1,720,992 research outputs found
Guest Editorial Special Section 'Advances in Lightning Modeling, Computation, and Measurement'
No full textLightning interaction with medium-voltage overhead power distribution systems
No full textDistribution lines located in areas with high ground flash densities are prone to lightning-caused power interruptions. Lightning overvoltages can be produced on medium-voltage (MV) systems when lightning hits either the line conductors (direct strokes) or a point in the vicinity of the distribution network (indirect strokes). The evaluation of the lightning electromagnetic environment is essential to mitigate its effects and improve the power system quality. This chapter presents initially, using the concepts given in Chapter 5 of Volume 1, a procedure for the estimation of the mean annual number of direct lightning strikes to a given overhead distribution line. Then, the basic features of the lightning overvoltages are discussed. Although some typical characteristics can be identified, the analysis of the overvoltages depends on various parameters relevant to the adopted model of the lightning return stroke, soil and power network. The influences of the most important ones are discussed in this chapter, with examples of measured and calculated voltage waveshapes. Then, the main protective measures that can be applied to improve the lightning performance of MV distribution lines, namely the increase of the line insulation withstand capability, the use of periodically grounded shield wires and the installation of surge arresters along the line, are addressed. The analysis of the effectiveness of each measure as a function of the type of phenomenon (direct or indirect strikes) and of various parameters, such as the soil resistivity, ground resistance and grounding spacing, is performed. After that, the procedure presented in Chapter 1 of this volume for estimating the mean annual number of line flashovers that an overhead MV line can experience, is applied to the case of lines with different protective measures and the relevant performances are compared. The case of urban lines, whose performance is affected by the presence of buildings in their vicinity, is also dealt with, as well as the case of hybrid configurations, in which MV and high voltage (HV) lines share the same structures
An Analytical Approach to Assess the Influence of Shield Wires in Improving the Lightning Performance Due to Indirect Strokes
No full textThe lightning performance of an overhead line due to indirect strokes is a measure of its ability to resist to surges produced by nearby lightning. The estimation of the lightning performance generally requires a huge number of calculations requiring computationally intensive simulations. In this paper, we present an approach that uses analytical expressions, leading to a very significant reduction of the required computer resources. By this analytical approach, we investigate the lightning performance under different line arrangements; the study is also devoted to assessing the improvement that can be obtained by addition of underbuilt shield wires to arrangements in which only upper shield wires are typically used: it is shown that such a measure can lead to significant enhancements in terms of lightning performance
Lightning-induced voltages on complex power systems by using CiLIV: The effects of channel tortuosity
No full text
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
Estimation of the Lightning Performance of Overhead Lines Accounting for Different Types of Strokes and Multiple Strike Points
No full textDealing with lightning performance computation, researchers usually consider only negative downward first return strokes and the relevant channel-base peak current distribution. The aim of this article is to account for the lightning flash mechanism, which means to include also downward negative subsequent strokes and downward positive strokes, taking into account available distributions of the number of strokes per flash (i.e., the flash multiplicity) and of the distance among the impact points of the strokes within the same flash (i.e., the channel terminations distance). In this regard, to the best of the authors’ knowledge, the present study is the first one taking into account the multiple ground channel terminations in the lightning performance assessment. The obtained results show that both the negative subsequent strokes and multiple channel termination determine a significant increase in the estimated number of flashovers per year, and they need to be considered in the lightning performance assessment. On the other hand, considering positive downward strokes, a less remarkable increase in the estimated number of flashovers per year is observed
Lightning protection of a multi-circuit HV-MV overhead line
Full text linkThis paper deals with the assessment of the lightning performance of a medium voltage (MV) power line sharing the same poles of a high voltage (HV) line. We focus on the case of a 15-kV line with compact configuration that is supported by the concrete poles of a 69-kV or a 138-kV transmission line. Due to the taller HV pylons and conductors, the number of direct events to the multi-circuit line is higher than for a MV line alone. The presence of the HV conductors and of the overhead ground wire reduces the amplitudes of the induced overvoltages in the MV conductors due to indirect lightning events. For the considered configurations, with a compact MV line, the number of flashovers per year expected in the double-circuit line due to both direct and indirect flashes is lower than in a MV line alone. It is shown that such a number depends mainly on the line withstand voltage, on the grounding resistance and on the number of surge arresters. The dependence is quantitatively discussed in the paper. The analysis of the overvoltages transferred on the LV side of the distribution transformers is also performed
- …
