1,720,956 research outputs found
Mechanisms Governing the occurrence of Partial Discharges in Insulation Liquids
Get PDFBreakdown of electrical equipment is unwanted and the use of reliable methods of insulation are therefore necessary. Liquid insulation is one preferred material for electrical devices used in subsea installations due to outstanding qualities related to insulation, heat transfer, safety and incompressibility properties. To ensure that the selected insulating liquid is suitable for the specific equipment it will be necessary to test its insulating quality. This is verified by stressing the liquid with voltages higher than the partial discharge inception voltage (PDIV). While there are existing tests for these properties, such as use of partial discharge measurements, this thesis examines an experimental approach to testing the properties of different liquids in a point-plane gap through the use of high AC voltage.Charges, like ions, in an electrical field creates a current flow. In a liquid, dissociative ionization under applied voltage is the mechanism that generates the most charges. Field emission originating from the negative polarity contributes in some liquids. Moving charges in the liquids results in measurable currents of the order 10 - 80 nA under applied AC voltage of 20 kV in a 20 mm point-plane gap in different liquids. Space charges are also found to affect the partial discharge (PD) behaviour in liquids. Different liquids have different chemical and physical properties and therefore different PD behaviour. It has been observed that PDs are stochastic, and in some sense correlated to earlier stress due to residual ions from previous half periods and permanently change of the chemical structure of some liquid molecules after a PD. The PD rate increases exponentially with increasing voltage, while the maximum charge per half period tends to increase linearly. This observed behaviour is in agreement with what is found in earlier reports.The presence of free electrons is important. A significant increase in the rate of PD at low magnitudes in both polarities occurred when the test cell was exposed to X-rays. PDs start under the influence of a strong field or an electron avalanche. It is essential to be able to test different liquids in laboratory conditions, for economical reasons, in order to identify the most suitable insulating media. Power electronics utilizing quality insulation is more reliable with a lower probability of downtime. The test setup and methods used in this thesis may easily be adapted for test of electrical components and equipment instead of the simple point-plane gap method.Different liquids have different PD phase patterns. This thesis intends to describe the reasons to explain this difference. It is found to be the result of different properties in the liquids for charge creation and differences in their electrical field threshold for PD initiation. PDs of a certain size are rarely occurring events and are strongly dependent on the prehistory of the stressed liquid gap. We do not know whether PDs in itself is harmful to insulating properties or not. It is therefore uncertain whether the IEC 61294 test method, based on PDs, provides any useful information or not. More research is therefore needed in order to fully understand the PD phenomenon
Mechanisms Governing the occurrence of Partial Discharges in Insulation Liquids
No full textBreakdown of electrical equipment is unwanted and the use of reliable methods of insulation are therefore necessary. Liquid insulation is one preferred material for electrical devices used in subsea installations due to outstanding qualities related to insulation, heat transfer, safety and incompressibility properties. To ensure that the selected insulating liquid is suitable for the specific equipment it will be necessary to test its insulating quality. This is verified by stressing the liquid with voltages higher than the partial discharge inception voltage (PDIV). While there are existing tests for these properties, such as use of partial discharge measurements, this thesis examines an experimental approach to testing the properties of different liquids in a point-plane gap through the use of high AC voltage.Charges, like ions, in an electrical field creates a current flow. In a liquid, dissociative ionization under applied voltage is the mechanism that generates the most charges. Field emission originating from the negative polarity contributes in some liquids. Moving charges in the liquids results in measurable currents of the order 10 - 80 nA under applied AC voltage of 20 kV in a 20 mm point-plane gap in different liquids. Space charges are also found to affect the partial discharge (PD) behaviour in liquids. Different liquids have different chemical and physical properties and therefore different PD behaviour. It has been observed that PDs are stochastic, and in some sense correlated to earlier stress due to residual ions from previous half periods and permanently change of the chemical structure of some liquid molecules after a PD. The PD rate increases exponentially with increasing voltage, while the maximum charge per half period tends to increase linearly. This observed behaviour is in agreement with what is found in earlier reports.The presence of free electrons is important. A significant increase in the rate of PD at low magnitudes in both polarities occurred when the test cell was exposed to X-rays. PDs start under the influence of a strong field or an electron avalanche. It is essential to be able to test different liquids in laboratory conditions, for economical reasons, in order to identify the most suitable insulating media. Power electronics utilizing quality insulation is more reliable with a lower probability of downtime. The test setup and methods used in this thesis may easily be adapted for test of electrical components and equipment instead of the simple point-plane gap method.Different liquids have different PD phase patterns. This thesis intends to describe the reasons to explain this difference. It is found to be the result of different properties in the liquids for charge creation and differences in their electrical field threshold for PD initiation. PDs of a certain size are rarely occurring events and are strongly dependent on the prehistory of the stressed liquid gap. We do not know whether PDs in itself is harmful to insulating properties or not. It is therefore uncertain whether the IEC 61294 test method, based on PDs, provides any useful information or not. More research is therefore needed in order to fully understand the PD phenomenon
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
Currents in AC stressed liquid insulated needle plane gap
Get PDFCurrents and space charge phenomena in 8 different dielectric liquids have been investigated for an ac stressed needle plane gap. Applied frequencies ranged from 0.1 Hz to 100 Hz. A high resolution analogue-digital converter and an active suppression of capacitive currents were used to extract the small conductive currents. Tip radii and voltages were varied. The currents varied non-linearly with voltage. At low frequencies the perfluorpolyeter and mineral oils had almost symmetrical current in both polarities, while cyclohexane, white oil and esters had asymmetrical current with lower peak current for positive polarity. Above a certain instantaneous voltage level - within the power cycle - the currents increased with the voltage squared, fitting a space charge limited current model. Clear indications of heterocharge space charges were revealed by varying tip radii and frequencies. The heterocharge will result in an increase of the electric field. The results show that at higher voltages a purely resistive model for the liquids becomes invalid. It is also evident that the presence of space charges will influence partial discharge behavior.Currents in AC stressed liquid insulated needle plane gapacceptedVersio
- …
