1,720,959 research outputs found
Effect of Thermal Stress over the Partial Discharge Inception Voltage on Twisted Pairs
No full textPartial discharges are one of the main causes for what concerns the degradation of the electrical machines insulation system and their onset can bring the devices in question to their end of life precociously. In addition, although an electric machine does not show partial discharges when it is put into service, it is possible that this phenomenon develops over time due to the aging of the insulating material caused by the electrical stress to which it is subjected. However, depending on the application in which these machines are used, electrical aging may not be the only cause that leads to partial discharges. Some electrical machines with type I insulating system may in fact be used in more prohibitive applications that involve high temperatures, therefore in such cases a possible cause of the onset of partial discharges may be the thermal ageing. In this study, twisted pairs with thermal class W-240 °C composed of copper wires covered with a polyimide-based enamel were analyzed. Thermal ageing tests were carried out on several specimens at different temperatures in order to measure the trend of their Partial Discharge Inception Voltage (PDIV) over time. To prevent electrical ageing phenomena from modifying and influencing the results, the repeated PDIV measurements were made in a very short time. In the here considered case, the study demonstrated that at both test temperatures the specimens showed a rapid decrease in the PDIV values in the first 65 hours while subsequently there was a flatter trend. From the reported graphs it is clear how a higher temperature can lead to a more marked decrease of the PDIV values
A review on the comparison of conventional and corona resistant nanofilled enamels
No full textThe low voltage rotating electrical machines, typically based on Type I random/wound windings, are normally controlled by power electronic drives. The power supply voltage waveforms are characterized by fast rise and fall time fronts and in case of resonance and reflection phenomena, overvoltage could appear at the connection terminals of the machine. These overvoltages can incept the partial discharges in the electrical insulation, leading to the total breakdown of the machine. The nanocomposite material used as enamel on the winding wires can lead to increase the performances of the insulating system. In recent years, new class of corona resistant materials based upon nano particles have been characterized. In this work, an overview of the research studies performed by the CMTEST laboratory staff of the University of Genova is presented to summarize the results obtained comparing the behavior of conventional and nano-filled enamels
Comparison of PD Patterns Obtained by Conventional Conducted Signals Measurement and TEV Sensor during Electrical Ageing Tests
No full textThe conventional Partial Discharges (PDs) measurement systems, IEC60270 compliant, normally use a quadripole as coupling device to convert the current pulses to output voltage signals when the system bandwidth is in the frequency range from 40 kHz to 1 MHz. This kind of measurement usually needs an off-line measure and an invasive procedure to connect the objects under test to the measurement system. Other measurement systems based on non-invasive sensors like High Frequency Current Transformer (HFCT) and Transient Earth Voltage (TEV) could be used if the provided results give comparable response. Focusing on TEV sensors, they can be coupled to measurement systems having a frequency bandwidth out of the range defined in IEC60270 standard. The comparison among TEV measurement system and those IEC60270 compliant can be based on the derived and statistical quantities computed starting from the PD pattern. PD Patterns have been collected during electrical ageing test applied to twisted pair located in an air insulated medium voltage switchgear (AIMVS)
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
Corona resistant insulating systems characterization for low voltage rotating machine
No full textThe type I insulating system for low voltage electric machines is characterized by organic enamel as primary insulation material and impregnating resin as secondary insulating materials. It is known that these machines are supplied by a PWM inverter and if there is a relative long distance between the machine and the power electronics, the resonance and the reflection phenomena could cause overvoltage in the power supply voltage. This overvoltage could incept partial discharge (PD) activity in the insulating systems. This kind of insulating system can't resist under PD activity and the machines quickly reach total breakdown conditions.In an insulating system composed by enamel and resin, it takes great importance the chemical compatibility between them and the adopted impregnation techniques. In this study, two kind of impregnating resin applied on twisted pairs with a conventional enamel, have been analyzed to compare the different behavior. Ageing tests have been performed until the specimen total breakdown, and the PD activity has been monitored. Analyzing the derived quantities of the PD activity, a predictive lifetime model has been obtained
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
DC component effect on Electrical Ageing tests
No full textThe Partial Discharge phenomenon represents one of the major causes of the deterioration and ageing of the insulating systems of electric machines and, consequently, leads to a considerable shortening of their lifetime. Although the theory and practice focus more on partial discharges caused by a sinusoidal power supply, this study aims to analyze the possible consequences due to the presence of a DC component. This objective in fact has been taken into consideration due to the numerous and continuously increasing use of power electronics in the electrical systems: this can introduce a DC offset into the power supplies. A circuit capable of combining a high voltage AC supply with a high voltage DC one has been therefore designed and realized. Several specimens based on twisted pair wires have been subjected to the resulting voltage. During the degradation process the Partial Discharges activity has been monitored. The obtained results suggest that, in the here considered case, the presence of a DC component does not significantly affect the specimen lifetime and the activity of Partial Discharges
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
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