Direct Liquid Cooling for an Outer-Rotor, Direct- Drive, Permanent-Magnet Synchronous Generator for Wind Farm Applications

Wind power is experiencing steady growth, and wind turbine technologies are continually developing. Currently, power ratings of up to 20 MW are being considered for future wind farms. Offshore applications, which call for the largest and most powerful wind turbines, demand a higher standard of reliability and maintainability. Because they offer better overall efficiencies and higher reliability, 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. Because of the lower rotational speeds, direct-drives must develop substantially more torque to generate higher powers. Torque is the product of air gap diameter and the integrated tangential forces. Therefore, to generate higher powers, direct-drive generators must either develop greater tangential stresses or be larger in diameter. Tangential stress is a function of stator windings current and magnetic flux density, both of which are limited in a traditional air-cooled permanent-magnet generator architecture. Magnetic flux density is produced by the rotor magnets and is essentially fixed. Current density is limited by the heat removal capabilities of the air-cooling system. Consequently, for traditional air-cooled generators, higher power generally means a much larger diameter. Dramatic cost savings can be realized with the development of a more effective stator windings cooling system that removes the limit on current density enabling the development of high-power direct-drive generators of substantially smaller diameters. This paper presents a direct liquid cooling (DLC) 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 with internal cooling liquid flow is also reported to demonstrate the workability of the designed cooling solution.