Committee:
     PRESIDENT: MONTAÑA PUIG, JUAN
     SECRETARI: ABOMAILEK RUBIO, BASEL CARLOS
     VOCAL: URRESTY BETANCOURT, JULIO CÉSAR
Thesis abstract: This thesis contributes to the field of partial discharges in low-pressure environments, aiming to improve electrical wire interconnecting systems (EWIS) protection in aerospace applications. A series of experimental studies have been conducted to evaluate the potential of corona and surface discharges as indicators of electrical insulation degradation. To this end, the influence of variables such as pressure, pressure drop, frequency, and geometry on corona discharge behaviour has been examined. Furthermore, the performance of various sensors, including CMOS image sensors, photoelectric UV sensors, and acoustic cameras, in detecting corona discharges has been investigated and compared. Additionally, by employing RGB image processing techniques, this thesis presents a novel method for the quantification of corona discharges. This method allowed studying the correlation between the light intensity of electrical discharges and the dissipated electrical energy, with a particular focus on how pressure and frequency variation impacts on this relationship. The feasibility of utilizing corona and surface discharges as an indicator for the degradation of wire insulation has also been explored, providing foundational knowledge for the future development of electrical protection tools aimed at predictive maintenance. The findings from this research contribute to the advancement of predictive maintenance strategies, offering potential for early detection of insulation failures, thereby enhancing the safety and reliability of aerospace electrical systems.

Committee:
     PRESIDENT: MOLINA GAUDÓ, MARIA PILAR
     SECRETARI: GALCERAN ARELLANO, SAMUEL
     VOCAL: CREBIER, JOEN-CHRISTOPHE
Thesis abstract: The proliferation of batteries for local storage and electric vehicles (EVs) is rising steadily. The EV market’s prospects point to exponential growth in the coming years. This trend has also contributed to the promotion of local storage using second-life batteries. The emerging electrical landscape aims for an environmentally friendly energy system. However, this new paradigm brings along various technological challenges. Electronic power converters are crucial in managing battery energy in both charging and discharging directions, offering grid flexibility services. Compliance with international safety standards mandates galvanic isolation, particularly in high-power systems. Furthermore, power density is significant in the automotive industry, where space and weight constraints are significant factors. The same holds for dwellings, where land value continues to escalate, especially in urban areas. In this context, the Dual Active Bridge (DAB) converter emerges as a highly suitable power electronics-based component that fulfills all the mentioned requirements. This Isolated Bidirectional DC-DC converter (IBDC) is appealing due to its impressive power density, galvanic isolation, and bidirectional power flow capability. Firstly, this thesis explores the potential of IBDC and addresses a state of-the-art. The next step focuses on the DAB converter, establishing the unification of criteria for the different DAB modulations and obtaining generalized equations for any DAB converter. Moreover, the different parasitic elements of the DAB converter are extensively analyzed. On the one hand, this thesis presents a Triangular Current Modulation with Zero Current Switching (TCM-ZCS). The analysis highlights that six of eight semiconductors operate under ZCS conditions, concentrating turn-off switching losses in the same leg regardless of the power flow direction. This finding suggests the potential for utilizing more cost-effective semiconductor technologies for those operating under ZCS conditions. As a result, a hybrid Si-SiC DAB configuration is considered the preferred hardware option to take advantage of this proposal. On the other hand, the work proposes two DAB modulations that consider all three degrees of freedom to optimize conduction losses and turn-off losses across the entire load and voltage rate range. The optimization algorithms and equations are presented and normalized for clarity, with nomograms as graphical representations. One key contribution of this thesis is the development of equations for implementing minimum conduction loss modulation while guaranteeing Zero Voltage Switching (ZVS) constraints and maintaining stability. The proposed approximation offers precise optimal solutions across the entire operating range, allowing real-time evaluation with low execution time and ensuring a fast dynamic response for any DAB converter. Finally, the last chapter focuses on the positive impact of the transformer’s magnetization inductance on extending the ZVS operation region of the DAB converter. The focus is on understanding the consequences of this extension, particularly in terms of increasing the Root Mean Square (RMS) current and conduction losses. The chapter provides a manageable analytical approximation for real-time modulation implementation.

Committee:
     PRESIDENT: ANTA MARTINEZ, ADOLFO
     SECRETARI: CEBALLOS RECIO, SALVADOR
     VOCAL: D\'ARCO, SALVATORE
Thesis abstract: The massive installation of converter-based resources, such as wind and solar photovoltaic power plants and High Voltage Direct Current (HVDC) systems, is resulting in a profound transformation of power systems and their operation. In this context of modern power-electronics-dominated power systems, the converters’ control will play a key role in their performance and stability. This thesis focuses on the stability analysis of converter-dominated power systems, encompassing both small-signal and transient stability, aiming to understand the fundamental operation and limitations of such modern power systems.First, the two most extended Voltage Source Converter’s (VSC) control approaches, grid-following (GFOL) and grid-forming (GFOR), are compared via time domain simulations under different disturbances to identify the main characteristics and potential limitations of both control strategies. Regardless of the converter’s control selected, power system stability must always be ensured, both for small-signal and transient disturbances. In this work, thorough fundamental small-signal analysis are conducted to understand the principles governing network stability in converter-dominated power systems. For this purpose, a methodology to build accurate EMT linear state-space models of large power systems has been implemented. Using these models, a detailed analysis is developed to identify the stability limits of grid-following operation in an essential system, investigating the influence of the VSC’s controllers on the system stability, revealing the main mechanisms of interaction and identifying the minimum synchronous generation to ensure system stability. This work is later extended to a larger power system with several generators in presence of an HVDC link. To this end, an index-based methodology has been developed, which determines the steady-state, small-signal and transient analysis regions of stability.Additionally, previous publications have proved that grid-forming operation can improve power system stability for high penetrations of converters. In this thesis, a comprehensive study of frequency dynamics in modern low-inertia power systems is provided, revealing that the VSC’s dominance can be an opportunity to shape frequency dynamics, which can be designed to follow the desired response. However, considering the GFOR converter’s limitations can lead to synchronisation loss. Therefore, this work analyses the risk of synchronisation loss during frequency excursions for droop-based GFOR converters when active power limitations are considered. In addition, a control modification is proposed to extend the operation frequency range of GFOR converters.Finally, a transient stability study is performed for a future HVDC system that will connect the Balearic power system to the Spanish peninsula. In this realistic scenario, both GFOL and GFOR operation modes are studied, considering several contingencies in the system, such as faults in different locations and line disconnections.

Committee:
     PRESIDENT: ANAYA LARA, OLIMPO
     SECRETARI: PRIETO ARAUJO, EDUARDO
     VOCAL: GEBRAAD, PIETER MARINUS OTTO
Thesis abstract: The significant worldwide offshore potential has garnered considerable interest from energy companies and oil and gas plant owners in recent years. With significant advancements in bottom-fixed structures, floating technologies have emerged to unlock potential in deeper waters, where wind resource is abundant. Although most existing systems today are pilot projects or parks with up to 11 turbines, such as the case of Hywind Tampen, it is estimated that in the coming years, the large-scale deployment of floating wind farms will become a reality. This evolution towards extensive integration of offshore wind energy not only poses a significant challenge for the wind sector but also for transmission and energy storage sectors, where hydrogen has emerged as a key technology to provide flexibility to excess generated energy and to compete not only in the electricity market but also in hydrogen/gas markets; such technology could provide stationary storage.However, there is limited research on how these offshore energy hubs could be developed or how their optimal operation would be. Many questions remain unanswered, such as how to optimize floating wind farms to reduce their costs? Is their combination with other energy sources such as wave energy optimal? What is the optimal allocation of floating wind energy to hydrogen for its operation?This thesis aims to address the paradigm of offshore energy hubs dominated by floating wind and their potential optimization and control, both individually and in combination with other existing energy sources such as wave or hydrogen, to reduce the total cost of installations and in many cases to utilize the same infrastructure. To achieve this, hybrid floating wind-wave technologies that can be installed near the coast and reversible hydrogen systems through solid oxide cell technology are primarily employed, allowing for both hydrogen production and bidirectional energy injection into the grid from hydrogen.Although this thesis presents conservative case studies for potential implementation feasible by Transmission System Operators (TSOs), it does not imply that these configurations are optimal or that they would necessarily be the options implemented by TSOs in hypothetical case studies. This thesis primarily presents case studies with data from locations in northern Spain, but it also explores the paradigm of energy islands in Denmark. These are the two potential implementation sites studied in this thesis. The main conclusions of this thesis are summarized in that the dynamics of floating offshore energy require advanced control techniques that allow for long-term optimization of their operation to avoid power losses while extending their lifespan.

Committee:
     PRESIDENT: ARNDT, BERNHARD
     SECRETARI: DÍAZ GONZÁLEZ, FRANCISCO
     VOCAL: BOHN, GUNTHER
Thesis abstract: Lithium-ion cells are commonly used in a variety of applications today. Depending on the power and energy requirements of the application, several cells are interconnected. Due to intrinsic and extrinsic factors, the parameters of each cell differ. Characterizing and monitoring individual cell parameters during operation and aging is a critical challenge to optimize battery utilization. The main objective of this thesis is to develop and evaluate an accurate and robust characterization methodology that determines the capacity and associated state of health of each series-connected cell in a battery during operation. The proposed method, called Iterative Learning State of Health Estimation Method (ILEM-SOH), extends the conventional use of an active balancing system to obtain additional information about the capacity differences between individual cells, thus providing a new approach to characterize individual series-connected cells. In order to evaluate the proposed method, several cells are systematically aged in various tests.Consequently, a second important aspect of this work is the investigation of the aging behavior and the resulting cell-to-cell variation. First, the capacity and internal resistance of 200 new cells from the same production batch are investigated and the initial cell-to-cell variation of key parameters are calculated. From this pool of cells, 8 cells are used to initially study the aging behavior and the associated development of cell-to-cell parameter variation of series-connected cells. The test setup is designed to minimize extrinsic battery design influences on cell aging, which can usually not be excluded when using completely assembled batteries.The main contribution of this thesis is the development of a novel iterative learning state of health estimation method. This method is described in detail, including the topology and algorithm of the applied active balancing system. The proposed methodology aims to provide a reliable state of health estimation while reducing the reliance on individual cell modeling and thus reducing the potential for errors due to inaccurate models, which is a critical limitation of most conventional methods. A first evaluation under steady state conditions is performed using the 8 aged cells from the aging study.To evaluate the novel ILEM-SOH in depth, two long-term aging tests are performed, each with a 12s1p battery, under varying conditions. The performance as well as the limitations of the method and the balancing hardware that may arise due to changing aging mechanisms over the lifetime of the batteries are investigated. Additionally, the tests are used to investigate the effects of active balancing on the aging behavior of the connected cells.The proposed ILEM-SOH provides good accuracy for almost the entire duration of the two long-term tests, despite the significant differences in the state of health values between the cells. A comparison with a benchmark state of health estimation method also shows that using the balancing charges of an active balancing system provides a more robust estimation than relying solely on measurable cell voltages.The thesis concludes with a summary of all results and suggestions for further optimization and investigation.

Committee:
     PRESIDENT: BERGAS JANE, JOAN GABRIEL
     SECRETARI: CAPELLI, FRANCESCA
     VOCAL: KADECHKAR, AKASH
Thesis abstract: In recent years, there has been a significant increase in the growth of electricity demand. This electricity demand requires retrofitting of lines or exploiting the maximum capacity of existing power lines. In addition, substation connectors are the critical components of power systems since the failure of substation connectors can lead to serve power outages and significantly affect the power transmission efficiency. Therefore, it is vital to have real-time information on key elements of electrical systems, such as connectors and conductors, in order to ensure the reliability and efficiency of power transmission systems. To this end, the Smartconnector project aims to combine ICT (Information and Communication Technologies), IoT (Internet of Things) and data-driven technologies to estimate the state of health of substation connectors and take advantage of the maximum capacity of lines. Over the past few years, several research projects have been carried out to develop a smart high voltage connectors prototype, Smartconnector, in order to collect and wirelessly transmit information from power connectors in real time. Moreover, a prediction model has been proposed to utilize the collected data to separately predict the remaining useful life of connectors. However, improvements, experimental validation and field application are still needed to verify the reliability and feasibility of the Smartconnector prototype. In addition, field application of the IoT device is required for both connectors and conductors.This thesis is dedicated to the development, improvement, and experimental validation of the IoT prototype and the extension of its application to further increase the efficiency of power transmission systems. It is divided into two cores, which include the optimization of the Smartconnector prototype, and the extension of its application. This thesis improves the power management system, which helps to prolong the lifetime of the device. This improvement is based on thermal energy harvesting together with the energy balance strategy. Also, it develops statistical filtering algorithms for data processing. The proposed algorithms are finally implemented on the embedded system of the Smartconnector device, ensuring the accuracy of the continuous measurements. This thesis also focuses on the application of the Smartconnector for the dynamic monitoring of the line capacity considering weather conditions. In conclusion, this thesis aims to provide improvements and developments for the Smartconnector, as well as to open its application to other fields.

Committee:
     PRESIDENT: DE LEÓN, FRANCISCO
     SECRETARI: CANDELA GARCIA, JOSE IGNACIO
     VOCAL: GARCIA DE BURGOS, BELEN
Thesis abstract: The main objective of this thesis is to develop and validate simple but sufficiently accurate mathematical models for single-phase and three-phase transformers, by estimating their parameters from simple laboratory tests and field measurements, in particular, from the information obtained from the inrush current. The energizing process of a transformer results in the generation of a high inrush current due to the saturation of the transformer core. This current can cause several problems, such as protections tripping and consequently being out of service. Clearing faults in the transmission network can also lead to transformer saturation. A part of this work aims to develop methodologies for the reduction of these high inrush currents.In the literature, a great effort has been dedicated to the modeling, identification and analysis of electrical transformers. The representation of a transformer can be very complex due to the different types of core configurations and the large number of transformer parameters, as well as the fact that some of these parameters are nonlinear and even frequency dependent. These include core and coil configurations, self and mutual inductances among coils, dispersion fluxes, skin and proximity effects in coils, magnetic core saturation, hysteresis cycles, losses due to eddy currents, and capacitive effects.The materials commonly used in transformer cores, like ferromagnetic materials, exhibit non-linear magnetic permeability, and usually work slightly saturated. The behavior of a non-linear core is given by the relationship between the magnetic field and magnetic induction, known as the magnetization or saturation curve. There are different ways to model the nonlinear operation of a magnetic core, ranging from finite elements to single-valued functions, through especially complex models such as Jiles-Atherton or Preisach hysteretic models.This work is focused on low frequencies models (up to a few kHz), whose parameters reproduce in detail those situations in which the nonlinearity of the magnetic circuit significantly influences its dynamic behavior. For example, inrush current can be predicted, for which modeling or estimation of the residual flux is necessary. The adjustment of the parameters will be based on experimental measurements to which the developed adjustment algorithms will be applied. These measurements will be obtained both in specific laboratory tests and in transient connection records in transformers connected to the distribution network.Although articles on transformers have been published for more than seventy years, the high number of current publications on their modeling and on the problems derived from their non-linear behavior is an indication that the issue has not been resolved satisfactorily and which is still relevant.

Committee:
     PRESIDENT: BONGIORNO, MASSIMO
     SECRETARI: EGEA ÁLVAREZ, AGUSTÍ
     VOCAL: BEERTEN, JEF JAN J.
Thesis abstract: There is a global effort to reduce the emission of greenhouse gases to mitigate the consequences of climate change. The decarbonisation of the electricty sector is already underway to meet this goal. A new energy mix driven by solar and wind energy resources are set to dominate the global capacity additions. These resources are typically connected to the AC grid by means of power converters, identified as key enablers for the rapid expansion of renewable energies and the transmission system development.Modern power systems are growing in complexity, characterised by the dominance of power electronics such as VSC technology. However, VSCs also bring new challenges due to the oscillatory phenomena caused by the interaction between their control and the grid. These oscillations can lead to instabilities, specially in poorly damped networks as reported in the literature. The knowledge of VSCs control structures is not always known due to the intellectual property protection over the controllers development. Studying their dynamic behaviour and possible impact over the network is becoming a more challenging task.It is desirable to have test facilities closer to reality to study these complex systems; however, it may not be feasible due to the high cost and risks involved. This can be addressed by performing offline computer•based EMT simulations of white and black•box models. Nevertheless, this can be a long and arduous task without adequate methods which enable to evaluate the system stability and identify the major players that might affect it. The development of frequency domain methods are currently of great interest as they can use frequency impedance curves obtained from impedance measurements of black•box models or real equipment. However, this approach alone might not provide all the necessary insights to understand the power system's complexity. This can be complemented with real•time simulations, where simulation and physical hardware are combined. One advantage is that hardware which cannot be included in a laboratory setup for practical limitations can be simulated instead.This document addresses these problems, making an emphasis on the development of methodologies which adapt to the current context and simplify the study the stability of modern power systems, both for offline and real•time studies. The methodologies developed are evaluated in the frequency domain and allow to study the oscillatory phenomena in poorly damped systems

Committee:
     PRESIDENT: ARANGUREN FINO, DANIEL
     SECRETARI: RIBA RUIZ, JORDI ROGER
     VOCAL: FABRÓ TAPIA, FERRAN
Thesis abstract: This research focuses on the characterization of optical emissions generated by various lightning processes, with the aim of improving the interpretation of optical detections made from space. Observations were conducted at the natural lightning laboratory in Barrancabermeja, Colombia, where a set of instruments has been deployed to complement space-based measurements. During the characterization, specific patterns in the optical emissions associated with different lightning processes were identified. For instance, some discharges exhibit oscillations in their optical emissions, possibly related to the initial vertical propagation of the leader within the storm cloud and its subsequent transition to horizontal propagation, a phenomenon observed in both intracloud and cloud-to-ground lightning. Additionally, fast-propagating lightning (less than 6 ms) was detected, generating a characteristic waveform in the optical emission, accompanied by VHF emissions that are captured by localization networks, showing a high correlation with the optical waveform. In the case of leaders propagating downward in cloud-to-ground lightning, their optical emissions overlap with the light detected along the channel, making it difficult to observe optical variations that directly correlate with their propagation.Furthermore, a detailed analysis of the waveforms of optical emissions of lightning with negative continuous current observed from ground was conducted to evaluate the detection capability of these currents by the Geostationary Lightning Mapper (GLM). The comparison of ground-based signals with those obtained from space provided a better understanding of the differences in lightning detections from space, particularly regarding the reduced duration of the reported continuous currents.Additionally, ground support was provided for the ALOFT project, a suborbital measurement campaign focused on investigating high-energy emissions from storm clouds. The ER-2 aircraft, flying at an altitude of 60,000 feet, was equipped with eight instruments, including an array of 25 photometers and two high-energy detectors. As part of the support for ALOFT measurements, an LMA station, a high-energy sensor triggered by VHF signals, a magnetic field LF antenna, and an interferometer were installed on the island of San Andrés, Colombia