1,720,954 research outputs found
Residual Current Protection of a Meshed DC Distribution Grid with Multiple Grounding Points
No full textDue to the emergence of renewable sources of energy and the progress in sustainable technologies, it could become very interesting in terms of energy savings and cost efficiency to switch the consumer level of the electrical network from AC to DC. This way, less power conversion would be required, and the meshing of the grid would be possible, hence increasing the reliability and flexibility of the power supply. Meshing the grid, however, is a challenge in itself. Indeed, it can be desirable to have several power sources connected to the same system, in order to provide for all the loads connected to the network when one of the sources fails. But having multiple sources in the network also means having several grounding points. Using the AC grounding methods, such as resistive grounding, proves problematic. As a matter of fact, as there are several grounding points in the network, they form a loop in the ground where the current can flow. If the voltage across the grounding points is not null, then a current will be able to flow through the ground. In AC, this was not such a problem, but DC ground currents will corrode the infrastructure around the network, which will prove harmful over time. It is thus necessary to devise a new way of grounding the system. The method proposed in this MSc thesis is capacitive grounding, which consists of using capacitors to ground the system, instead of resistors or inductors. This will ensure that no current flows through the ground in steady-state, and will consequently prevent corrosion. Grounding the grid through capacitors enables the use of the residual current measurement method to protect the meshed network against ground faults. Coupled to a smart communication system that divides the network in protection zones, this method will ensure the selectivity of the protection scheme, and will also discriminate net currents circulating through the grid from actual ground fault currents. The protective relays will be able to determine where the ground fault is on the poles with the polarity of the residual current measurement, and will only disconnect that pole, leaving the other half of the network in operation. This selective disconnection will also help improve the reliability of the system, and the consumers will be able to use the network safely.Electrical Engineering, Mathematics and Computer ScienceElectrical Sustainable EnergyDC systems, Energy conversion & Storag
Steps towards the universal direct current distribution system
No full textThe traditional ac power system is challenged by emerging distributed renewable energy sources and an increase in installed load capacity, e.g., electric vehicles. Most of these new resources use inherently dc as do more and more appliances. This poses the question, if they should still be connected on ac in the low voltage grid, which was chosen a century ago, because at that time dc could not be easily transformed to higher voltage levels. In this dissertation, steps are set towards a universal dc distribution system that has the capability of replacing current low voltage ac grids. Standardization is very important at this voltage level due to the high number of connected devices. Therefore, an analysis of the future power system requirements is made and a modular architecture is proposed that consists of connected nano- and microgrids. The dc distribution grid could be meshed and these nano- and microgrids could be connected without a converter separating them, which has significant implications on the overall design of the system. Modular bipolar voltage levels can increase the efficiency of the system, but complicate its operation as well. The exact optimal power flow is formulated for bipolar dc distribution grids with asymmetric loading. It can be used to manage congestions that could affect only one pole. Congestions in distribution grids are likely to increase, due to the increase in installed capacity. They are also more severe in dc grids due to the use of power electronic converters that have very hard limits in comparison with ac transformers. A general method to calculate locational marginal prices between any two nodes in the dc grid is formulated. The optimal power flow formulation is extended to multiple periods in order to include storage operation. Protection is one of the main challenges in creating large dc grids, as short-circuit currents can be very high and there is no current zero crossing as in ac. A low short circuit current protection philosophy is formulated to deal with this. It also addresses the challenge of very low fault current contribution in case of islanded operation. Solid-state circuit breakers are proposed as the main protection devices for dc distribution grids in combination with fast fault detection and clearance. The challenges regarding fault discrimination and selectivity are addressed. Additionally a classification of protection zones in dc distribution grids based on risk is proposed. Experimental measurements of a developed prototype using current derivative tripping are shown. Finally, dc ready devices, that can operate on dc as well as ac, are introduced as a means of simplifying the transition towards dc distribution grids. The losses in the rectification components, when operated on dc instead of ac, are analyzed and it is found that the rectification components of wide input range devices do not need to be enhanced.DC systems, Energy conversion & Storag
Power Balance Control of DC Microgrids
No full textControl techniques for DC microgrid.Electical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
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
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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