Direct liquid cooling for an outer-rotor direct-drive 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, direct-drive 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-PMSG. 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.