Control Techniques For Small-scale Vertical Axis Wind Turbines Based On Doubly Fed Induction Generators

Wind fluctuations in urban and sub-urban environments poses a challenge to the wind energy industry and requires that various mechanisms be put in place to overcome the drawbacks of such an environment.One challenging aspect is the control of such turbines to maximize the conversion of wind energy in turbulent conditions.One turbine that is chosen for this environment is the Darrieus vertical axis wind turbine.This turbine,which we have examined in this study,is able to admit wind from any direction and achieve good performance in the fore-mentioned conditions.In addition,it,has reduced noise and is structurally stable due to the position of the generator at the base.However,the modelling and controller design of these wind turbines is such a challenging task owing to the highly nonlinear aerodynamic characteristics of the rotor blades.This dissertation studies and examines the control of a vertical axis wind turbine based on a doubly fed induction generator.Robust control techniques have been employed for this study because of their ability to suppress disturbances and ensure stability against a variety of uncertainties.Such control techniques include linear parameter varying(LPV)control,H_? control,?-synthesis and loop-shaping control alongside the classical PI control that has been used as a base controller.Modelling and design of the turbine and generator configurations have been carried out within the Matlab2019R&Simulink environment.The study has provided that linear robust control methods for instance,linear parameter varying control,are suitable for application in the nonlinear control of the pitch angle of the Darrieus NACA 0018 blade profile ensuring quick adaptation to wind changes and limiting the loading on the blades as a protection mechanism.Furthermore,the gain scheduled controller allows for real time computation of the control signal and hence allows for rapid response.H_? control remains a powerful and robust technique for linear control and this has been determined by the studies on the control of the DFIG over the entire range of operation and at steady states.Other robust control techniques such as ?-synthesis,LPV and loop-shaping control are equally powerful tools although they show limitations such as less ability to decouple interference from other signals which is an advantage of H_? control.Inertia plays a big role in the operation of a turbine.It both determines the starting efficiency of the turbine and the frequency support suitability of a grid connected wind energy conversion system.More so,it influences how quick a turbine responds to wind speed changes.The set-up uses a VAWT-DFIG connection to study this phenomenon alongside the well understood LPV technique.It can be deduced that a low inertia turbine is more acceptable over a high inertia turbine for its suitability in maximum power conversion.In conclusion,the studies have made several contributions including proposing a novel pitch control approach for small scale VAWTs,a novel DFIG based VAWT control through active and reactive powers,a comparison of linear robust control techniques and their suitability for the plant in study and finally the role of inertia in control of VAWTs.The findings lead to our finer understanding of the details of linear control techniques and their applicability to wind energy conversion systems.Various aspects such as pitch control,torque control and the use of Matlab&Simulink provides us with interesting insights into their use for further studies and in industrial application.