High-Voltage Hybrid Generator and Conversion System for Wind Turbine Applications

This paper presents the design of a high-voltage hybrid generator (HG) and conversion system for wind turbine applications. The HG combines wound field (WF) and permanent magnet (PM) rotor excitations. At any given speed, the PM induces a fixed stator voltage, while the WF induces a variable controlled stator voltage. The HG alternating output is rectified via a passive rectification stage; hence, the machine net dc output voltage is controlled over a prescribed, but a limited range. The split ratio between PM and WF rotor sections is considered as varying from a fully WF rotor, or traditional synchronous generator, to some ratio of PM to WF excitation. The turbine operational characteristics and maximum wind velocity variations between turbines in a wind farm are used to define the WF to PM split ratio. Both a three-phase and a nine-phase stator winding design are investigated. The nine-phase winding results in 4.2% higher output RMS voltage that yields a more power dense solution. It further yields lower rectified dc-link voltage ripple. The HG mass, loss audits, and efficiency discussions are presented. In order to investigate the feasibility of the HG concept, a small scale laboratory prototype is designed, and operational test results presented that show good agreement with the simulation model results.