127,507 research outputs found
Enhanced pump absorption of active fiber components with skew rays
No full textWe propose a model and then demonstrate a technique that can be used to optimize pump absorption in active multimode fibers with different cladding geometries, by varying the angle of pump light excitation. The analysis of the contribution of different ray groups at different wavelengths enables the design optimization of active and passive fiber components in fiber lasers.George Y. Chen, Christophe A. Codemard, Michalis N. Zervas, Tanya M. Monro, and David G. Lancaste
Calibration of the Pulsed Electroacoustic Technique in the Presence of Trapped Charge
No full textThe influence of pulse voltage on the accuracy of charge density distribution in the pulsed electroacoustic technique (PEA) is discussed. It is shown that significant error can be introduced if a low dc voltage and high pulse voltage are used to calibrate charge density. However, our main focus in the present paper is to deal with one of the practical situations where space charge exists in the material prior to any measurements. The conventional calibration method can no longer be used to calibrate charge density due to the interference by the charge on the electrode induced by space charge. A method has been proposed which is based on two measurements. Firstly, the sample containing charge is measured without any applied voltage. The second measurement is carried out with a small external applied voltage. The applied voltage should be small enough so there is no disturbance of the existing charge in the sample. The difference of the two measurements can be used for calibration. An additional advantage of the proposed method avoids the influence of the pulse voltage on calibration and therefore gives a more accurate representation of space charge. The proposed method has been validated
The effect of degassing on morphology and space charge
No full textIt is believed that space charge buildup in cross-linked polyethylene (XLPE) insulation is the main cause for premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is one of the material treatment process commonly employ in cable manufacturing to improve insulation performance. In this paper, investigation on the effect of degassing period has on the morphology and space charge was carried out. Planar XLPE samples of the same composite were subjected to different degassing time. It is discovered that apart from removing volatile by-products, degassing also anneal XLPE material; changing the morphology as a result
Effect of polyethylene interface on space charge formation
No full textThis paper reports the results of an investigation into the space charge formation and decay at different material interfaces. In particular, the influence of the interface between electrode and polymer or polymer and polymer on the space charge dynamics has been studied. Planar samples were subjected to high dc electric stresses for extended periods of time and space charge measurements taken using the pulsed electroacoustic (PEA) technique. It has been found that the types of interface between electrode and polymer play a significant role in determining the charge distribution in the insulation and that the interface between polymer and polymer acts as a potential barrier to electrons whilst allowing positive charge carriers through easily
A novel calibration method for PD measurement in power cables and joints using capacitive couplers
No full textPartial discharge (PD) measurements are universally accepted as a technique giving some indication of the state of the insulation in high voltage apparatus. Cable end users are keen to adopt an on-line PD monitoring during commissioning of systems. However, because of noisy environments and the problems of interference the conventional methods are difficult to implement. As a consequence, ultra-high frequency (UHF) / very high frequency (VHF) techniques of on-site detection of partial discharges have been developed. A technique based on the capacitive coupling has received much attention. It has been demonstrated that it is possible to using the technique to monitor the partial discharge in cables, particularly in joints/terminations. However, in order to obtain quantitative information about PD, calibration is required for this technique. Existing calibration methods are difficulty to implement on-site. In this paper a novel method is proposed and compared with the conventional method on a short piece of cable. It has been shown that individual capacitive coupler can also be accurately calibrated on-site and online using the new method, therefore it provides quantitative information about the amount of apparent discharge. In practice this is important for electricity utilities as the quantitative information about PD can be used to determine the quality of cable system and to decide whether the system needs to be repaired or replaced
Space charge and charge trapping characteristics of cross-linked polyethylene subjected to ac electric stresses
No full textThis paper reports on the result of space charge evolution in cross-linked polyethylene (XLPE) planar samples of approximately 220 ?m thick. The space charge measurement technique used in this study is the PEA method. There are two phases to this experiment. In the first phase, the samples were subjected to dc 30 kVdc/mm and ac (sinusoidal) electric stress level of 30 kVpk/mm at frequencies of 1 Hz, 10 Hz and 50 Hz ac. In addition, ac space charge under 30 kVrms/mm and 60 kVpk/mm electric stress at 50 Hz was also investigated. The volts off results showed that the amount of charge trapped in XLPE sample under dc electric stress is significantly bigger than samples under ac stress even when the applied ac stresses are substantially higher. The second phase of the experiment involves studying the dc space charge evolution in samples that were tested under ac stress during the first phase of the experiment. Ac ageing causes positive charge to become more dominant over negative charge. It was also discovered that ac ageing creates deeper traps, particularly for negative charge. This paper also gave a brief overview of the data processing methods used to analyse space charge under ac electric stress
Mechanisms for surface potential decay on fluorinated epoxy in high voltage DC applications
Full text linkEpoxy resin has been extensively used for decades as an insulation material in high voltage transmission systems. However, this insulation material does suffer from bulk and surface charging when used as insulating spacer, mainly in high voltage DC applications. By applying fluorination treatment, the surface of polymeric insulation is chemically treated and so modifies charge transport characteristics of the material. In doing so, excellent surface properties can be obtained without compromising the bulk characteristics of the polymeric insulation. In this paper, the authors investigate the surface potential decay performance of non-fluorinated and fluorinated epoxy resin samples. The surface decay performance of insulating material is a crucial parameter in dissipating accumulation of surface and bulk charge that can lead to premature breakdown of the insulating material. The epoxy samples were characterised by Energy Dispersive X-Ray (EDX) analysis to determine the changes in chemical composition of the samples before and after fluorination treatment. Surface potential decay measurement using positive corona discharging was then performed, followed by bulk DC conductivity measurement to further explain the mechanisms which govern the surface potential decay. The existence of surface-fluorinated layer on the treated samples had been found to play a major role in dictating the movement of charges away from the surface during the decay process. The influence of fluorination treatment on the decay mechanisms was discussed
Temperature effect on space charge dynamics in XLPE insulation
No full textThis paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kVdc/mm under four temperature 25 oC, 50 oC, 70 oC and 90 oC for 24 hours. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The dc threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown
A Rosary of Rubies: The Chronicle of the Gur-rigs mDo-chen Tradition from South-Western Tibet
Full text linkThe mDo-chen bKa’-brgyud-pa school represents a little known Buddhist tradition from Mang-yul Gung-thang in south-western Tibet. It goes back to a Buddhist yogin known as Ma-bdun-pa or Ma-bdun ras-chen (12th/13th c.) and was later mainly spread by members of the Gur family. Although belonging to the “Upper ’Brug” (stod ’brug) branch of the ’Brug-pa bKa’-brgyud-pa school, the mDo-chen tradition has always been deeply infused with the “spoken teachings” (bka’ ma) and “treasure teachings” (gter ma) of the rNying-ma-pa school, and the cult of the “Seven Ma-mo Sisters” (ma mo mched bdun) was particularly practised and transmitted by its members. This book presents a critical edition, an annotated translation and a photographic reproduction of a manuscript copy of a rare chronicle of the Gur-rigs mDo-chen tradition written by Brag-dkar rta-so sPrul-sku Chos-kyi dbang-phyug (1775–1837). The text provides us with an overview of the tradition’s development mainly through biographical accounts but also through prophecies, prayers and praises for individual masters. The study concludes with two appendices based on the mDo chen bka’ brgyud gser ’phreng, a lineage history composed in the 15th century, and the “records of teachings received” (thob yig) of three important members of the Gur family, thus allowing us to gain an insight into the transmissions of the mDo-chen bKa’-brgyud-pa school and the interactions of its representatives with other important Buddhist teachers up to the 18th century. The present work is a further outcome of the author’s investigations into the cultural and religious traditions of south-western Tibet and the neighbouring Himalayan valleys
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