Hybrid excited synchronous generator design and comparison of direct drive wind turbines

Akgemci, Aysel
Various types of electrical generators are used in wind turbines and there is not an agreement on the best generator type. Although, high speed Doubly Fed Induction Generators (DFIGs) are still the most common generator topology utilized in wind turbine systems, there is a trends toward direct-drive Permanent Magnet Synchronous Generators (PMSGs), as they are more efficient and reliable. However, permanent magnets (PMs) induce uncontrollable voltage due to the fixed flux resulting from PM excitation. Conventional synchronous direct-drive generators are controllable, but they usually have a higher mass. In the last decade, the idea of using hybrid excitation to effectively control the output voltage emerged. The hybrid term come from the employment of both PM and DC-excited field winding excitations. By this way, it would be possible to control the load angle and power factor, and an uncontrolled diode rectifier can be used. Furthermore, the amount of required magnet reduces, which helps to reduce overall cost. In this thesis, several topologies of the hybrid excited generators in the literature will be introduced and a 2.5 MW two rotor hybrid excited generator is designed for direct drive wind turbine application. The design is evaluated using finite element method (FEM) simulations. At the same ratings, the generator topology is compared with the PMSG and Electrically Excited Synchronous Generator (EESG) in terms of short circuit characteristics, torque ripple and induced voltage harmonics.