A low-cost ac-dc rectifier for variable-speed wind energy conversion systems

In this thesis, a novel AC-DC rectifier is adopted to convert the variable AC voltages from wind generator to a constant dc-link voltage. A DC-AC inverter is used to transfer the energy to fixed ac grid as well as maintain dc-link voltage constant. The rectifier is connected to the generator, which has two sets of three-phase windings. Electrical exited or permanent magnetic synchronous generators are suitable for this rectifier. The AC-DC rectifier features low cost, low power losses and simple control structure. The major content of this thesis consists of four parts. The first part investigates the operation of the AC-DC rectifier. The analysis illustrates the current paths in the rectifier and the remarkable voltage-boosting capability. A simulation model of the converter is built and verified. The second part models the synchronous generator with two sets of windings. The model is developed in the d-q synchronous reference frame. The simulation results from the model are verified by the experimental results.Then the third part is the design of the control system, which involves the generator, the AC-DC rectifier and constant dc-link voltage source. The control system composites of the inner current loop and the outer speed loop. The control system achieves the variable-speed operation of the generator and the regulation of the dc-link current. Finally, a prototype of wind turbine system is established in the lab to verify the analysis of rectifier, synchronous generator and control system design. The steady-state and transient responses of the wind system are compared. Simulation and experiment verify the analysis in this thesis.