Διερεύνηση αιολικών συστημάτων με χρήση σύγχρονης γεννήτριας για την τροφοδοσία με ηλεκτρική ενέργεια ασθενών δικτύων αυτόνομα και σε συνεργασία με υδροηλεκτρικό σταθμό

Abstract

In this thesis, the analysis and control of wind energy conversion systems is studied. The chosen wind energy converter consists of a horizontal-axis three-blade pitchable wind turbine, a six-phase synchronous generator and an AC/DC/AC link. The operation is based on a variable-speed constant-frequency scheme that is achieved through two different AC/DC/AC topologies and the appropriate pulsation of the IGBT converters. The aerodynamic torque on the wind turbine is modeled by a special nonlinear equation, which also includes the pitch angle as an extra variable for the control of the aerodynamic power. The simulation program SIMULINK has been used for the modeling and the study of the various systems under large variations of the wind speed. Usually, the wind generators in Greece are connected to the middle voltage grid. Small wind parks are connected to points with very low short-circuit capacity, which means that the voltage is highly susceptible to the variations of the mechanical power on the axis of the wind turbine due to the fluctuations of the wind speed. These disturbances make difficult the supply of real power into the AC grid. The first system studied in this thesis is a wind turbine connected to a middle voltage grid with low short-circuit capacity. The AC/DC/AC topology consists of two IGBT converters: the first is a rectifier that drives the wind turbine into the right angular velocity so that the produced real power is maximized and the second is an inverter, which secures that the frequency and the rms value of the voltage at the point of common coupling are kept constant as the real power is supplied into the AC grid. Three PI controllers, which are designed based on the linearized model of the system, are proposed for the successful control of the velocity of the wind turbine, the produced real power and the voltage stability. The study of the system, during expected major disturbances, has been carried out with the aid of a simulation program. The supply of isolated loads through only a wind generator is of great interest for applications and research. The proposed wind turbine is studied supplying an autonomous system with an AC/DC/AC topology consisted in this case of a rectifier with simple diodes and an IGBT inverter. The total load has been categorized into several load steps according to their importance and a special load shedding control system has been developed for the successful supply of each load step according to the wind speed. A fuzzy controller acts on the pitch angle and a simple PI controller on the pulsation of the inverter so that the right amount of real power is produced and the voltage of the load is kept constant. The validation of the system design has been carried out under expected disturbances by means of the simulation program. The supply of a small grid through a wind generator and a hydro power station is the third system under study. The topology of the wind turbine is the same as in the previous system and the maximization of the real power produced from the wind turbine is now achieved through the appropriate pulsation scheme of the inverter, which includes a PI controller designed by the optimal control theory. The frequency of the load voltage is determined from the hydro power station. The response of the system under large disturbances proved, through the simulation program, to be very good.