BUSCADOR RECOLECTA

Doubly-fed induction generator (DFIG) wind turbines connected to capacitive series-compensated transmission lines are prone to exhibit oscillatory behavior. The phenomena is called sub-synchronous resonances (SSRs), as these oscillations occur at frequencies below the fundamental component. This paper first develops a modeling methodology for DFIG wind turbines, based on impedance matrices, that is applied to model a real wind farm where SSRs were reported. The stability analysis performed shows how the interaction between the grid-side converter and the rotor-side converter contribute to the instability of DFIG wind energy conversion systems connected to series compensated grids. With this model, we propose a simple sub-synchronous resonance control strategy based on an orthogonal proportional action applied to the rotor currents, and a variable gain in the PI controller adjusted as a function of the DFIG rotational speed. This control strategy depends only on the rotor currents, which are local and already measured variables in any DFIG wind turbine, and is implemented in the rotor side converter, so it does not imply an additional cost at wind farm or wind turbine level and can be applied to any DFIG wind energy conversion system (WECS). Additionally, it proves to be robust for any line impedance series compensation level, and it does not need real-time information concerning the grid at which the wind turbine is connected, or its parameters. A real case study is considered, where the sub-synchronous resonance damping strategy presented in this work is able to stabilize the system for every possible line impedance compensation level., This work was supported by the Agencia Estatal de Investigacion (AEI) (Spanish State Research Agency) under Grant PID2019-110956RB-I00/AEI/10.13039 and Grant DPI-2016-80641-R.

Several active damping strategies have been proposed in the literature for grid-connected converters with LCL filter but there are not specific strategies for DFIG wind turbines. In this system, there is an interaction between the two converters of the back-to-back conversion structure, which must be properly modeled in order to design effective damping strategies for the LCL filter resonant poles. This paper proposes a robust active damping strategy for DFIG wind turbines with LCL filter that considers the special features of this system. In this technique the filter capacitor current is fed back through a lag compensator that adjusts the delay of the feedback loop to emulate a virtual impedance that has dominant resistive behavior in the range of possible resonance frequencies. It is shown that a similar damping of the LCL filter resonance is achieved when the strategy isimplemented in either of the two converters., This work was supported by the Spanish State Research Agency (AEI) under grants PID2019-110956RB-I00 /AEI/10.13039 and DPI-2016-80641-R.

Doubly-fed induction generator (DFIG) wind turbines connected to series compensated grids are prone to sub-synchronous resonance (SSR) instability. In this paper we develop a model to analyze SSRs and propose a damping strategy based on the stator voltage feedback that is implemented in the rotor-side converter (RSC). The control strategy is based on local variables that are already measured, so it is applicable to any new or existing DFIG wind turbine. Simulation results performed fora real wind farm where sub-synchronous resonances were reported validate the proposed damping strategy., This work was supported by the Spanish State Research Agency (AEI) under grants PID2019-110956RB-I00 /AEI/ 10.13039 and DPI-2016-80641-R.

Efficiency and power density are important parameters in the design of power electronics converters. In many applications, low-frequency transformers are being substituted for medium-frequency and high-frequency transformers in order to reduce the volume and therefore the cost of the transformer. However, preventing their saturation is a complex task. This paper studies the medium-frequency transformers' flux balancing in a novel three-phase topology for cascaded conversion structures.Based on the modulation technique of the converter, a method to directly measure the magnetizing current of the medium-frequency transformers is proposed in this paper. A control loop to regulate the dc value of the magnetizing current is also designed and developed. Simulation results validate the correct operation of the control loop, which prevents the transformer saturation., This work was supported by the Spanish State Research Agency (AEI) under grants PID2019-110956RB-I00 /AEI/ 10.13039/501100011033 and DPI-2016-80641-R, and by the Public University of Navarre through a doctoral scolarship.

El contexto energético actual se caracteriza por un fuerte crecimiento de las energías renovables, motivado por el notable descenso de coste que han experimentado en los últimos años, especialmente en el caso de la energía solar fotovoltaica. Las plantas fotovoltaicas de gran tamaño son las que presentan un menor coste, utilizándose tradicionalmente en ellas la configuración de inversor central. Sin embargo, esta configuración, por sus propias características, tiene un potencial limitado en cuanto a futuras reducciones de costes. En esta tesis, se propone una nueva topología de conversión con la configuración de inversor en cascada que, entre otras ventajas, permite la reducción del cableado de la planta fotovoltaica. Además, gracias a su funcionamiento secuencial, reduce el número de etapas de conversión en comparación con las soluciones investigadas hasta la fecha. Asimismo, su modulación hace que se realicen conmutaciones sin pérdidas en todas sus etapas de conversión.
Las principales líneas de investigación que se abordan en esta tesis son:
- Análisis de la configuración de inversor central en una gran planta fotovoltaica;
- Estudio de las ventajas de la configuración de inversor en cascada y realización de un estado del arte de esta configuración;
- Propuesta de la nueva topología de conversión;
- Análisis del funcionamiento y desarrollo del control de la topología propuesta;
- Validación experimental de la topología propuesta;
- Estudio de la estabilidad de un sistema de control: formulación de un nuevo criterio de estabilidad y aplicación al diseño de los lazos de control de la topología propuesta;
- Dimensionado óptimo de un sistema fotovoltaico con la topología propuesta y evaluación de su competitividad., Esta tesis ha sido subvencionada por el programa de Formación de Personal Investigador (FPI) de la Universidad Pública de Navarra, a través de una beca predoctoral. Además, se ha recibido financiación de la Agencia Española de Investigación (AEI) y el Fondo Europeo de Desarrollo Regional (FEDER-UE) a través de los proyectos de investigación y desarrollo DPI2016-80641-R y PID2019-110956RB-I00., Programa de Doctorado en Tecnologías de las Comunicaciones, Bioingeniería y de las Energías Renovables (RD 99/2011), Bioingeniaritzako eta Komunikazioen eta Energia Berriztagarrien Teknologietako Doktoretza Programa (ED 99/2011)

Photovoltaic (PV) conversion systems are in continuous development due to their increasingly competitive prices. The traditional configuration of large-scale PV plants is based on high-power central inverters, which have reduced their cost by increasing their power rating. However, this cost reduction is expected to saturate in the near future, mainly due to an increase in the cost of the dc wiring. Cascaded conversion systems have appeared as potential solutions to continue reducing the PV plant cost. They consist of several conversion units whose ac outputs are connected in series. This enables the power-rating reduction of each individual conversion unit, while maintaining the power rating of the conversion structure. Thus, the conversion units are placed closer to the PV panels, reducing the dc wiring cost. In this paper, a novel three-phase topology for medium-voltage cascaded conversion systems is presented. The proposed topology is formed of several conversion units, each one with a reduced number of conversion stages, namely, dc/ac, medium-frequency isolation and ac/ac. Moreover, thanks to its sequential operation and modulation technique, zero-voltage switching and zero-current switching are achieved in all conversion stages. In this way, with respect to the configuration with central inverters, the proposed topology has the advantages of cascaded conversion systems. In comparison to previously investigated cascaded topologies, the proposed topology also presents promising characteristics, representing a potential cost reduction and efficiency increase. An experimental validation of the topology is carried out in a laboratory prototype consisting of three conversion units., This work was supported in part by the Spanish State Research Agency (AEI) under Grant PID2019-110956RB-I00 /AEI/10.13039/501100011033, and in part by the Public University of Navarre through a Ph.D. Scholarship.

Inductors are cornerstone components in power electronics converters. Since winding loss is the dominant loss mechanism in these components, its accurate measurement is fundamental for the validation of the inductor's operation and design. The techniques for the winding resistance R_{w} measurement in power inductors can be classified into two groups, indirect and direct. Both techniques use coupled inductors to separate winding and core power losses. If coupled inductors with non-zero winding mutual resistances R_{w,m} are used, invalid results are obtained with these techniques. Understanding the meaning of R_{w,m} in coupled inductors is complex. In this paper, the impact of R_{w,m} on the inductor R_{w} measurement techniques is demonstrated and practical guidelines for the design of the zero R_{w,m} coupled inductors are given. Particularly, the location of the auxiliary winding for the direct technique is investigated. In order to compare the R_{w} measurement techniques and to validate the coupled inductor's R_{w,m} impact, two different inductors are built and tested. The results are compared with the values for R_{w} calculated by FEA simulation. It is found that only the direct technique with an auxiliary winding carefully designed and located following the guidelines given in this paper makes the accurate measurement of R_{w} in power inductors possible., This work was supported in part by the Spanish State Research Agency (AEI) and the FEDER-UE under Grant PID2019-110956RB-I00/AEI/10.13039/501100011033, and in part by the Ingeteam Power Technology.

Doubly fed induction generators (DFIGs) with an LCL filter are widely used for wind power generation. In these energy conversion systems, there is an interaction between the grid-side converter (GSC) and the rotor-side converter (RSC) control loops, the generator and the LCL filter that must be properly modeled. Such interaction between the GSC and the RSC proves to have a significant influence on the stability. Several active damping (AD) methods for grid-connected converters with an LCL filter have been proposed, nevertheless, the application of these techniques to a DFIG wind turbine is not straightforward, as revealed in this article. To achieve a robust damping irrespective of the grid inductance, this article proposes an AD strategy based on the capacitor current feedback and the adjustment of the control delays to emulate a virtual impedance, in parallel with the filter capacitor, with a dominant resistive component in the range of possible resonance frequencies. This work also proves that, by applying the AD strategy in both converters simultaneously, the damping of the system resonant poles is maximized when a specific value of the grid inductance is considered. Experimental results show the interaction between the GSC and the RSC and validate the proposed AD strategy. © 1986-2012 IEEE., This work was supported by the Spanish State Research Agency (AEI) under Grant PID2019-110956RB-I00/AEI/10.13039.

Toroidal inductors are present in many different industrial applications, thus, still receive researchers' attention. AC winding loss in these inductors have become a major issue in the design process, since switching frequency is being continuously increased in power electronic converters. Finite element analysis software or analytical models such as Dowell's are the main existing alternatives for their calculation. However, the first one employs too much time if different designs are to be evaluated and the second one lacks accuracy when applied to toroidal inductor windings. Looking for an alternative that overcomes these drawbacks, this paper proposes an accurate, easy-to-use analytical model, specifically formulated for calculating high-frequency winding loss in round-wire toroidal inductors., This work was supported by the Spanish State Research Agency (AEI) under grants PID2019-110956RB-I00/AEI/10.13039 and DPI-2016-80641-R.

In three-phase grid-connected PV inverters, regulating the input voltage is a fundamental requirement. In order to reduce the influence of the PV non-linear behavior and ensure stability in the whole operating range, the input capacitance is currently oversized. This paper reveals the important effect of the inner current loop in the voltage stability and proposes to use a Proportional (P) controller instead of a PI controller. If tuned following the guidelines provided in this paper, the P controller makes it possible to design a stable voltage loop without increasing the input capacitance, thus reducing the converter cost., This work was supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00/AEI/10.13039 and grant DPI-201680641-R.