1,720,970 research outputs found
Approximate Kron Reduction Methods for Electrical Networks With Applications to Plug-and-Play Control of AC Islanded Microgrids
No full textKron reduction (KR) is a methodology for analyzing an electrical network by replacing it with a simpler circuit having less nodes but the same terminal behavior of voltages and currents at target vertices. Existing approaches to instantaneous KR, however, can fail in preserving the structure of transfer functions representing power lines. Therefore, even if the original lines are passive circuits, reduced lines might have transfer functions that do not correspond to a physically realizable passive system. To overcome this drawback, in this paper we focus on line models and propose two approximate KR algorithms producing reduced lines with the first-order transfer functions and capable of representing exactly the asymptotic behavior of electric signals, even if they are unbalanced. Then, we show how to apply these KR methods to the design of decentralized voltage and frequency controllers for AC islanded microgrids. Notably, we focus on the plug-and-play algorithm previously proposed by some of the authors, which assumes loads connected to the inverter outputs, and generalize it to networks where loads appear in arbitrary positions. Theoretical results are validated with numerical examples and the application of KR for designing PnP controllers is assessed through a simulation on a 21-bus microgrid.SCI-STI-GF
Plug-and-play control of AC islanded microgrids with general topology
No full textn this paper, we provide an extension of the scalable algorithm proposed in (Riverso et al., 2015) for the design of Plug-and-Play (PnP) controllers for AC Islanded microGrids (ImGs). The method in (Riverso et al., 2015) assumes DGUs are arranged in a load-connected topology, i.e. loads can appear only at the output terminals of inverters. For handling totally general interconnections of DGUs and loads, we describe an approach based on Kron Reduction (KR), a network reduction method giving an equivalent load-connected model of the original ImG. However, existing KR approaches can fail in preserving the structure of transfer functions representing transmission lines. To avoid this drawback, we introduce an approximate KR algorithm, still capable to represent exactly the asymptotic periodic behaviour of electric signals even if they are unbalanced. Our results are backed up with simulations illustrating features of the new KR approach as well as its use for designing PnP controllers in a 21-bus ImG derived from an IEEE test feeder.SCI-STI-GF
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
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
Direct high-temperature MOCVD growth of GaN on foreign substrates
No full textGaN-based devices are ubiquitous in everyday life, ranging from InGaN/GaN light-emitting diodes (LEDs) for illumination to AlGaN/GaN high-electron-mobility transistors for compact power supplies. Nevertheless, despite the tremendous development in the past thirty years, GaN technology is far from reaching maturity, and many challenges are yet to be solved. Among them, the realization in a practical way of fully-vertical GaN-on-Si power devices and the demonstration of high-efficiency red InGaN microLEDs are crucial from an industry perspective.
The growth and quality of a III-nitride film critically depend on the choice of the substrate and on the design of the intermediate buffer layer. This suggests that a novel approach in selecting the substrate and buffer may be the key to solve the open challenges.
In this thesis, we propose an innovative and original technique to directly grow GaN at high temperature (HT) on a foreign substrate by metalorganic chemical vapor deposition (MOCVD), without the use of any low-temperature (LT) buffer, by simply employing an in-situ preflow with trimethylaluminum (TMAl).
We investigated for the first time this method to directly growth GaN on ScAlMgO4 (SAM), a novel substrate with a small lattice mismatch with respect to GaN. Our proposed technique resulted in mirror-like Ga-polar GaN layers on SAM with morphology and crystal quality comparable to the use of conventional LT GaN buffers. At the same time, our method enabled the growth of arbitrarily thin GaN layers directly on SAM, thanks to the absence of the initial 3D growth step. The effect of the TMAl preflow parameters (duration, flow, and carrier gas) was studied in detail, and a model for the growth mechanism was proposed.
This technique was applied to grow an ultra-thin GaN layer on SAM as a buffer for the subsequent growth of a thick In0.17Ga0.83N layer, with the same lattice constant of SAM, for high-indium-content InGaN quantum wells, which presented a higher crystal quality and photoluminescence efficiency compared to reference samples grown with a conventional LT InGaN buffer. Therefore, our growth technique may contribute to the achievement of red InGaN microLEDs.
Moreover, as an alternative to the in-situ TMAl preflow, we demonstrated that GaN could be grown on SAM at HT also using an ex-situ Al deposition, resulting in smooth films with an even better crystal quality compared to a TMAl preflow. We clarified the growth mechanism, revealing the role of the thermally-dewetted Al layer on the improvement in GaN quality.
Finally, we demonstrated that the direct HT GaN growth with TMAl preflow was successful also in the case of sapphire and Si substrates. In particular, we found that n-GaN layers directly grown on n-Si with a TMAl preflow not only present a better crystalline quality compared to the use of thin AlN buffers, but also exhibit orders-of-magnitude improvement in vertical current conduction between GaN and Si, thanks to the absence of highly resistive AlN layers. Therefore, our proposed technique opens a new pathway for the effective realization of fully-vertical GaN-on-Si devices.
In conclusion, in this thesis we proposed a new technique to directly grow GaN on foreign substrates (SAM, sapphire and Si) by simply using a TMAl preflow, without any intentional buffer layer. The obtained results confirmed that our proposed growth method could contribute to solve the open challenges holding back the next generation of GaN devicesPOWERLA
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
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