10 MW Wind Turbine Direct-Drive Generator Design with Pitch or Active Speed Stall Control

The objectives of this paper are to investigate the feasibility of a 10 MW generator for a direct-drive wind turbine and to compare the generator systems for pitch control and for active speed stall control. The idea behind the active speed stall control concept is to make a rotor that is as simple as possible, and therefore very robust and suitable for offshore wind turbines. This is done by removing the pitch control of the blades. Above rated wind speed, the power is not controlled by controlling the pitch, but by controlling the rotor speed: the rotor speed is so much reduced that the aerodynamic power is limited to the rated value. A rough 10 MW permanent-magnet direct- drive generator design is presented, indicating that such a generator is feasible. It is shown that for a thorough evaluation of active speed stall control, more knowledge is required about changes in the wind speed. However, a considerable increase in generator system cost is necessary to enable active speed stall control.