Comparing the performance of two control strategies for a variable speed wind turbine

Abstract

Energy generation through burning of fossil fuels is considered as the most contributor to greenhouse gases emissions in Europe. Renewable energy sources is one of the options for reducing such emissions and out of the various options, wind energy is proving to be one of the most preferred choice. This fact is fuelling research on variable speed wind turbine control systems to ameliorate energy yields such that the return of investment is improved. This study is based on a comparative analysis between two variable speed wind turbine control schemes mainly the traditional control scheme (control scheme one) and the alternative control scheme (control scheme two). The latter is effectively the reversal of the former. Additionally an auxiliary control algorithm was implemented as an inertia response controller. This controller was added on top of either control scheme to investigate the behaviour of the wind turbine when grid support is demanded. The two control schemes are employed on a 2MW wind turbine model, developed in MATLAB® Simulink®. The model consists of a mechanical system comprising of the rotor and drivetrain, an electromechanical component being the permanent magnet synchronous generator and an electrical system consisting of a back to back voltage source converter which is connected to the grid through a filter. Two filter variants (L and LCL filter) were designed to compare the harmonic distortion levels present on the output current waveform. The simulation outcome favours the LCL filter, which produced a cleaner output current waveform; however both filters are grid compliant. Different wind profiles were chosen to compare the two control schemes mainly, a wind step, a wind gust according to the IEC2 standard and a turbulent wind environment. Simulation results show that both control schemes work almost identical except for some small discrepancy. The grid output power for both schemes is almost the same. The two control schemes also settle at approximately the same generator rotor rotational speed. The ineptness of the wind turbine to track fast changing wind speeds is particularly visible when the IEC2 standard wind gust was simulated. Both control schemes, support the grid with the extra power needed when a grid support command is issued to the auxiliary inertia response controller.