Doubly-Fed Induction Machines: Model, Control and Applications

Renewable energy resources far outweigh fossil fuels in several terms of benefits, i.e. environmentally friendly, and economically. Wind Energy Conversion Systems are the fastest grown units among renewables within recent years. Due to large penetration of wind units in nowadays power systems, some specific regulations have been issued through modern national grid codes to manage their technical commitments. Low Voltage Ride Through (LVRT), as one of the most important requirements, asks wind units to ride through some predefined grid low voltage conditions in terms of amplitude reduction and time duration, mainly caused by different types of balanced and/or unbalanced power network faults. Doubly-Fed Induction Generators (DFIGs) as the most popular machines among the current driven wind turbines, are electrically connected to the grid through a three-phase winding placed at stator, while rotor is electromagnetically connected to stator. Hence, a sudden reduction of voltage profile, will trigger large current/flux oscillations in the machine, may hit the physical limits and consequently, violate grid codes. The main topic of this thesis is modeling and control of DFIG-based wind turbine systems to substantiate LVRT requirements without imposing any additional hardware to installed components. To achieve this objective, system/control theory tools are applied to investigate the effects of grid faults on DFIG dynamics, and design proper control-based countermeasures. More specifically, taking advantage from analyzing the internal dynamics of DFIG, various feedforward-feedback controllers have been designed to deal with line faults having increasing complexity. A crucial role in such approach is played by a suitable state reference trajectory design, based on the feature of the DFIG internal dynamics. Such kind of method has been applied to deal with the mechanical dynamics, as well. Numerical realistic simulations validate the benefits of the proposed controller, in crucially improving the machine response under severe grid faults.