1,720,955 research outputs found
Design and Simulation of a Novel Modular Converter-Transformer for AC/DC, DC/AC and DC/DC Operations
No full textThis manuscript describes the research and the development of a novel family of modular AC/DC, DC/AC and DC/DC converters for high-voltage interconnections between AC and DC lines or between two DC lines. Such devices are bidirectional and they integrate a three-phase power transformer. The relationship existing between AC and DC voltages in each module for rectifier and inverter operations is analogous to the one set in single-phase thyristor bridges. Due to the novelty of this topic, a low-voltage, low-power prototype was sized to make a first validation of its predicted behavior. This converter should realize all the operating modes allowed by the new family of devices, namely, AC/DC, DC/AC and DC/DC conversions. Its rated phase voltage is 230 V rms and it should be able to carry at least 1 kW. Thus, a switching model of the device was realized first. The simulated behavior of this prototype is deeply discussed in this paper during steady-state, transient and DC-fault operations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Study and design of a bidirectional AC/DC converter working as an active front end
No full textLAUREA MAGISTRALEI convertitori elettronici sono sempre più utilizzati e le nuove regolamentazioni impongono limiti stringenti in termini di contenuto armonico delle correnti assorbite dalla rete. Inoltre, molte applicazioni richiedono la capacità di invertire il flusso di potenza del convertitore. L’obbiettivo di questa tesi è realizzare un convertitore bidirezionale “active front end (AFE)”, ovvero un convertitore bidirezionale che assorbe una corrente in fase con la tensione di rete e con un basso THD. Il focus sarà sulla realizzazione del sistema di controllo.
Nel primo capitolo, sarà fatto un excursus dei diversi tipi di convertitori elettronici che possono fungere da raddrizzatori, evidenziando le loro caratteristiche principali. Inoltre, saranno selezionate le topologie per realizzare due AFE, monofase e trifase, che sono rispettivamente il VSC (Voltage Source Converter) trifase e il VSC full bridge. Successivamente, questi due convertitori saranno modellizzati usando le teorie “sliding average” e “sliding harmonics”, oltre che la trasformata dq. A questo punto, verrà presentata un’implementazione in Simulink dello schema di controllo che agisce sulle variabili dq. Inoltre, siccome il controllo in assi dq monofase ha mostrato delle criticità, verrà creato uno schema di controllo alternativo, basato sui controllori proporzionali-risonanti. La taratura dei controllori è stata fatta principalmente in modo sperimentale, aiutandosi anche con la funzione “pidtune” di MATLAB.
Tutti i sistemi di controllo sono stati testati in simulazione con l’ausilio di Simulink per verificare le prestazioni dinamiche, il THD della corrente e il fattore di potenza. Inoltre, è stato verificato l’impatto delle principali specifiche del convertitore sul THD e sul ripple della tensione lato DC. Successivamente, il controllo proporzionale risonante monofase e il controllo in assi dq trifase sono stati testati in un setup sperimentale in scala ridotta. Queste due tipologie di controllo si sono dimostrati validi dal punto di vista delle prestazioni dinamiche e per ottenere l’operatività a fattore di potenza unitario. Di conseguenza, sono adatte per realizzare convertitori elettronici che operano da AFE.The electronic converters are used more and more these days and new regulations impose more stringent limits on current harmonics injected into the grid. In addition, several applications demand the capability of power regeneration to the power supply. This thesis aims to realize a bidirectional converter working as active front end (AFE); thus, the studied converter must absorb a current in phase with the grid voltage and with low harmonic content (unitary power factor operation). The focus will be on the development of the control that runs on such type of converter.
In the first chapter, an excursus on different type of electronic converters acting as rectifier will be done: their main characteristics are highlighted. The topologies to realize two AFE converters (for single-phase and three-phase operations), will be selected, namely the full bridge VSC (Voltage Source Converter) and the three phase VSC. Then, the modelling of such converters is done, using the sliding average theory, the sliding harmonics theory and the dq transformation. After that, a possible implementation in MATLAB Simulink of the control schemes, acting on the dq variables, is presented. Moreover, since the single-phase dq control shows some criticalities, an alternative control scheme based on proportional-resonant controller is created. The tuning of the controller parameters is done mainly experimentally, using also the “pidtune” function in MATLAB to get first-guess values.
All the proposed control systems are tested in MATLAB Simulink to verify the dynamic performances and the unitary power factor operation. Moreover, the impact of the main system parameters on the AC current THD (Total Harmonic Distortion) and the DC voltage ripple is investigated. Then, the proportional resonant control and the three phase dq control are tested in a small-scale experimental setup. These two controls show good performance both in term of unitary power factor operation and dynamic responses to load or voltage variation. Thus, they are suitable to realize an electronic converter working as active front end
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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