Design of Direct-driven Permanent-magnet Generators for Wind Turbines

This thesis presents an investigation of how a direct-driven wind turbine generator should be designed and how small and efficient such a generator will be. Advantages and disadvantages of various types of direct-driven wind turbine generators are discussed, and a radial-flux permanent-magnet generator connected to a forced-commutated rectifier is chosen for a detailed theoretical investigation. Further, a design method is developed for the electromagnetic part of the chosen generator type. The generator is optimized with a simplified cost function which, besides including the cost of the active generator parts and the cost of the structure, also includes the cost of the average losses. Therefore, a method to calculate the average losses is derived. The design method is used to investigate the optimization of a 500 kW generator, and the size, efficiency and active weight of optimized generators from 30 kW to 3 MW are presented. A result of the investigation is that the outer diameters of the direct-driven generators are only slightly larger than the width of conventional wind energy converter nacelles. A comparison of average efficiency shows that direct-driven generators, including the losses in the frequency converters, are more efficient than conventional wind energy converter drive trains. Compared with other direct-driven generators, the proposed generator type is small, mainly because of the forced-commutated rectifier and because the generator is not required to produce a pull-out torque higher than the rated torque.

[1]  Xian Liu,et al.  Modeling and Design Optimization Of Permanent Magnet Motors , 1992 .

[2]  F. Caricchi,et al.  Design Criteria of a Low-Speed Axial-Flux PM Synchronous Machine , 1991 .

[3]  John Hindmarsh.  Electrical Machines and Drives , 1996 .

[4]  E. J. Davies.  Airgap winding for large turbogenerators , 1971 .

[5]  E. Spooner,et al.  Damping the power-angle oscillations of a permanent-magnet synchronous generator with particular reference to wind turbine applications , 1996 .

[6]  Thomas Hartkopf,et al.  Direct-drive generators for megawatt wind turbines , 1997 .

[7]  Kjeld Thorborg.  Power Electronics , 1988 .

[8]  J. Perho,et al.  Analysis of a low-speed permanent-magnet wind generator connected to a frequensy converter , 1996 .

[9]  Ezio Santini,et al.  Performance evaluation of an axial-flux PM generator , 1992 .

[10]  Ronald G. Harley,et al.  The General Theory of Alternating Current Machines , 1975 .

[11]  J. G. Kassakian,et al.  A STATIONARY THERMAL MODEL FOR SMOOTH AIR-GAP ROTATING ELECTRIC MACHINES , 1979 .

[12]  Theodore de Koning,et al.  The cooling of electric machines and cables , 1955 .

[13]  Anders Grauers.  Synchronous generator and frequency converter in wind turbine applications : system design and efficiency , 1994 .

[14]  S. Jöckel.  Gearless wind energy converters with permanent magnet generators : an option for the future? , 1996 .

[15]  O. Honorati,et al.  Gearless Wind Energy Conversion System using an Axial-Flux PM Synchronous Machine , 1991 .