BUSCADOR RECOLECTA

The application of droop control techniques without inner current control loops to doubly-fed induction generator (DFIG) based wind turbines does not allow to provide a stable response at all operating points in terms of rotational speed and active and reactive power. After modeling the system dynamics and analyzing the causes of instability, this paper proposes a control strategy that allows to stabilize the system response at all possible operating points. Simulation results performed in MATLAB/Simulink validate the proposed control strategy proving its effectiveness., This work was supported by the Spanish State Research Agency (AEI) under Grant PID2019-110956RB-I00/AEI/10.13039.

For the controller design and stability analysis of power electronic converters, the Bode stability
criterion and its subsequent revisions are the most practical tools. However, even though the control of the
power converter is usually implemented in a microprocessor, none of these methods is infallible when applied
to a discrete system. This article therefore proposes a new stability criterion, named the Discrete Generalized
Bode Criterion (DGBC). This method is based on the Nyquist criterion but developed from the open-loop
Bode diagram, evaluated also at 0 Hz and at the Nyquist frequency. The proposed criterion combines the
advantages of the Nyquist and Bode criteria, since it is always applicable and provides an interesting and
useful tool for the controller design process. The method is applied to design an active damping control of
an inverter with LCL filter, showing how the proposed criterion accurately predicts stability, in contrast to
the existing Bode criteria. The theoretical analysis is validated through experimental results performed with
a three-phase inverter and an LCL filter., This work was supported in part by the Agencia Estatal de Investigación (AEI) under Grant
PID2019-110956RB-I00/AEI/10.13039/501100011033, and in part by the Ingeteam Power Technology.

Due to the replacement of synchronous generators,
grid operators are currently demanding to control grid-connected
inverters in grid–forming mode to make them participate in the
maintenance of the grid. To carry this out, the traditional droop
controls based on the active and reactive powers are usually
adopted, achieving a satisfactory performance in normal
operation. Nevertheless, the power-frequency (P-ω) droop may
become transiently unstable under voltage dips. This is because of
the modification of the active power response caused by the
inverter current limitation together with the voltage reduction. To
enhance this, the power angle-frequency (δinv-ω) droop is
proposed, consisting in employing an estimation of the inverter
power angle as input to obtain the inverter frequency. The
proposed method provides the inverter with the same performance
as the P-ω droop in normal operation, while enhancing
considerably the transient stability margins under current
limitation. This is thanks to the higher variation of the inverter
power angle with the phase difference between the inverter and
the grid. Simulation results show the transient stability problems
of the P-ω droop as well as the superior performance of the
proposed δinv-ω droop control, which has also been verified by
means of HIL results., This work has been supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00 /AEI/ 10.13039/501100011033, and by the Public University of Navarre through a doctoral scholarship.

In this paper a dynamic model for the simulation of pressurized alkaline water electrolyzers is presented. The model has been developed following a multiphysics approach, integrating electrochemical, thermodynamic, heat transfer and gas evolution processes in order to faithfully reproduce the complete dynamical behavior of these systems. The model has been implemented on MATLAB/Simulink and validated through experimental data from a 1 Nm3/h commercial alkaline water electrolyzer. Validations have been performed under real scenarios where the electrolyzer is working with power profiles characteristic from renewable sources, wind and photovoltaic. The simulated results have been found to be consistent with the real measured values. This model has a great potential to predict the behavior of alkaline water electrolyzers coupled with renewable energy sources, making it a very useful tool for designing efficient green hydrogen production systems., This work has been supported by the Spanish State Research Agency (MCIN/AEI/10.13039/501100011033) under grants PID2019-111262RBI00, PID2019-110956RB-I00 and TED2021-132457B-I00 (also funded by the European Union NextGenerationEU/PRTR), by Ingeteam R&D Europe and by Ingeteam Power Technology.

Grid-forming power converters are controlled as voltage sources to regulate the grid voltage and frequency. These converters can increase power system strength if they impose a voltage waveform resilient to grid transients. For this reason, in this paper, we propose a deadbeat control strategy of the capacitor voltage for high power converters with LCL filter. To damp the LCL resonant poles, an active damping strategy is developed, based on a modification of the deadbeat control law. With this purpose, a notch filter is applied to the electrical variables allowing to emulate different damping resistances for the fundamental component and the harmonics. As a result, the active damping does not introduce tracking errors of the fundamental frequency component, while it provides damping to the filter resonance. The proposed strategy does not require knowledge of the grid impedance, an interesting feature in grid-connected power converters because the grid impedance is generally unknown. Experimental results validate the proposed strategy., This work was supported in part by Spanish State
Research Agency (AEI) under Grants PID2019-110956RB-I00 /AEI/ 10.13039
and TED2021-132604B-I00, in part by the Spanish Ministry of Universities
through “Programa Estatal de Promoción del Talento y su Empleabilidad en
I+D+i, Subprograma Estatal de Movilidad, del Plan Estatal de Investigación
Científica y Técnica y de Innovación 2017-2020” Program, in part by the
Advanced Center for Electrical and Electronics Engineering (AC3E) under Grant
ANID/FB0008, in part by Solar Energy Research Center (SERC) under Grant
ANID/FONDAP/15110019, and in part by the ANID/Fondecyt under Regular
Grant 1211826.

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.

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.

Reducing carbon emissions is essential to stop
climate change. The grid-share of renewable generation plants
is increasing, being wind and photovoltaic plants the most
common ones, whereas conventional plants are the only ones
that provide the necessary services to maintain the grid stability
and keep the generation-demand balance. However, with the
aim of achieving carbon-neutral generation, conventional plants
are being dismantled. This leads to the imminent need of
providing these services with renewable plants. Due to this
challenge, this proposal analyses a hybrid plant composed by
wind and photovoltaic generation with two types of storage,
lithium-ion batteries and a thermal storage system based on
volcanic stones. In order to compare both strategies, a technoeconomic methodology is explained that allows to optimally size
the plant, using the current prices of each technology. The most
cost-competitive proposal turns to be the hybrid plant with
thermal storage, composed by 623.9 MW installed power and
21.9 GWh of storage, which could replace a 100 MW, 24/7
conventional power plant, with an LCOHS (levelized cost of
hybrid system) of 118.38 €/MWh, providing identical grid
services and an equivalent inertia in a way committed with the
environment. This is in turn a zero-carbon emissions solution
perfectly matched to a second life plan for a conventional power
plant., This work has been supported by Siemens Gamesa Renewable Energy, by the Spanish State Research Agency (AEI/ 10.13039/501100011033) under grants PID2019-111262RB-I00 and PID2019-110956RB-I00, and by the Public University of Navarra under project ReBMS PJUPNA1904.

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.

Many thriving applications where isolation is
required, such as LED drivers, traction and EV fast charging,
implement LLC resonant converters, particularly when voltage
regulation is not required or an additional conversion stage is in
charge of it. The LLC converter can be operated under discontinuous conduction mode (DCM), due to its advantages such as
unregulated and sensorless operation, fixed switching frequency
and voltage gain, and zero-current switching (ZCS). However,
ZCS results in EMI and switching losses in the primary converter,
particularly for≥1200-V devices. Alternatively, zero-loss switching
(ZLS) can be accomplished by means of a proper design of the
LLC converter, overcoming the drawbacks of ZCS. The focus
of this paper is to perform an exhaustive research on the LLC
converter under DCM-ZLS: discontinuous conduction mode with
lossless switching in the primary and secondary sides. As a result of
this analysis, a set of design boundaries are deduced for parameters
such as the magnetizing inductance, the leakage inductance, and the
gate resistance. A comprehensive, step-by-step design methodology
is proposed and applied to a 18-kW, 200-kHz test bench. The
designed parameters are implemented in the converter and several
experiments are conducted, including a test at rated input voltage
and rated power (600 V, 18 kW). The conduction states studied
theoretically in the analysis of the LLC converter are identified in
the experimental results, and the operation of the test bench under
DCM-ZLS is verified., This work was supported by the Public University of Navarra under Grant PID2019-
110956RB-I00, funded by MCIN/AEI/10.13039/501100011033/, and in part
by the under a PhD Scholarship.

The increasing wind power penetration in electrical power systems results in a reduction of operative conventional power plants. These plants include synchronous generators directly connected to the grid. Facing a change in grid frequency, these generators inherently respond by varying their stored kinetic energy and their output power, which contributes to grid stability. Such a response is known as inertial response. Wind turbines (WTs) are mostly based on Doubly-Fed Induction Generator (DFIG) or Permanent Magnet Synchronous Generator (PMSG) machines. Their power electronics interface decouples the electromechanical behaviour of the generator from the power grid, making their inertial response null or insignificant. Therefore, in order not to weaken the frequency response of the power system, WTs must be able to react to frequency variations by changing their output power, i.e., emulating an inertial response. Common techniques for inertia emulation in WTs rely on pitch control and stored kinetic energy variation. This contribution proposes a strategy (applicable for both DFIG and PMSG) which uses the energy stored in a supercapacitor connected to the back-to-back converter DC link to emulate the inertial response. Its performance is compared by simulation with aforementioned common techniques, showing ability to remove certain limitations., This work has been supported by Siemens Gamesa Renewable Energy, by the Spanish State Research Agency (AEI/ 10.13039/501100011033) under grants PID2019-111262RB-I00 and PID2019-110956RB-I00, and by the Public University of Navarra under project ReBMS PJUPNA1904

This paper addresses the dual problem (electrical-mechanical) of DFIG-based wind turbines under voltage sags and proposes an optimized solution that, starting from a design aimed to mitigate the mechanical loads in the structural components of the wind turbine, provides the electrical capability to meet the most demanding grid codes., This work has been supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00 /AEI/ 10.13039/501100011033

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.

In this paper a dynamic model for the simulation of pressurized alkaline water electrolyzers is presented. The model has been developed following a multiphysics approach, integrating electrochemical, thermodynamic, heat transfer and gas evolution processes in order to faithfully reproduce the complete dynamical behavior of these systems. The model has been implemented on MATLAB/Simulink and validated through experimental data from a 1 Nm3h-1 commercial alkaline water electrolyzer, and the simulated results have been found to be consistent with the real measured values. This model has a great potential to predict the behavior of alkaline water electrolyzers coupled with renewable energy sources, making it a very useful tool for designing efficient green hydrogen production systems., This work has been supported by the Spanish State Research Agency. (AEI/10.13039/501100011033) under grants PID2019-111262RB-I00 and PID2019-110956RB-I00, and by Ingeteam R&D Europe.

This paper presents an asymmetrical firing angle modulation strategy for 12-pulse thyristor rectifiers aimed at supplying high-power electrolyzers, which allows to reduce the size of the passive filter and the static compensator (STATCOM) required to comply with grid harmonic regulations and achieve unity power factor. Usually, 12-pulse thyristor rectifiers follow a symmetric modulation strategy in which the same firing angle is applied to both 6-pulse bridges. In this case, large passive ac-side inductances are required to reduce grid current harmonics, which increase the reactive power consumption and thus the required STATCOM size. However, this paper demonstrates that by applying different firing angles to the two 6-pulse bridges it is possible to comply with the harmonic regulation limits using smaller filtering inductances and therefore reducing the STATCOM size. The methodology to find the optimal firing angle values that should be applied in order to minimize the filtering inductance and the STATCOM size for a given electrolyzer is explained. This strategy is validated by simulation, and results show that the required filtering inductance and the apparent power of the STATCOM can be effectively reduced by 62% and 31%, respectively, using this asymmetrical firing angle modulation., This work is part of the projects PID2019-110956RB-I00 and TED2021-132604B-I00, funded by MCIN/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR. It has also been supported by Ingeteam Power Technology.

In order to decide whether passive or active damping is required in a three-phase inverter, a previous step is to assess the intrinsic damping of the system. However, few works focus on modeling this damping for DC/AC operation. This paper proposes two models to reproduce the damping sources: the simulation model, used to validate the system in large-signal, and the small-signal model, which can be employed for the controller design. Both models have been validated by means of experimental results for a 1.64 MVA inverter., This work was supported by the Spanish State Research Agency (AEI) under grants PID2019-110956RB-I00/AEI/10.13039 and DPI-2016-80641-R, and by the Public University of Navarre through a doctoral scholarship.

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.

Many thriving applications where isolation is required, such as traction and EV fast charging, implement solid-state transformers (SST). Half-cycle discontinuous-conduction-mode series resonant converters (HC-DCM-SRC) are suitable for these applications. The focus of this paper is to perform a comprehensive approach to HC-DCM-SRC and provide straight-forward requirements in order to ensure zero-loss switching (ZLS) of semiconductors. In addition, these requirements can be expressed as design boundaries for the transformer. Finally, the paper shows that, due to ZLS, silicon devices may have larger power capability than silicon-carbide switches. Therefore, IGBTs can be used instead of SiC MOSFETs, resulting in a significant cost reduction of the converter., This work was supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00 and by the Public University of Navarre (UPNA) under a PhD scholarship

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)

Traditionally, to characterize the response of droop-controlled systems RMS models have been used. However, as it is demonstrated in this work, when droop control is applied to doubly-fed induction generators, RMS models do not allow to predict the system stability and dynamic response. Thus, in this article, a linearized small-signal model that overcomes the limitations of RMS models is presented. The proposed model is validated by simulation in MATLAB/Simulink demonstrating that it allows to accurately analyze the stability and dynamic response of the system under study. This model is an interesting tool that can be used in future works to design and adjust grid-forming controllers for doubly-fed induction generators., This work was supported by the Spanish State Research Agency (AEI) under Grant PID2019-110956RB-I00/AEI/10.13039.

Toroidal inductors are used in many industrial applications in which they are key components regarding cost and volume. In the inductor design process, it is paramount to accurately estimate its high-frequency winding loss. Finite-element analysis (FEA) software and analytical models can be used for this purpose. However, the former employs too much time and the latter lacks accuracy when applied to toroidal windings, leading to an overestimation that can exceed 200%. As a consequence, designers would benefit from a reliable method to calculate high-frequency loss in toroidal windings. This article proposes an analytical model that considers the 2-D characteristic of the magnetic field and the geometrical particularities of toroidal windings. Furthermore, it provides an easy-to-use method, which avoids the unaffordable computational cost of FEA software. Simulations and experimental measurements are carried out for seven toroidal power inductors, from 10 Hz up to 200 kHz. Three different well-known state-of-the-art analytical models are used for comparison purposes. The results obtained with the proposed model are in good agreement with those from FEA and the experiments. The proposed model shows a maximum deviation below 20% while the overestimation of the existing analytical methods reaches values from 93% to 226%., This work is part of the project PID2019-110956RB-I00, funded by MCIN/AEI/10.13039/501100011033/, and has also been supported by the Public University of Navarra (UPNA) under a Ph.D. scholarship.

In the case of photovoltaic (PV) inverters, an adequate dc voltage regulation is fundamental to maximize or limit the power injected into the grid. However, the traditional control requires a large dc capacitance to ensure stability in the whole operating range while the existing alternatives, despite achieving a stable control with a small capacitance, become too slow in the open-circuit area. This paper proposes two control methods to improve this performance. Firstly, a new voltage control with virtual impedance emulation is presented, showing that the response becomes faster in all operating points. Secondly, the control with impedance emulation is combined with a feed-forward compensation, further improving the dynamic response. Both methods are very simple to implement and their superior performance when using a small dc capacitance is verified by means of simulation results., This work was supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00/AEI/10.13039.

The rapid uptake of renewable energy sources is causing synchronous generators (SG) to be replaced by power electronic inverters meaning these inverters need to offer the characteristics traditionally associated with SG. As a result, it has been proposed that the inverters should be controlled in grid-forming mode in order to support the voltage of the microgrids. Given that these inverters are controlled as a voltage source, temporary events such as short-circuits or overloads could cause currents that are far higher than the rated current. As the semiconductors used in power electronics are highly sensitive to overcurrents, this paper proposes a dual voltage-current control that provides the grid-forming inverters with the capability to quickly limit the current under any overload or short-circuit condition. The proposed method has been validated through experimental tests in stand-alone mode., This work hasbeen supported by theSpanish State Research Agency (AEI) under grantPID2019-110956RB-I00/AEI/10.13039/501100011033, and by the Public University of Navarre through a doctoral scholarship.

The energy transition towards renewables must be accelerated to achieve climate targets. To do so, renewable power plants, such as wind power plants (WPPs) must replace conventional power plants (CPPs). Transmission System Operators require this replacement to be made without weakening the frequency response of power systems, so it must be ensured that WPPs match the response of CPPs to grid frequency variations. CPPs consist of grid-tied synchronous generators that inherently react to frequency variations by modifying their stored kinetic energy and their output power, thereby contributing to grid stability. Such response is known as inertial response. By contrast, wind turbines (WTs) are mostly based on either doubly-fed induction generators (DFIG) or permanent magnet synchronous generators (PMSG). Their power electronics interface decouples the electromechanical behavior of the generator from the power grid, leading to a negligible inertial response. Therefore, in order to replace CPPs with WPPs, WTs must be able to react to frequency variations by changing their output power, i.e., emulating an inertial response. Currently implemented inertia emulation strategies in WTs rely on pitch control and stored kinetic energy variation. This paper proposes an alternative strategy, using the energy stored in a supercapacitor directly connected to the back-to-back converter DC link to emulate inertia. Its performance is validated by means of simulation for both DFIG and PMSG. Compared to state-of-the-art techniques, it allows a more accurate emulation of grid-tied synchronous generators, favoring the replacement of these generators by WTs., This work was supported in part by Siemens Gamesa Renewable Energy, in part by the Spanish
State Research Agency, AEI/10.13039/501100011033 under Grants PID2019-
111262RB-I00 and PID2019-110956RB-I00, and in part by the Government
of Navarre under Grant 0011-1411-2022-000049 through EnVeNA Project.

The design of EV fast chargers faces a new challenge
due to the boost in the battery voltage of electric cars and heavyduty
electric vehicles. Two-stage converters, that consist of an
isolated dc-dc stage and an extra regulated dc-dc converter, are
attracting an increasing attention thanks to their outstanding
performance. The potential benefits of multilevel converters,
such as lower power losses and more compact filters, can
be incorporated to two-stage architectures at the expense of
simplicity due to the need of a voltage balancing method. In this
article, a novel dc-dc two-stage three-level (2S3L) architecture is
presented, which guarantees that the multilevel input dc voltages
are balanced without any specific balancing technique or extra
components. Moreover, it accomplishes lossless switching in the
isolated dc-dc stage, enabling a high efficiency. A 15 kW test
bench is built in order to experimentally verify the inherentlybalanced
voltages. The experimental tests demonstrate that the
dc-link voltages are inherently-balanced (no control needed) in
both transient and steady states, and that it is robust against
tolerances and faulty operation. The test bench is able to provide
a wide output voltage, from 200 to 900 V, and reaches a high
peak efficiency of 98.2% at rated power., This work is part of the projects PID2019-110956RB-I00 and TED2021-132604B-I00, funded by MCIN/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR. It was also been supported by the Public University of Navarre (UPNA) under a postdoctoral scholarship.

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.

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.

Currently, inverter-based stand-alone microgrids are gaining interest due to the advantages of obtaining energy
from renewable sources. To manage the operation, these microgrids include storage systems connected in par-
allel to the PCC through electronic inverters that are controlled as voltage sources in order to support the fre-
quency and voltage at the PCC. For the purpose of ensuring P and Q sharing among inverters and also the
synchronization stability of the microgrid, droop control is widely used, achieving a satisfactory performance in
normal operation. Nevertheless, in the presence of overloads or short-circuits, the inverters must limit the current
for self-protection, thereby modifying the performance of the system that then becomes prone to suffer transient
stability problems. In this paper, first the performance of the inverter-based stand-alone microgrids with the
conventional P-f and Iact-f droops is analyzed, obtaining the stability boundaries during current limitation. In
order to always ensure the synchronization stability of the system, this paper then proposes the G-f droop that
consists in employing the equivalent conductance seen by each inverter for its frequency droop control.
Furthermore, as this variable always correctly represents the inverter power angle, the system dynamics are not
affected by the operating conditions. The theoretical results have been validated by means of simulation and
Hardware-In-the-Loop results, showing the superior performance of the proposed G-f droop, This work has been supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00/AEI/ 10.13039/501100011033, and by the
Public University of Navarre through a PhD scholarship. Open access funding provided by Public University of Navarre.

Inductor filters, such as the ones implemented in
DC-DC buck-boost converters for electric vehicle chargers,
have a major impact on the converter weight, volume and cost.
Thus, their design is key in order to obtain an optimal design of
the whole converter. This paper proposes a design methodology
for powder core toroidal inductors, which is based on a holistic
approach of the design of the inductor, where losses due to highfrequency effects are computed by means of specific loss model
for toroidal windings, and saturation, geometrical and thermal
constraints are considered. The convenience of the design tool is
shown through an analysis over a wide variation of parameters,
including converter topology, parallelization, switching
frequency and inductance. The analysis demonstrates the
relevance of high-frequency effects on the inductor design, so
certain misconceptions can be avoided, such as that the inductor
volume monotonically decreases when the inductance value is
decreased or that paralleling inductors always results in more
compact designs. A design example is presented for a 15-kW,
three-level electric vehicle battery charger. The algorithm is
used to obtain an optimal design of the converter, including the
inductors and SiC MOSFET devices. Finally, an easy method to
obtain a commercial inductor design from the theoretical one
provided by the algorithm is presented., This work was supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-100 and by the Public University of Navarre (UPNA) under a PhD scholarship.

Selective harmonic mitigation pulsewidth modulation (SHMPWM) combined with model predictive control (MPC) is a promising approach for grid-connected power converters. SHMPWM can guarantee grid code compliance in steady state, e.g. grid harmonic injection, with a reduced output converter filter, while MPC improves dynamic response and allows grid code compliance in the event of grid transients. This paper presents a survey of the MPC strategies already published in the literature developed for their use with SHMPWM. The existing strategies fall into two categories: direct model predictive control with an implicit selective harmonic mitigation modulator, and direct model predictive control based on finite control set (FCS-MPC). One representative control strategy of each group is compared to each other and to the performance of classical proportional- integral (PI) controllers combined with SHMPWM. The goal is to identify the potential benefits of MPC for grid-connected power converters, and determine the main advantages and limitations of the two selected state-of-the-art control strategies. Their performance is assessed through Hardware-in-the-Loop (HIL) experimental results in terms of real-time implementation, harmonic content grid code compliance, dynamic response and performance under grid transients., This work is part of the projects PID2019-110956RB-I00 and TED2021-
132604B-I00, funded by MCIN/AEI/10.13039/501100011033 and by the European
Union NextGenerationEU/PRTR. It has also been partially supported
by Ingeteam Power Technology and the Public University of Navarre.

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.

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.

When grid-forming droop control strategies are implemented in grid-connected power converters, two control strategies are widely used: the single-loop and multiloop droop controls. However, only multiloop droop control strategies with inner control loops have been implemented in doubly fed induction generator (DFIG)-based wind turbines so far. This article proposes the application of a single-loop droop control strategy to a DFIG wind turbine, which has not been previously explored or implemented. As shown in the article, the application of the conventional droop control without inner control loops to DFIG-based wind power systems does not ensure a stable response. After modeling the system dynamics and evaluating its stability, two causes of instability have been identified: a resonance at the rotor electrical frequency relevant at high slips and a phase margin reduction at low slips. To solve these instability issues two control solutions are proposed: the emulation of a virtual resistor and a phase rotation. The proposed control strategy allows stabilizing the system and achieving a fast and damped dynamic response. The effectiveness of the proposed control strategy is validated by experimental results., This work was supported by the Spanish State Research Agency under Grant PID2019-110956RB-I00/AEI/10.13039.

In the case of photovoltaic (PV) inverters, an adequate input voltage regulation is fundamental to maximize or limit the power. When employing the traditional control, the input capacitance requires to be oversized in order to reduce the influence of the PV generator and achieve a stable control in the whole operating point. This paper proposes a voltage control method which permits reducing the capacitance by a factor of 5, thereby reducing the system cost. The control includes a feed-forward compensation of the PV voltage, making it possible to achieve a fast and stable control with a simple implementation. The proposed method is verified by simulation, showing the problems of the traditional control and the superior performance of the proposed control., This work was supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00/AEI/10.13039.

La crisis medioambiental actual está potenciando el aumento de la demanda de fuentes renovables y sistemas de almacenamiento. La conexión de estas unidades de generación, que se realiza a través de inversores de alta potencia para minimizar costes, está causando el reemplazo completo de los generadores síncronos, dando lugar a que aparezcan más frecuentemente microrredes basadas en inversores de alta potencia que pueden operar conectadas a red, aisladas o en ambos modos. En este escenario, los inversores electrónicos ya no pueden ser controlados en modo grid-following, ya que su funcionamiento como fuentes de corriente podría comprometer la estabilidad del sistema de potencia. Para evitar esto, se ha propuesto controlar los inversores en modo grid-forming, de forma que se comporten como fuentes de tensión, contribuyendo siempre al mantenimiento de la frecuencia y la tensión y siendo capaces de operar de forma aislada.
Esta tesis se centra en el desarrollo de estrategias de control grid-forming para inversores de alta potencia. El principal objetivo es diseñar e implementar técnicas que permitan garantizar la operación fiable de los inversores grid-forming en cualquier condición. Por ello, se han abordado los siguientes aspectos:
• Modelado del amortiguamiento intrínseco de inversores de alta potencia para diseñar los lazos de control de forma robusta.
• Desarrollo de un método que proporcione al control de tensión basado en un único lazo, generalmente implementado en inversores de alta potencia, capacidad para limitar sobrecorrientes.
• Análisis de estabilidad de una microrred basada en inversores cuando se utiliza el control droop como estrategia grid-forming.
• Diseño de un control droop de frecuencia que garantice la estabilidad transitoria de la microgrid en presencia de sobrecargas y cortocircuitos.
• Diseño de un control droop de frecuencia que mejore la estabilidad transitoria de los inversores grid-forming durante huecos de tensión., The present environmental crisis is driving the increase in demand for renewable sources and energy storage systems. The connection of these generation units, made through high-power inverters to minimize costs, is leading to the total replacement of synchronous generators, giving rise to the more frequent appearance of high-power inverter-based microgrids that can work tied to the main grid, islanded or in both operating modes. In this scenario, electronics inverters can no longer be controlled in grid-following mode since their performance as current sources would compromise the stability of the power systems. To avoid this, the concept of controlling the inverters in grid-forming mode has been proposed, in such a way that they perform as voltage sources, always contributing to the maintenance of frequency and voltage and having capability of working under stand-alone mode.
This thesis aims to develop grid-forming control strategies for high-power inverters. The main objective is to design and implement techniques that permit ensuring the reliable operation of grid-forming inverters at all possible conditions. Therefore, the following issues are addressed:
• Modeling of the inherent damping of high-power inverters to robustly design the control loops.
• Development of a method that provides the single-loop voltage control, generally implemented in high-power inverters, with an overcurrent limiting capability.
• Stability analysis of inverter-based microgrids when using the conventional droop control as grid-forming strategy.
• Design of a frequency droop control that guarantees the transient stability of inverter-based microgrids in stand-alone mode in the presence of overloads or short-circuits.
• Design of a frequency droop control that enhances the transient stability of grid-forming inverters in grid-connected mode under voltage dips., This thesis has been funded by the Public University of Navarre through a PhD scholarship. The company Ingeteam Power Technology has supported economically and technically this project under OTRI contracts 20200901027 and 2018901116. Moreover, the Spanish State Research Agency (AEI) and FEDER-UE have supported the investigation under grants PID2019-110956RB-I00 and DPI2016-80641-R., 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)

This contribution presents a technical analysis of the Lithium-ion batteries (LIBs) used in the WindSled project. In this project, an expedition has been carried out by means of a 0-emission vehicle that have covered more than 2500 kilometers in Antarctica Eastern Plateau pulled by kites. This adventure allowed the performance of 10 scientific experiments with a minimal disturbance of the polar environment. The required electricity for the survival and the scientific experimentation was delivered by flexible PV panels installed on the sled and commercial LIBs. The study performed in this contribution aims at the quantification of the LIBs degradation after the expedition. The results show a capacity fade of 5 % and an internal resistance increase of 30 %. Based on these results, it can be claimed that the LIBs used in the WindSled Project can successfully operate under -40°C. Moreover, these batteries can be used in upcoming expeditions, entailing an improvement from an economical and environmental point of view compared to primary batteries., Spanish State Research Agency (AEI) and FEDER-UE under grants DPI2016-80641-R, DPI2016-80642-R, PID2019-111262RB-I00 and PID2019-110956RB-I00, the Government of Navarra through research project 0011-1411-2018- 000029 GERA and the Public University of Navarre under project ReBMS PJUPNA1904.