1,720,974 research outputs found
Avoiding Voltage Rise in Distribution Grids Using Energy Storage Systems
Full text linkAs a consequence of the combination of increased worldwide energy consumption and rising concerns regarding environment, the amount of renewable electric power plants are rapidly increasing. These are often connected directly to the electric power grid at lower voltage levels than traditional power plants are, causing severe problems regarding power quality and security of supply, in addition to challenges such as voltage rise. Among others, voltage rise is an obstacle for wind farms also in Norway, as such power plants often are connected in weak grids. One solution to avoid voltage rise, is integration of energy storage systems. A range of storage systems are available, and also in this field technology is improving.
In addition to investigate the theory behind voltage rise and possible support provided by storage systems, a simulation model has been in Matlab Simulink build to analyse this. The model consist of a \SI{11}{kV} medium voltage grid with a wind farm connected at it's far end and a storage system possible to connect at several locations. Initial tests only having the wind farm connected, clearly show increase in voltage, the highest impact on buses closest to the wind farm.
The storage system was connected at two different locations, providing the grid with active and reactive power according to it's voltage reference, set equal to nominal voltage. Depending on initial state of the grid and amount of wind farm generation, active power was consumed or delivered by the storage system, lowering or rising voltage. A more even voltage profile was also obtained, proofing the ability of the storage system to both lower a too high voltage but also increase it if initially low.
The storage system was able to best support the grid close to it at both locations tested. However, for improving the overall grid state with the aim of mitigate voltage rise, a location close to the wind farm is the most beneficial. At high voltage levels, change in voltage is most dependent on change in reactive power. To better understand the impact of the storage system, after analysing results it was clear that for lower voltage levels, also active power plays an important role regarding change in voltage
Avoiding Voltage Rise in Distribution Grids Using Energy Storage Systems
No full textAs a consequence of the combination of increased worldwide energy consumption and rising concerns regarding environment, the amount of renewable electric power plants are rapidly increasing. These are often connected directly to the electric power grid at lower voltage levels than traditional power plants are, causing severe problems regarding power quality and security of supply, in addition to challenges such as voltage rise. Among others, voltage rise is an obstacle for wind farms also in Norway, as such power plants often are connected in weak grids. One solution to avoid voltage rise, is integration of energy storage systems. A range of storage systems are available, and also in this field technology is improving.
In addition to investigate the theory behind voltage rise and possible support provided by storage systems, a simulation model has been in Matlab Simulink build to analyse this. The model consist of a \SI{11}{kV} medium voltage grid with a wind farm connected at it's far end and a storage system possible to connect at several locations. Initial tests only having the wind farm connected, clearly show increase in voltage, the highest impact on buses closest to the wind farm.
The storage system was connected at two different locations, providing the grid with active and reactive power according to it's voltage reference, set equal to nominal voltage. Depending on initial state of the grid and amount of wind farm generation, active power was consumed or delivered by the storage system, lowering or rising voltage. A more even voltage profile was also obtained, proofing the ability of the storage system to both lower a too high voltage but also increase it if initially low.
The storage system was able to best support the grid close to it at both locations tested. However, for improving the overall grid state with the aim of mitigate voltage rise, a location close to the wind farm is the most beneficial. At high voltage levels, change in voltage is most dependent on change in reactive power. To better understand the impact of the storage system, after analysing results it was clear that for lower voltage levels, also active power plays an important role regarding change in voltage
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
Optimal Operation Voltage for Maximal Power Transfer Capability on Very Long HVAC Cables
Full text linkAbstractAn analytical approach towards the operation of very long HVAC cables has been developed. The main physical phenomena have been expressed with simplified analytical equations. A comparison with numerical simulations has shown acceptable accuracy. A set of long distance cable parameters has been introduced. The main conclusion is that beyond a certain specific cable length, the operation voltage should be optimised rather than set to the rated voltage value. Operation at the power factor fx1 has been found to be optimal with regards and power transmission capability.The developed equations can help to gain a better understanding of the subject matter, and the introduced long distance parameters enable to easily compare the long distance capabilities of different cables. It is concluded, that for very long HVAC cable projects, it makes sense to consider cables with very high voltage rating, even though they might not be operated at rated voltage. Such cables can be beneficial due to their thicker insulation layer resulting in lower capacitance. The results also highlight the importance of HVAC cables with low capacitance for very long distance applications, which might lead to a new niche market of specialised cables
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
Optimization of Marine Energy Storage Systems for Desired Lifetime, Energy Saving and Safety - result report
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