to marine currents. The analytical model used in the paper is presented at first. In a second step, the innovative and original method that allows solving at the same time the design optimization and the control strategy (dq stator currents and rotor current) are presented. It shows how it is possible to minimize both the lost energy during the working cycle and the mass while fulfilling all the constraints (especially the thermal constraint with its transient temperature response) and keeping a reduced computation time. The case of a 2 MW tidal wave turbine is chosen to illustrate this study. Finally, the optimal design selected is validated by a 2D magnetic Finite Element Analysis (FEA).