Abstract

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.

Highlights

  • 1.1 Wind EnergyThe demand for electrical power Increases 1n the world

  • The wind energy conversion system (WECS) is modeled using small signal model and transfer functions are developed for regulator design

  • A new rectifier is adopted in the wind energy conversion system

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Summary

Introduction

1.1 Wind EnergyThe demand for electrical power Increases 1n the world. Accompanying with that, the concern of environmental impact, such as natural resource exploitation, greenhouse emission and hazard nuclear waste, has been raised. In small-scale wind turbine generator systems, an AC-DC-AC converter is the most commonly interface circuit to connect wind turbine generator to grid This configuration allows wind turbine generator operate at variable speeds and extract maximum power from wind. In order to provide high quality electricity to grid, the DC-link voltage in AC-DC-AC converter has to be maintained higher than the peak value of DC-AC converter output voltage Due to this reason, AC-DC rectifier is required to have capability to boost the DC-link voltage, when wind turbine generator operates at low speed. Compared with other AC-DC rectifiers proposed in previous research, the novel AC-DC rectifier studied here provides a higher cost-efficient solution for wind energy conversion systems. A feedback control system is designed to regulate the generator speed according to optimal speed of wind turbine. The steady states and the dynamic responses of the system are measured and compared to the simulation results

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