Abstract

The main goal of this Ph.D. thesis is the study and application of harmonics reduction techniques in small wind power generation systems. The problem is addressed from the point of view of the wind turbine, i.e., focusing the study around the input stage of the power electronic converter, which consists of a diode three-phase rectifier, a DC-DC boost converter and a grid-connected inverter. In the case of AC-DC front-ends connected to the grid, operating at constant nominal values of the AC voltage amplitude and frequency, the power factor of a diode rectifier followed by a boost DC-DC converter can be corrected by operating the DC-DC converter in discontinuous conduction mode. This Ph.D. thesis shows that this technique can be extended to the input stage of the wind energy conversion system in order to reduce the total harmonic distortion of the wind turbine output currents (THDi), which in turn reduces the vibrations caused by torque harmonics and increases the power factor.

The challenge of the extension of this principle to wind turbines operating at a variable speed is that the three-phase rectifier operates from an AC voltage with a wide variation of both the amplitude and the frequency. To operate in discontinuous conduction mode it is required to add an LCL filter to the turbine output, so that the rigorous study of the current mode control of the DC-DC converter placed after the diode rectifier is critical.

Additionally, other relevant issues for this context have been studied, such as the comparison of several wind speed estimation algorithms working from noisy measurement signals produced by the discontinuous conduction mode, and the study of a control structure that allows the implementation of maximum power point tracking algorithms for the wind turbine without significantly degrading the performance achieved by the proposed techniques. The analytical results have been validated experimentally using a prototype of a 2kW wind generation system, which comprises a permanent magnet synchronous generator and the power converter described previously. This generation system is powered by a wind turbine emulator that allows taking into account various factors such as the blades inertia, the aerodynamic characteristics, and so on.