Committee:
     PRESIDENT: LASHAB, ABDEREZAK
     SECRETARI: MONTESINOS MIRACLE, DANIEL
     VOCAL: TAN, SEN
Thesis abstract: Global electricity generation increasingly incorporates Distributed Generation (DG) resources, such as solar and wind, into distribution systems (DS), offering benefits like improved reliability, power quality, rapid integration, and reduced payback periods, while lowering greenhouse gas emissions. However, their integration presents challenges, including overvoltage, voltage fluctuations, and imbalances caused by improper synchronization with the grid. DGs alter short-circuit currents, necessitating updates to protection relay settings. As DG penetration rises, distribution networks become more complex, requiring advanced protection systems to handle bidirectional power flows, which challenge existing schemes. Inverter-based DGs, such as solar and wind, generate lower fault currents due to inverter power electronics, diminishing the effectiveness of traditional fault detection methods, leading to potential protection blinding or false tripping. These challenges highlight the need for precise fault detection, accurate localization, and rapid protective responses. Disconnecting DGs during faults is increasingly undesirable, requiring innovative protection schemes to minimize unnecessary disconnections and address limitations like fault resistance, pre-fault load conditions, and noise interference. Traditional fault location techniques, often computationally intensive, struggle with accuracy, prolonging restoration times and increasing downtime, further emphasizing the need for advanced fault protection systems. Total Harmonic Distortion (THD) analysis has proven effective for fault detection in systems with complex harmonic profiles caused by DG integration. Faults induce increased harmonic distortion, making THD monitoring a valuable indicator. Despite its promise, protection systems for grids with high DG penetration, especially those using inverter-based DGs, are underexplored, and existing protection algorithms rarely incorporate THD. To address this, three novel protection systems utilizing grid voltage harmonic content for fault detection and localization in medium-voltage (MV) DS are proposed. The first system combines THD measurements with voltage amplitude and zero-sequence components using a finite state machine (FSM)-based algorithm. It focuses on third harmonic (triple-n) components, unique to inverter neutral points and unaffected by other grid harmonics. Fault-induced voltage dips excite harmonic components, amplifying THD, making it an effective fault indicator. THD is calculated using the Multiple Second Order Generalized Integrator (MSOGI) method. However, this system relies on communication channels, which could fail, limiting its robustness. To mitigate this, a two-layered protection system is introduced. The first layer employs the SOGI-FLL grid monitoring technique, optimizing computational efficiency by reducing the number of required SOGIs while maintaining accurate THD calculations. Fault detection is achieved by filtering the THD signal and comparing pre-fault and fault-time averages, with significant deviations indicating faults. The second layer implements a communication-less fault localization algorithm based on positive and negative voltage sequence components to determine fault symmetry. This approach enables each protection device (PD) to operate independently, ensuring reliable fault localization even without communication, albeit with slightly slower detection times compared to communication-based methods. To enhance overall reliability, especially during communication failures, a third system, priority system, is proposed. It integrates the two-layered protection, with the first layer as the primary fault detection and communication-based trip signal initiator. If communication fails, the second layer provides backup protection by analyzing voltage sequence components locally. The effectiveness of these systems is validated against different protection method under various conditions.

Committee:
     PRESIDENT: BEERTEN, JEF JAN J.
     SECRETARI: GRIÑO CUBERO, ROBERTO
     VOCAL: RAULT, PIERRE
Thesis abstract: High Voltage Direct Current (HVDC) offers clear advantages for long distance electrical transmission. This is why its implementation is rising in the interconnection of remote renewable energy and interconnection between islanded Alternating Current (AC) grids, as is the case in Europe. The majority of these systems are currently developed as point-to-point systems. However, Multiterminal High Voltage Direct Current (MT-HVDC) systems will be built shortly. The critical elements in these systems are the Interconnecting Power Converters (IPCs) that enable the transformation between AC and DC. These converters mainly use the Modular Multilevel Converter (MMC) technology, which can be controlled by leveraging the energy stored inside its cells to achieve its control objectives.The control strategy of these converters is diverse, and different control objectives can be implemented. Two subgroups exist: grid-forming (GFM) and grid-following (GFL) strategies. The main aim of the first one is to form the grid (AC and/or DC), while the second assumes a stable grid (AC and/or DC) with which to interchange power. The control strategy applied to be grid-forming or grid-following is not unique, and different control architectures are already proposed in the literature. Thus, studying the effect of combining these control architectures on the same system’s stability and performance is crucial.The main objective is to solve the question of how to properly assign and combine these roles in the emerging MT-HVDC grids to ensure stability and proper system response.Firstly, the stability of point-to-point links is studied. This includes a study on the interaction between AC and DC sides when AC-GFM and AC-GFL converters are in the same point-to-point link. It is seen how the different levels of energy buffering capability affect the level of interactions between AC and DC sides. Then, the extension of point-to-point links to MT-HVDC systems is studied. A crucial role in this case is the DC-GFM control. Thus, the stability of HVDC systems is studied when two different DC-GFM control strategies are applied.Secondly, MT-HVDC systems are studied. These systems introduce additional complexities. A key challenge in these systems is the optimal assignment of control roles to IPCs. One of the studies presents a novel methodology to optimize the control role assignment of IPCs by considering both small-signal stability and control performance. This methodology accounts for multiple power flow scenarios and analyzes the steady-state deviations and small-signal stability following selected events. The results of this study highlight the importance of control role assignment in enhancing system stability and performance.From the results extracted from the previously mentioned study, a dynamic control role assignment is proposed to solve the problems previously identified. Traditionally, the control role of IPCs has been static, with little consideration given to dynamically changing the control mode of these converters in real-time operations. However, this study advocates for a dynamic approach to IPC control role assignment, suggesting that allowing transmission system operators to adjust the control roles in response to varying conditions can lead to improved system performance, enhanced stability, and greater operational flexibility.Finally, the research also addresses the advantages of dual-port GFM control in hybrid AC/DC systems. Unlike traditional GFM and GFL controls, which must be carefully assigned to individual IPC terminals, dual-port GFM control imposes a stable voltage on both AC and DC terminals, making it suitable for deployment across all IPCs regardless of network configuration. The findings of this study indicate that dual-port GFM control offers several significant benefits, making it a promising solution for enhancing HVDC systems’ stability and dynamic performance.

Committee:
     PRESIDENT: URRESTY BETANCOURT, JULIO CÉSAR
     SECRETARI: BERGAS JANE, JOAN GABRIEL
     VOCAL: ABOMAILEK RUBIO, BASEL CARLOS
Thesis abstract: This thesis contributes to the field of design and optimization of magnetic core devices, specifically transformers for power electronics. A calculation software has been developed for the design of transformers, based on an analytical-parametric geometric, electromagnetic, thermal model of the main components of a transformer: the core and windings. Experimental studies have been carried out to analyze the magnetic losses in transformer cores, evaluating the contribution of the different constructive parts as well as the impact of voltage harmonics on the magnetic losses of the core. In addition, hysteresis and other loss models have been developed to improve the accuracy of core loss estimation. For the coils, electromagnetic (skin and proximity) and thermal effects have been modelled. Through experimental studies and simulations, the interaction between the temperature and frequency-dependent effects on winding resistance have been analyzed. These studies have demonstrated that winding losses can be, in fact, analytically and accurately calculated. Once a detailed design has been obtained, the core and winding losses are entered into a thermal model to predict the operating temperature of the transformer. After concluding the analytical-parametric model, a real-world industrial applications-oriented optimization system has been implemented. This system allows both single-objective and multi-objective optimization on any desired transformer parameter, always ensuring compliance with the technical and operational needed requirements through constraints. The optimization system has been validated using real cost-optimized prototypes and the results obtained have been compared with experimental tests, demonstrating the accuracy and reliability of the calculation system developed, and allowing its applicability in actual industrial manufacturing processes.

Committee:
     PRESIDENT: COLAS, FRÉDÉRIC
     SECRETARI: ROCABERT DELGADO, JOAN
     VOCAL: FREYTES, JULIAN
Thesis abstract: Modern power systems are increasingly integrating a larger share of renewable energy resources to mitigate climate change. These renewable resources are gradually replacing conventional power plants, which employ synchronous machines for power generation. Synchronous generators behave as voltage sources governed by their electromechanical dynamics. Moreover, they possess intrinsic inertia, which stabilizes the network frequency. In contrast, renewable energy sources primarily use power electronics converters to connect to the electrical grid. The behavior of power electronics is dominated by their implemented control strategies, as their physical dynamics are substantially faster.Currently, the most widely used control approach for converters is grid‐following control. This strategy drives converters to behave as current sources. However, the AC network requires voltage sources for operation and stability. Thus, the integration of grid-following converters displaces voltage-source generators with current-source devices, potentially affecting system stability and dynamics.To improve overall network stability, grid‐forming control was introduced. Grid-forming converters operate as voltage sources and may incorporate virtual inertia. Virtual inertia is emulated because power electronics lack rotating mass. Moreover, by behaving as voltage sources, grid-forming converters contribute to system stability.The present thesis provides a fundamental assessment of both grid‐following and grid‐forming control strategies. First, the internal controls of grid-following converters are analyzed, and their key components are defined. Then, different implementations of the internal control strategies of grid-forming converters (synchronization control and voltage control) are explored, and the most suitable configuration is assessed in different chapters. Specifically, a tuning methodology for the voltage control is presented, which accounts for the power electronics' inherent limitations. This methodology is employed to identify the optimal control structure based on the specific configuration of the implemented AC connection filter. Later, the industry-class synchronization controls are examined, focusing on their behavior when employing a frequency estimator. Afterwards, the limitations of renewable resources when operating the power electronics in grid‐forming mode are investigated. To allow renewable energy sources to operate in grid-forming mode despite their limitations, this thesis introduces a Resource‐Aware Grid‐Forming (RAGFM) control. RAGFM strategy aims to maximize the frequency response, considering the resource limitations. The proposed control is validated through various analytical studies and simulations. RAGFM control is also simulated for a single-stage photovoltaic power plant. This topology exhibits a non-linear relationship between the DC-link voltage and power generation, which introduces significant control challenges. Thus, RAGFM operation is validated under this specific operating condition. Finally, the proposed control structure is further validated through experimental results.This work contributes to enhancing the integration of renewable resources through grid-forming control, supporting the transition to a more sustainable energy future.

Committee:
     PRESIDENT: DEBUSSCHÈRE, VINCENT
     SECRETARI: CHEAH MAÑÉ, MARC
     VOCAL: KAZMI, HUSSAIN SYED
Thesis abstract: Recent technological developments in renewable energy have enabled a shift in the energy generation capacity closer to the consumption. This evolution has led to a decentralisation process that is required for the coordination of generation and demand in electric power systems. Consequently, the energy sector is transitioning towards a more decentralised control due to these prosumers and active consumers, who cooperate for the management and control of storage systems and flexible demand. A resulting framework attempting to solve the challenges associated with this decentralisation is that of local energy communities (LECs) representing local, self-organising entities that operate autonomously or semi-autonomously within an electricity grid. At the same time, digitalisation has rendered machine learning a key tool for improving processes in several sectors, as in the case of electrical power systems. This thesis starts by analysing LECs and their integration to smart distribution grids. Ultimately, developing a definition for these entities. Then, an analysis on machine learning is assessed to generate an understanding of its capabilities, aiming to discover a relationship between the operation of LECs and existing machine learning algorithms. Finally, with this knowledge, a framework is developed to forecast congestions lines with machine learning in smart distribution with renewable and local energy communities presence, showing the capabilities machine learning can have in complex power systems.

Committee:
     PRESIDENT: RAISON, BERTRAND
     SECRETARI: HEREDERO PERIS, DANIEL
     VOCAL: MELENDEZ FRIGOLA, JOAQUIM
Thesis abstract: Driven by increasingly strict climate goals, the need for modernization of the electric power system has intensified in recent years. The transition towards a distributed and decarbonized smart energy system requires modernizing power grids to accommodate the widespread adoption of renewable energy sources (RES), electric vehicles (EVs), and distributed energy resources (DERs). One key factor to enable this is the advancements in power electronics and their widespread deployment. The Power Router (PR) technology is crucial for this transition, as it facilitates flexible and efficient management of electricity as well as integration between AC and DC grids. It is a device composed of multiple ports that provides a seamless interface of different elements of a power grid by controlling the power between ports.In the first half of this thesis, different kinds of PR concepts are investigated and a novel grid concept based on PRs has been defined, named Power Router Grid (PRG). The PR concept adopted consists of coupling a set of voltage source converters to a common DC bus, in which each converter functions as a different input or output port. The converter model design used is the Modular-Multilevel-Converter (MMC) and is adaptable for PRs with any number of ports and any power level. Then, the theory behind the PRG is presented. First, a set of rules is proposed to ensure the correct configuration and operation of the PRG. Secondly, each PR role is defined based on their tasks within the PRG. Finally, in combination with graph theory methods, a new concept is introduced called Slack Tree (ST). The ST is the backbone that regulates and ensures power balance and the feasibility of the PRG operation, and is a connection path between all ports operating as a slack element.In the second half, all of these novelty concepts behind the PRG are combined with optimal power flow (OPF) models, convexity techniques and converter loss modelling. The goal is to create a Python-based convex OPF formulation suitable for any hybrid AC-DC network Based on PRs. The mathematical formulation is based on a second-order cone relaxation of the traditional power flows equations applied to radial networks. The developed PRG-OPF however, due to the decoupling characteristics of the PRs, is demonstrated to be suitable for any network meshed through PRs. In the last part of thesis, this formulation is further expanded to include the effects of converter losses. The loss model developed is defined as a set of linear constraints that are scalable and easy to implement inside an OPF. Additionally, other constraints regarding DC lines and AC grid integration are developed and integrated. The proposed OPF formulation is loss-aware and utilizes the full potential of PRs to integrate different systems.Throughout this doctoral thesis, seven case studies are presented in order to demonstrate the validity of the proposed concepts. Specifically, the power flow analysis show the viability of the highly-flexible PRG design. Different sensitivity analysis are conducted in order to assess the impact of converter losses and also the ST selection in the optimal operation of the PRG. Among the key remarks, the results demonstrate that the choice of ST does not significantly affect line losses. It is also shown that, despite the added converter losses, the PRG is more efficient than a traditional network in most scenarios, particularly in the presence of loads with low power factor.

Committee:
     PRESIDENT: ESTEVE PUJOL, JOAN
     SECRETARI: ALVAREZ FLOREZ, JESUS ANDRES
     VOCAL: FAUNDEZ ZANUY, MARCOS
Thesis abstract: In this thesis, important ideas like electricity, magnetism, electric motors, special relativity, and history are explained. This is because, on the one hand, the historical memory of Francesc Aragó, a 19th-century scientist whose experiments were essential in the development of electromagnetism and special relativity, is recovered. His technical knowledge and curiosity allowed him to conduct two crucial experiments of the 19th century, the Aragó disk and the determination of the constancy of the speed of light. At first, these experiments seem completely disconnected from each other, as they did in the 19th century. However, as presented in this thesis, they are related by the common denominator of relative speed, an essential aspect of special relativity.The Aragó disk was a controversial experiment on rotational magnetism, forgotten today, but it caused great excitement in the 19th century. So much so that Faraday abandoned his experiments on chemical electrolysis to study the Aragó disk. The final result was the Faraday disk and, therefore, the origin of all generators and electric motors. However, the Faraday disk also presents controversies, such as the so-called Faraday paradox, which we have developed in this thesis and resolved through its relationship with the principle of conservation of angular momentum and the application of the second law of thermodynamics.With the purpose of experimenting, deducing, and analyzing to achieve a better theoretical understanding of rotational motion related to magnetism, an Aragó disk and two Faraday disks have been constructed, one as a generator and the other as a motor. Based on these results, new electrodynamic devices have been designed and built, based on the research on the Faraday disk conducted by Dr. Albert Serra. These experiments have provided indications for the construction of new electric motors that operate at high intensities and without an externally applied magnetic field. We have built this electric motors and called them Electrodynamic Turbines.In its study, the theory of special relativity has been applied to understand the relativistic origin of the magnetic field and has been applied in the construction of an experiment that we have called the relativistic coil. Concluding that relativity provides a new vision of electromagnetic behavior in a coil.A historiographical study has been carried out on the attempts to determine the relative speed between the Earth and the stars. Observations that influenced Aragó and his measurement of the speed of light as a universal constant.Einstein, in his 1905 article "On the Electrodynamics of Moving Bodies," only refers to one experiment, which is the Faraday disk. And regarding the principle of the constancy of the speed of light, which Einstein adopts, it is based on the experiment to determine the speed of light conducted by Aragó and Fresnel. We conclude that Francesc Aragó decisively influenced the science of his time in a key aspect for subsequent centuries, electromagnetism. So much so that two of his experiments were essential for Einstein's inspiration in developing the theory of special relativity

Committee:
     PRESIDENT: TEIGELKÖTTER, JOHANNES
     SECRETARI: PAWELLEK, ALEXANDER
     VOCAL: DIETZ, ARMIN
Thesis abstract: When using voltage source converters (VSC) to control the load flow between the DC voltage circuit and n-phase AC consumers/generators, distortion currents/voltages occur due to the principle involved. These describe the deviation of the real, measurable signal from its ideal, desired course. The distortion voltages occurring at the AC-side connection points of the VSC correspond to the deviation from the specified set voltage. At the DC-side connection point strong distortion currents are superimposed on the direct current emitted or absorbed by the VSC.Within the scope of this work, the emergence of the DC-side distortion currents, caused by the switching mode of operation of the VSC, based on corresponding literature sources, is discussed. The consideration here is limited to the differential mode components of the currents mentioned, a common mode consideration is not carried out. Criteria for describing the DC-side distortion currents and factors for influencing these criteria are derived from the results of the investigations carried out.For a given system and operating range, the distortion current stress caused by the VSC can be varied by adapting the control method used. For this purpose, different control methods found in the literature are selected and the operating point-dependent distortion current stress caused by them is determined analytically. To verify the results, a simulation model is developed and tests are carried out on an experimental test setup. The VSC is operated with a modulator-based control method as well as with a direct current control method. The results are compared regarding the DC-side distortion current. Namely, the control methods used are Space Vector Modulation (SVM) and Scalar Hysteresis Control (SHC). The latter was developed at the Technical University of Applied Sciences Wuerzburg-Schweinfurt.If the DC-side distortion current stress caused by the VSC is known, a filter can be designed that reduces the interference emission of the VSC to a specified level. As a level for the remaining interference emission, the remaining voltage ripple at the DC-side connection point of the filter is usually used in regulative specifications. Passive filters consisting of one or more capacitors of the same or different type are usually used for this purpose. This filter structure is described below as conventional structure. The different variants are presented accordingly and the filter capacitor is designed analytically/numerically on a concrete example for the DUT operated with SHC as well as SVM.Concepts known from literature for reducing the passive filter effort are presented and their effects on the DC-side distortion current load are described analytically. The concepts presented are divided into the categories of horizontal and vertical extension of the VSC as well as extension by downstream active components.In the second part of the work, a non-linear hybrid filter consisting of an actively controlled four-quadrant controller and two passive filter stages is designed and a methodology for the design of the components is developed. Furthermore, a modulation scheme with superimposed control for the use of the four-quadrant controller as an active filter is developed. The methodology for designing the filter is applied to the DUT used in this work as an example. The verification of the calculated results is carried out using a simulation model and an experimental test setup. Finally, the presented and more detailed passive filter variants as well as the developed non-linear hybrid filter are compared with each other in terms of construction volume and interference immunity. This shows that the presented non-linear hybrid filter has a significantly better interference immunity and a lower tendency to oscillate compared to passive filters. As a result, a more stable filter effect can be expected, especially when used in dynamic systems or systems with unknown dynamic behaviour.

Committee:
     PRESIDENT: ARBOLEYA ARBOLEYA, PABLO
     SECRETARI: GALCERAN ARELLANO, SAMUEL
     VOCAL: HERRAIZ JARAMILLO, SERGIO
Thesis abstract: Due to the exponential growth of electric vehicles and distributed generation in distribution networks, potential congestion in power lines, transformers, and voltage deviations is expected to increase in the coming years. For this reason, the development of modern distribution planning strategies to strengthen grid infrastructure needs to be studied, considering its dynamics and impacts.This thesis proposes a simulation-based optimization model for active distribution network expansion, capturing nonlinear complexities through AC power flows, voltage stability, and operational constraints. The formulation includes an objective function, constraints, and both traditional and flexible planning actions, focusing on determining the optimal battery power and capacity over projected horizons to defer infrastructure investments.Additionally, this thesis reviews and compares smart grid measurement devices based on their technical specifications and potential applications in distribution network operation and planning. A recurrent neural network model is also presented for medium- and long-term load forecasting using historical consumption data from medium-voltage (MV) transformer stations. This forecasting model was later integrated with an investment planning model to compare passive and flexible reinforcement actions in a real case study in Spain.Finally, supervised learning models (Random Forest, XGBoost, LSTM, and SVM) were trained using the proposed optimization model to predict and minimize investment costs, thereby identifying the most effective approach for DSO planning applications. This methodology was tested on a CINELDI network under various electric vehicle charging expansion scenarios and is intended for use in connection request applications.