Employing a very sparse matrix converter for improved dynamics of grid-connected variable speed small wind turbines

Abstract

The amount of energy obtained from a wind turbine depends not only on the wind regime but also on the control technique used to govern the turbine. When wind speed changes, the turbine moves away from its optimal tip speed ratio until maximum power point tracking forces the turbine to operate at its optimal speed. For rapid wind speed changes, the maximum power point tracking controller can take longer than the necessary time to return the turbine to its maximum power coefficient which reduces the energy harvest, especially in gusty conditions. Proposed in this paper is a motoring-generating control technique for grid-connected small wind turbines in order to accelerate the turbine to its optimal operating speed in the case of increasing wind speed. In the case of decreasing wind speed, the turbine decelerates by loading the generator at its maximum torque. The generator torque is controlled through the backward bidirectional very sparse matrix converter where the grid is connected to the rectifier stage and the generator is connected to the inverter stage. The generator torque is regulated by controlling the generator current based on the principle of field oriented control where a decoupling between the field and torque current components is achieved. The direct axis current is set to zero while the quadrature axis current is used to control the generator torque and speed. Then the generator reference voltage and the converter switching signals are synthesized using the principle of space vector modulation. In order to show the feasibility of the proposed control technique, a 5.7kW grid-connected variable speed wind turbine is simulated using Matlab/Simulink.