1,721,015 research outputs found
Study of surface discharge behaviour at the oil-pressboard interface
No full textThis thesis is concerned with the surface discharge behaviour at the oil-pressboard interface. For large transformers this is classified as a serious failure mode because it can lead to catastrophic failure under normal AC voltage operating conditions. To increase understanding on this failure mode, a surface discharge experiment at the oil-pressboard interface has been conducted on different moisture levels in pressboard by applying a long period of AC voltage stress. The processes in the surface discharge at the oil-pressboard interface until the appearance of a first full discharge have been recognised and correlated with the measured data. The results show that the different moisture levels within the pressboard play an important role on the partial discharge (PD) activity of certain processes. The decreasing trend in the PD data during the surface discharges cannot be treated as a reliable condition monitoring measure of health because it is the key indicator of white marks propagation toward the earth point. The characteristics of full discharge events have been analysed to develop knowledge for condition monitoring of surface discharge at the oil-pressboard interface. Full discharges are corona-like events in which their random occurrences are dominated by accumulated charges on the pressboard surface along the white marks rather than the polarity of applied AC voltage. A 2-D axial symmetry surface discharge model has also been developed using COMSOL Multiphysics, a finite element analysis (FEA) software package. The model considers the pressboard region near the interface (a transition region) as porous, whilst in the bulk region of pressboard as a perfect insulator. The model is developed using continuity equations and coupled with the Poisson’s equation to study the problem in terms of charge transport mechanisms and electric field distributions. The thermal conduction equation is included to study the thermal effects of surface discharge activity at the oil-pressboard interface. The behaviour of surface discharge is studied by validating the simulated surface discharge current pulse with the measured current. The simulation results show that a field dependent molecular ionisation mechanism plays an important role in the streamer propagation during the period of the rising front of the current pulse, whilst during the period of decaying tail of the current pulse, the contribution of an electron attachment process is dominant. The modelling results suggest that degradation marks (white and black marks) are due to high energy over long periods of partial discharge events that lead to thermal degradation at the oil-pressboard interface
Partial discharges in bulk oil and at oil-pressboard interface under negative lightning impulse voltages
No full textThe presence of oil-pressboard interface has been associated with creepage discharge in large power transformers. Creeping discharge or surface discharge is considered as a serious fault condition that can lead to catastrophic failure under normal operating conditions. The studies on this fault condition are normally on the inception and extinction voltage under AC voltages. Recently, there are interest on the white marks formation and leakage current during the surface discharges. On the other hand, most studies related to creepage discharges under lightning impulse (LI) voltages focused on the pre-breakdown streamer propagation. The use of LI voltages is important in standard material breakdown test such as multiple-level method in IEC 60060-1. This paper investigates the partial discharge (PD) behaviour in the bulk oil and at the oil-pressboard interface from accumulative effect of breakdown test by using different configurations of electrodes. A point-plane electrode is used to study the PD behaviour in the oil bulk, whilst two setup of needle-bar electrode, i.e. needle placed in parallel with the pressboard surface and needle place at an angle close to the horizontal of pressboard are used to study the creepage discharge behaviour at the oil-pressboard interface. In addition, the PD behaviour under certain level of negative LI voltage that less than the statistical withstand voltage (SWV) is also studied by measuring the PD current using radio frequency current transformer (RFCT)
Modelling the Non-equilibrium Electric Double Layer at Oil-pressboard Interface of High Voltage Transformers
No full textIn large oil-filled power transformers, cellulose-based pressboard and paper are used throughout for electrical insulation. Microscopic views have shown that pressboard insulation is a fibrous and porous structure with non-homogeneous surface. It has been recognised that the pressboard structure is more porous towards the edge [1]. The pores within the pressboard allow oil absorption during impregnation process and provide paths for oil to penetrate until saturation is reached. The ratio of fibre and oil changes as the material structure changes from a medium of bulk oil-pressboard composite toward the bulk oil medium. The porosity of pressboard can also result in impurities within the oil being drawn into the pressboard. It has also been recognised that physicochemical process of a liquid in contact with solid wall leads to the formation of electric double layer (EDL) in the liquid region [2, 3]. The material properties and geometry of pressboard thus lead to a complex oil-pressboard interface. A 2-D model of oil-pressboard interface has been constructed using Comsol Multiphysics Finite Element Analysis software and this is shown in Figure 1. The mathematical model considers the dissociation of a generic impurity in the oil into positive and negative ions and considers the role of the porous and non-homogeneous wall of pressboard in the formation of the EDL. The pressboard, which is represented by different arrays of fibre, promotes preferential adsorption and desorption processes between ions in the oil and unoccupied fibre surfaces of oil impregnated pressboard. The model studies the non-equilibrium charge density profile in the EDL at the oil-pressboard interface when the oil is in the stationary condition
Measurement of Leakage Current at the Oil-Pressboard Interface during Surface Discharge
No full textSurface discharge at the oil-pressboard interface is classified as one of the most dangerous fault conditions of large transformers. Surface discharges can be sustained for long periods without surface flashover or breakdown as long as the surface breakdown voltage is not exceeded [1]. Damage at the pressboard surface due to surface discharge is a function of applied voltage and time. If an applied voltage, above the surface partial discharge inception voltage (PDIV) but less than surface flashover voltage persists, the resulting discharges can develop a creeping path due to the drying out of the pressboard by localized heating (forcing oil and water out of the pressboard pores) and the carbonization of the oil and cellulose [1]. The drying of pressboard during the surface discharge process has been observed as the development of white marks on the pressboard surface [1]. A surface discharge experiment using a needle-bar electrode configuration has been developed. A shunt resistor that has been extensively used to measure leakage current for outdoor insulation systems [2] is used in the experiment. The leakage current is also measured using a commercial partial discharge detection system to allow comparison of the leakage current waveform from the oscilloscope against the corresponding phase resolved partial discharge (PRPD) pattern obtained. Figure 1 shows an example of measured leakage current during the occurrence of full-length discharge. It has been found that after long periods of surface discharge, temporary full-length discharges may occur across the pressboard surface bridging the needle tip and the earth without resulting in complete electrical breakdown. The full-length discharges occur when the white marks have reached the earth bar and travel along the same path as the white marks [3]. The occurrence of this full-length discharge leads to small leakage current (mA range) flowing to earth that is unlikely to trip the earth fault protection relays [3]. Although the currents are relatively small, identifying their presence and relating them to forms of PD activity at the pressboard oil interface is important as it may provide a mechanism for earlier detection of creep stress failure
Characteristics of leakage current during surface discharge at the oil-pressboard interface
No full textSurface discharge at the oil-pressboard interface leads to the development of a conducting path that is generally accepted as white marks due to localized heating. The visible white marks are believed to be due to partial discharge (PD) energy that is high enough to dry out the pressboard through vaporizing the moisture and breaking oil molecules to generate gases in the pressboard pores. A long period of electrical stress applied to a surface discharge experiment using a needle-bar electrode configuration has led to the formation of full-gap white marks on the pressboard surface that bridged the needle tip and earth bar. During the surface discharge, leakage current may occur intermittently that sometimes is visible in the form of full discharge of arcing at the oil-pressboard interface bridging the needle tip and earth bar. This condition is expected to be similar to that of dry-band arcing from inclined-plane tracking experiments for outdoor insulation. This paper explains the surface discharge experiment and details the obtained leakage current characteristics in terms of waveforms and frequency spectra. Correlation between leakage current measurements using a shunt resistor and PD signal detection has been undertaken in order to identify the useful information for condition monitoring of surface discharge in a large transformer. An equivalent circuit to represent the leakage current occurrence at the oil-pressboard interface without considering the superimposed pulses due to individual PD events is also proposed. The effect of moisture on the experimental results is also discussed
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
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