Nonlinear constrained and saturated control of power electronics and electromechanical systems

Power electronic converters are extensively adopted for the solution of timely issues, such as power quality improvement in industrial plants, energy management in hybrid electrical systems, and control of electrical generators for renewables. Beside nonlinearity, this systems are typically characterized by hard constraints on the control inputs, and sometimes the state variables. In this respect, control laws able to handle input saturation are crucial to formally characterize the systems stability and performance properties. From a practical viewpoint, a proper saturation management allows to extend the systems transient and steady-state operating ranges, improving their reliability and availability. The main topic of this thesis concern saturated control methodologies, based on modern approaches, applied to power electronics and electromechanical systems. The pursued objective is to provide formal results under any saturation scenario, overcoming the drawbacks of the classic solution commonly applied to cope with saturation of power converters, and enhancing performance. For this purpose two main approaches are exploited and extended to deal with power electronic applications: modern anti-windup strategies, providing formal results and systematic design rules for the anti-windup compensator, devoted to handle control saturation, and “one step” saturated feedback design techniques, relying on a suitable characterization of the saturation nonlinearity and less conservative extensions of standard absolute stability theory results. The first part of the thesis is devoted to present and develop a novel general anti-windup scheme, which is then specifically applied to a class of power converters adopted for power quality enhancement in industrial plants. In the second part a polytopic differential inclusion representation of saturation nonlinearity is presented and extended to deal with a class of multiple input power converters, used to manage hybrid electrical energy sources. The third part regards adaptive observers design for robust estimation of the parameters required for high performance control of power systems.

Il contributo dell’Unione europea all’affermazione dello stato di diritto nella comunità internazionale

This thesis is aimed at analysing EU external relations from the perspective of the promotion of the rule of law in order to evaluate the effectiveness and consistency of its action within the international community. The research starts with an examination of the notion of the rule of law from a theoretical point of view. The first chapter initially describes the historical-political evolution of the establishment of the notion of the rule of law. Some of the most significant national experiences (France, the UK, Germany and Austria) are discussed. Then, the focus is put on the need to propose interpretations which explain the grounds of the rule of law, by highlighting the different formal and substantive interpretations. This philosophical-historical analysis is complemented by a reconstruction of how the notion of the rule of law was developed by the international community, with a view to searching a common notion at the international level by comparing theory and practice within the main international organisations such as the UN, OECD and the Council of Europe. Specific mention is made of the EU experience, whose configuration as a Community based on the rule of law is often debated, starting from the case law of the European Court of Justice. The second chapter deals with the conditionality policy and focuses on the development and scope of democratic conditionality according to the dominant approach of the doctrine. First, the birth of conditionality is analysed from an economic point of view, especially within international financial organisations and the different types of conditionality recreated in the scientific sector. Then an analysis is provided about the birth of democratic conditionality in the EC – in relation to its external relations – firstly as a mere political exercise to be then turned into a standardised system of clauses. Specific reference is made to the main scope of conditionality, that is to say enlargement policy and the development of the Copenhagen criteria. The third chapter provides further details about the legal questions connected to the use of democratic clauses: on the one hand, the power of the EC to include human rights clauses in international agreements, on the other, the variety and overlapping in the use of the legal basis. The chapter ends with an analysis of the measures of suspension of agreements with third countries in those rare but significant cases in which the suspension clause, included in the Lomè Convention first and in the Cotonou Agreement then, is applied. The last chapter is devoted to the analysis of democratic clauses in unilateral acts adopted by the European Union which affect third countries. The examination of this practice and the comparison with the approach analysed in the previous chapter entails a major theoretical question. It is the clear-cut distinction between conditionality and international sanction. This distinction is to be taken into account when considering the premises and consequences, in terms of legal relations, which are generated when democratic clauses are not complied with. The chapter ends with a brief analysis of what, according to the reconstruction suggested, can be rightly labelled as real democratic conditionality, that is to say the system of incentives, positive measures developed within the community GSP. The dissertation ends with a few general considerations about the difficulties experienced by the EU in promoting the rule of law. The contradictory aspects of the EU external actions are manifold, as well as its difficulties in choosing the most appropriate measures to be taken which, however, reflect all the repercussions and tension resulting from the balance of power within the international community. The thesis argues that it is difficult to grant full credibility to an entity like the EU which, although it proclaims itself as the guardian and promoter of the rule of law, in practice, is too often biased in managing its relations with third countries. However, she adds, we must acknowledge that the EU is committed and constantly strives towards identifying new spaces and strategies of action.

Da alleati vittoriosi ad acerrimi rivali: i processi di ri-allineamento dopo i conflitti egemonici. Teorie a confronto e studio di casi

Why some powers manage to coordinate their security efforts while others confront each other as rivals is still one of the most relevant and debated questions in the field of IR theory. The dissertation wants to give a contribution to this important debate. In particular, the main goal of the research is to analyse the dynamics of great power interactions after the end of hegemonic conflicts, that is to understand why, following the defeat of the common enemies, some of the winning allies continue to cooperate, while others begin to engage in political and military competition. In order to understand this difference, the study compares the explanatory value of two rival theoretical perspectives: neorealism, in its main version of the balance of power framework, and a liberal approach focused on domestic politics. The thesis is divided in two sections. In the first, I do summarize the main assumptions and predictions of the theories, from which I derive two different sets of hypotheses on the evolution of post-war great power relations. In the second part, I test the hypotheses by focusing on two cases of post-war alignment dynamics: 1) the relations among Austria, Prussia, Russia, Great Britain and France after the Napoleonic wars; 2) the relations among the US, the UK, France and Italy after the end of WWI. The historical cases disconfirm the logic of the balance of power and confirm the liberal hypotheses, seeing that the results of the analysis show changes in the domestic structures of the great powers had a much larger impact on the emergence of new alliances and rivalries than did the international distribution of power. In the conclusion of the dissertation, I provide the reader with a discussion of the main theoretical implications of the empirical findings.

L'altro eccezionalismo: William Appleman Williams tra storia, storiografia e impegno civile

La storiografia statunitense, a partire dagli anni Cinquanta, vide l’affermarsi di una nuova interpretazione della politica estera americana. Archiviata la storia diplomatica come storia dei trattati o storia delle interazioni delle élites dominanti, abbandonata una visione incentrata sull’equilibrio di potenza, il dibattito storiografico si arricchì della cosiddetta interpretazione «revisionista», antitetica rispetto a quella che, fino a quel momento, aveva predominato. Soggetto di analisi storica restava sempre lo Stato ma l’enfasi maggiore era posta sui fattori economici che ne influenzavano l’azione: si metteva in rilievo l’interazione tra l’interesse privato e il soggetto statale. Capofila di questa nuova scuola fu William Appleman Williams. Questa ricerca si pone l’obiettivo di delineare il contesto storiografico dal quale emersero gli studi di Williams e di cui egli ne roviesciò alcuni assunti fondamentali. Si intende tracciare il suo percorso intellettuale – storiografico e pubblico – al fine di restituire la complessità di un personaggio che divenne un vero e proprio «intellettuale pubblico». I quesiti, a cui questa ricerca vuole dar risposta riguardano l’evoluzione del percorso intellettuale di Williams tanto in ambito storiografico quanto, più in generale, in quello pubblico; il contributo alla ridefinizione dell’identità statunitense e del suo ruolo internazionale; il lascito della sua riflessione nella storiografia. Prendendo le mosse dall’idea di frontiera proposta da Turner, Williams sostenne che la fine dell’espansione territoriale «interna» aveva obbligato gli Stati Uniti a cercare nuovi mercati per il proprio surplus. Era stata tale necessità a catalizzare la Open Door Diplomacy, guidata da ragioni economiche, che presto identificarono l’interesse nazionale per trasformarsi in una vera e propria ideologia nel XX secolo.L’esito di tale politica estera fu la creazione di un impero non più territoriale ma frutto dell’espansione economica. E proprio questa riflessione sull’impero influenzò, negli anni Sessanta, la protesta studentesca che chiese un ripensamento del ruolo internazionale degli Stati Uniti.

Giochi diplomatici. Sport e politica estera nell'Italia del secondo dopoguerra (1943-1953)

Questa tesi di dottorato, partendo dall’assunto teorico secondo cui lo sport, pur essendo un fenomeno periferico e non decisivo del sistema politico internazionale, debba considerarsi, in virtù della sua elevata visibilità, sia come un componente delle relazioni internazionali sia come uno strumento di politica estera, si pone l’obiettivo di investigare, con un approccio di tipo storico-politico, l’attività internazionale dello sport italiano nel decennio che va dal 1943 al 1953. Nello specifico viene dedicata una particolare attenzione agli attori e alle istituzioni della “politica estera sportiva”, al rientro dello sport italiano nel consesso internazionale e alla sua forza legittimante di attrazione culturale. Vengono approfonditi altresì alcuni casi relativi a «crisi politiche» che influirono sullo sport e a «crisi sportive» che influenzarono la politica. La ricerca viene portata avanti con lo scopo primario di far emergere, da un lato se e quanto coscientemente lo sport sia stato usato come strumento di politica estera da parte dei governi e della diplomazia dell’Italia repubblicana, dall’altro quanto e con quale intensità lo sviluppo dell’attività internazionale dello sport italiano abbia avuto significative ripercussioni sull’andamento e dai rapporti di forza della politica internazionale.

Development and characterization of a distributed measurement system for the evaluation of voltage quality in electric power networks

The research activity carried out during the PhD course in Electrical Engineering belongs to the branch of electric and electronic measurements. The main subject of the present thesis is a distributed measurement system to be installed in Medium Voltage power networks, as well as the method developed to analyze data acquired by the measurement system itself and to monitor power quality. In chapter 2 the increasing interest towards power quality in electrical systems is illustrated, by reporting the international research activity inherent to the problem and the relevant standards and guidelines emitted. The aspect of the quality of voltage provided by utilities and influenced by customers in the various points of a network came out only in recent years, in particular as a consequence of the energy market liberalization. Usually, the concept of quality of the delivered energy has been associated mostly to its continuity. Hence the reliability was the main characteristic to be ensured for power systems. Nowadays, the number and duration of interruptions are the “quality indicators” commonly perceived by most customers; for this reason, a short section is dedicated also to network reliability and its regulation. In this contest it should be noted that although the measurement system developed during the research activity belongs to the field of power quality evaluation systems, the information registered in real time by its remote stations can be used to improve the system reliability too. Given the vast scenario of power quality degrading phenomena that usually can occur in distribution networks, the study has been focused on electromagnetic transients affecting line voltages. The outcome of such a study has been the design and realization of a distributed measurement system which continuously monitor the phase signals in different points of a network, detect the occurrence of transients superposed to the fundamental steady state component and register the time of occurrence of such events. The data set is finally used to locate the source of the transient disturbance propagating along the network lines. Most of the oscillatory transients affecting line voltages are due to faults occurring in any point of the distribution system and have to be seen before protection equipment intervention. An important conclusion is that the method can improve the monitored network reliability, since the knowledge of the location of a fault allows the energy manager to reduce as much as possible both the area of the network to be disconnected for protection purposes and the time spent by technical staff to recover the abnormal condition and/or the damage. The part of the thesis presenting the results of such a study and activity is structured as follows: chapter 3 deals with the propagation of electromagnetic transients in power systems by defining characteristics and causes of the phenomena and briefly reporting the theory and approaches used to study transients propagation. Then the state of the art concerning methods to detect and locate faults in distribution networks is presented. Finally the attention is paid on the particular technique adopted for the same purpose during the thesis, and the methods developed on the basis of such approach. Chapter 4 reports the configuration of the distribution networks on which the fault location method has been applied by means of simulations as well as the results obtained case by case. In this way the performance featured by the location procedure firstly in ideal then in realistic operating conditions are tested. In chapter 5 the measurement system designed to implement the transients detection and fault location method is presented. The hardware belonging to the measurement chain of every acquisition channel in remote stations is described. Then, the global measurement system is characterized by considering the non ideal aspects of each device that can concur to the final combined uncertainty on the estimated position of the fault in the network under test. Finally, such parameter is computed according to the Guide to the Expression of Uncertainty in Measurements, by means of a numeric procedure. In the last chapter a device is described that has been designed and realized during the PhD activity aiming at substituting the commercial capacitive voltage divider belonging to the conditioning block of the measurement chain. Such a study has been carried out aiming at providing an alternative to the used transducer that could feature equivalent performance and lower cost. In this way, the economical impact of the investment associated to the whole measurement system would be significantly reduced, making the method application much more feasible.

Nonlinear Characterization and Modelling of GaN HEMTs for Microwave Power Amplifier Applications

Semiconductors technologies are rapidly evolving driven by the need for higher performance demanded by applications. Thanks to the numerous advantages that it offers, gallium nitride (GaN) is quickly becoming the technology of reference in the field of power amplification at high frequency. The RF power density of AlGaN/GaN HEMTs (High Electron Mobility Transistor) is an order of magnitude higher than the one of gallium arsenide (GaAs) transistors. The first demonstration of GaN devices dates back only to 1993. Although over the past few years some commercial products have started to be available, the development of a new technology is a long process. The technology of AlGaN/GaN HEMT is not yet fully mature, some issues related to dispersive phenomena and also to reliability are still present. Dispersive phenomena, also referred as long-term memory effects, have a detrimental impact on RF performances and are due both to the presence of traps in the device structure and to self-heating effects. A better understanding of these problems is needed to further improve the obtainable performances. Moreover, new models of devices that take into consideration these effects are necessary for accurate circuit designs. New characterization techniques are thus needed both to gain insight into these problems and improve the technology and to develop more accurate device models. This thesis presents the research conducted on the development of new charac- terization and modelling methodologies for GaN-based devices and on the use of this technology for high frequency power amplifier applications.

The role of bifidobacteria in newborn health and the intestinal microbial balance

Gut microbial acquisition during the early stage of life is an extremely important event since it affects the health status of the host. In this contest the healthy properties of the genus Bifidobacterium have a central function in newborns. The aim of this thesis was to explore the dynamics of the gut microbial colonization in newborns and to suggest possible strategies to maintain or restore a correct balance of gut bacterial population in infants. The first step of this work was to review the most recent studies on the use of probiotics and prebiotics in infants. Secondly, in order to prevent or treat intestinal disorders that may affect newborns, the capability of selected Bifidobacterium strains to reduce the amount of Enterobacteriaceae and against the infant pathogen Streptococcus agalactiae was evaluated in vitro. Furthermore, the ability of several commercial fibers to stimulate selectively the growth of bifidobacterial strains was checked. Finally, the gut microbial composition in the early stage of life in response to the intrapartum antibiotic prophylaxis (IAP) against group B Streptococcus was studied using q-PCR, DGGE and next generation sequencing. The results globally showed that Bifidobacterium breve B632 strain is the best candidate for the use in a synbiotic product coupled to a mixture of two selected prebiotic fibers (galactooligosaccharides and fructooligosaccharides) for gastrointestinal disorders in infants. Moreover, the early gut microbial composition was affected by IAP treatment with infants showing lower counts of Bifidobacterium spp. and Bacteroides spp. coupled to a decrement of biodiversity of bacteria, compared to control infants. These studies have shown that IAP could affect the early intestinal balance in infants and they have paved the way to the definition of new strategies alternative to antibiotic treatment to control GBS infection in pregnant women.