Diseño de estrategias de control para optimizar el comportamiento del generador asíncrono doblemente alimentado ante tensiones de red no-lineales

Zusammenfassung

The goal of the present PhD Thesis consists in the development of controlalgorithms for the two power converters of a doubly-fed induction generator(DFIG)-based wind turbine. Such algorithms are intended to optimizethe performance of the wind turbine under distorted grid voltage conditions|e.g., imbalances, harmonics|, not only ensuring the safe operation ofthe latter, but also trying to be minimally detrimental to the power networkto which it is connected.For that purpose, the evolution that the control systems of both windturbines and wind farms have experienced during the last years |mainlyoriginated by the increasingly demanding grid codes| is analyzed first.Next, and based on the well-known three-phase DFIG model, the necessarymathematical transformations are applied in order to derive twoadditional representations of the system which are required throughout therest of the Thesis: the DFIG model expressed in vector space, and thespace-vector model represented in symmetrical sequences.The major contribution of the present work is tackled afterwards. Basedon the sliding-mode control theory, a first approach to the rotor- and gridsidepower converter control algorithms is presented, named original 1-SMCcontrollers. Despite these controllers present improvements with respect towhat can be found in the literature, the commutation frequency of the converters'transistor gates turns out to be variable. Consequently, a secondapproach, whose control algorithms are referred to as adapted 1-SMC controllers,is proposed aiming at solving such a drawback. In both controlschemes, besides governing the active and reactive powers generated by thesystem, the following additional goals have been established for each powerconverter under distorted grid voltage conditions: the rotor-side convertereliminates the oscillations that arise in the electromagnetic torque, whilethe grid-side converter can be commanded to attain either non-fluctuatingtotal active and reactive powers or harmonic-free total currents, being bothoptions incompatible to each other.Next, as a first stage in analyzing to what extent the DFIG capabilityto generate power is reduced under distorted grid voltage conditions, theparticular case of imbalances is assessed. Hence, several equations have beenderived to calculate the controlable rotor current and stator power limitsunder different types of imbalance. Making use of such expressions, boththe feasibility regions of the DFIG and new rotor current and stator powerlimiters have been presented.Later on, a rigorous simulation study validates the designed controlalgorithms, as well as the aforementioned capability analysis.Finally, the report concludes by listing the main conclusions extractedand future research goals, followed by the bibliography.