Direct liquid cooling for an outer-rotor directdrive permanent-magnet synchronous generator for wind farm applications

Offshore applications, which call for the largest and most powerful wind turbines, demand a higher standard of reliability and maintainability. Direct-drive permanent-magnet synchronous generators (DD-PMSGs) are increasingly being specified for these applications. The major shortcoming to traditional high-powered direct-drive generators is extraordinary size and mass leading to extraordinary cost. To generate higher powers at low rotational speeds, directdrive generators must either develop greater tangential stresses or be larger in diameter. For traditional air-cooled generators, higher power generally means a much larger diameter. Dramatic cost savings can be realised with the development of a more effective stator windings cooling system that puts further the limit on current density enabling the development of high-power direct-drive generators of substantially smaller diameters. This study presents a direct liquid cooling system design for an 8 MW outer-rotor DD-P MSG. The approach is new for wind turbine generators, so its impact on the thermal behaviour and reliability for the total electrical machine has been evaluated and reported here. Testing of a stator coil prototype (1/72nd of the complete stator) with internal cooling liquid flow is also reported to demonstrate the workability of the designed cooling solution.