Search

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.

This paper presents a novel control algorithm for the rotor-side converter of Doubly-Fed Induction Generators. The main goal is to endow the system with effective Low Voltage Ride Through capability, under harsh balanced and unbalanced grid voltage sags, without relying on dedicated auxiliary hardware, which is commonly adopted to sustain severe line faults. In this respect, nonlinear control theory arguments are applied to design a controller capable of mitigating oscillations (particularly on rotor currents and voltages) arising during line faults, therefore preventing the system from disconnecting for protection. The proposed solution adopts both feedforward and feedback terms. The former stems from a thoughtful analysis of the system internal dynamics, taking into account the effects of line voltage perturbations, which is exploited to design feasible state trajectories for the generator electromagnetic variables. Specifically, such references do not contain poorly-damped oscillatory modes of the machine natural dynamics, expressed in synchronously rotating frames (such components turn into slowly varying DC ones in a stationary frames). Then, a state feedback unit is designed according to modern saturated control techniques, accounting for constraints on rotor voltage, and steering real variables toward references, where priority given to rotor currents, to avoid rotor-side converter tripping due to overcurrent. In addition, a non standard line voltage reconstruction and dip detection scheme, based on adaptive state observers, is designed, to reliably cope with challenging faulty conditions. Detailed numerical simulations validate the proposed method benefits under severe symmetric and asymmetric dip scenarios.

A novel control solution for Doubly-Fed Induction Machines is proposed to enable new applications of such kind of machines. This strategy guarantees a fully decoupled motion control and contactless power transfer between stator and rotor, by using controlled inverters on both stator and rotor side. For large and complex rotary apparatus with relevant electric loads and actuators placed on-board the mobile part, the proposed solution allows to exploit direct-drive versions of Doubly-Fed Induction Machines for both moving and feeding independently the rotating part, where the rotor-side converter has to be hosted. Power transfer is attained through two different working principles in order to achieve decoupling from the torque references and to suitably deal with voltage saturations on power converters. Global asymptotic stability proof is provided for this control strategy. Simulation results are reported in order to validate the promising theoretical developments.

In this paper, the stator/rotor currents control problem of doubly-fed induction generator under faulty line voltage is carried out. Common grid faults cause a steep decline in the line voltage profile, commonly denoted as voltage dip. This point is critical for such kind of machines, having their stator windings directly connected to the grid. In this respect, solid methodological nonlinear control theory arguments are exploited and applied to design a novel controller, whose main goal is to improve the system behaviour during voltage dips, endowing it with low voltage ride through capability, a fundamental feature required by modern Grid Codes. The proposed solution exploits both feedforward and feedback actions. The feedforward part relies on suitable reference trajectories for the system internal dynamics, which are designed to prevent large oscillations in the rotor currents and command voltages, excited by line perturbations. The feedback part uses state measurements and is designed according to Linear Matrix Inequalities (LMI) based saturated control techniques to further reduce oscillations, while explicitly accounting for the system constraints. Numerical simulations verify the benefits of the internal dynamics trajectory planning, and the saturated state feedback action, in crucially improving the Doubly-Fed Induction Machine response under severe grid faults.