1,723,143 research outputs found
Degradation by partial discharge in cavities under AC electric field
No full textPartial discharge (PD) is a localized electrical breakdown event inside solid or fluid insulating materials. PD does not link the two conductors directly. However, it can cause damage to the insulating material, and eventually build up a damaged channel leading to final breakdown.In general, PD events can be classified based on their discharge mechanisms, such as surface discharge, corona discharge, and free space cavity discharge which is the main focus of this work. Cavity discharge studied in this work is generated by an air-filled spherical cavity inside the insulating material, which is epoxy resin. Such small cavities exist in many high voltage equipment where polymeric insulating materials are involved. Detection of cavity PD is generally believed to be very concerned by industrial operators, and sometimes an immediate replacement of the plant is required.However, with the growing understanding of material degradation induced by cavity PD, it is possible to assess the degradation level by PD behaviour. By understanding the link between PD behaviour and degradation level, equipment’s’ service life may be extended to their maximum.Cavity PD in epoxy resin was found to develop itself through five stages in this work, defined by clear variations in terms of their phase resolved partial discharge (PRPD) patterns. Optical and scanning electron microscopic observations at degradation surfaces also confirmed the physics behind PD stage transition. Previous researchers usually focus on PRPD patterns against cavity air consumption for short life materials such as polyethylene, or singular pulse characters against degradation surface with long life materials such as epoxy resin. In this work, the feedforward (usually termed as feedback by other researchers, however, considering the fact that after each discharge event the cavity suffers deterioration and become a slightly different system, feedforward is a more accurate term from system control theory) or memory effect between adjacent pulses, short term surges, and long term behaviours were the main focus of discussion. The cavity losing its feedforward capability or memory capacity, as well as PD events getting localized onto specific damaged points, are experimentally shown to be the driving force from one degradation stages to another showed by PRPD pattern transitions.Results achieved in this work successfully established the link between various aspects of information which were usually studied in a less interactive manner. The results suggest that accumulative PD behaviour transition along degradation time is governed heavily by the cavity surface condition, the two of which form a closed loop. Pulse train analysis studies the capability of feedforward capability of degraded cavity surfaces. Both PRPD analysis and pulse train analysis take step changes in a synchronized manner with the discovery of new-born surface condition features. Some of the surface condition features can induce later stage discharge patterns such as swarming and treeing (“wing-like”) patterns, feedforward capability study of such patterns can directly be utilized to determine degradation level at such surface locations.Such information could be used to qualitatively, and quantitatively determine the level of degradation induced by cavity PD. At this stage, PRPD patterns can be used to predict degradation surface conditions, and a number of factors related to the PD feedforward capability of such degraded cavities are found. The feedforward capability was never investigated to such depth, and was never used as a condition monitoring tool in the past. This thesis contributes towards a new way of thinking for cavity PD degradation assessment, and if materialized, a longer and more reliable service life of polymeric insulating materials if cavities are involved
Large deviations for processes with long-range dependence, with queueing applications
No full textChang, Cheng-Shang; Yao, David D.; Zajic, Tim. (2000). Large deviations for processes with long-range dependence, with queueing applications. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/3473
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
Qiang guan lian xi tong zhong de chang cheng xu
No full textLong range order (LRO) is one of the most important properties of physics systems, especially for a strongly correlated system. In this thesis, the long range order in a few strongly correlated systems is investigated both rigorously and numerically.Magnetic orders in the two-dimensional periodic Anderson model (PAM) were investigated in the project. Several numerical methods including exact diagonalization, mean field methods and the constrained path Monte Carlo (CPMC) method were used here. We studied the effect of the dispersion of the impurity band on the magnetism and gave estimated phase diagrams on the band filling and impurity chemical potential plane, by comparing the ground state energies and by studying the Fourier transformation of the spin-spin correlations.The Neel long range order in various quantum spin models was studied. A spatially anisotropy antiferromagnetic Heisenberg system was studied and a critical point Jup⊥S was obtained. When J⊥ is larger than Jup⊥S , the Neel long range order was proved to exist in the ground state of the system. Then an onsite single-ion anisotropy D-term was imposed on the above system and its effect on the existence of the long range order was studied. We obtained a critical line on the DJ⊥ plane such that above which the long range order exists. To get insight into the long range order in the two-dimensional isotropic Heisenberg model, the XXZ model and the Heisenberg system with next nearest neighbors interaction were studied. For the XXZ model, two critical couplings DeltaXY and DeltaIsing were obtained such that when 0 ≤ Delta ≤XY or Delta ≥ DeltaIsing, the Neel order appears. For the antiferromagnetic Heisenberg model with next nearest neighbors interaction both spin S = 1/2 and S = 1 were studied. For S = 1/2, a critical next nearest neighbors ferro-coupling Jc2 was obtained such that when J2 ≤ Jc2 , the Neel long range order appears, while for S = 1 an improved Jc2 was obtained such that when J2 ≤ Jc2 the Neel order still exists even with frustration.Wang Yongqiang = 强关联系统中的长程序 / 王永强."June 2005."Adviser: Haiqing Lin.Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 3861.Thesis (Ph.D.)--Chinese University of Hong Kong, 2005.Includes bibliographical references (p. 156-162).Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.Text in English; abstracts in English and Chinese.School code: 1307.Wang Yongqiang = Qiang guan lian xi tong zhong de chang cheng xu / Wang Yongqiang
On-line monitoring trihalomethanes in chlorinated water by membrane introduction–fast gas chromatography mass–spectrometry
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