1,858 research outputs found
Transformer Oil Passivation and Impact of Corrosive Sulphur
In recent years a significant volume of research has been undertaken in order to understand the recent failures in oil insulated power apparatus due to deposition of copper sulphide on the conductors and in the insulation paper. Dibenzyl Disulfide (DBDS) has been found to be the leading corrosive sulphur compound in the insulation oil [1]. The process of copper sulphide formation and the deposition in the paper is still being investigated, but a recently proposed method seems to be gaining some confidence [1]. This method suggests a two-step process; initially the DBDS and some oil soluble copper complexes are formed. Secondly the copper complexes are absorbed in the paper insulation, where they then decompose into copper sulphide [2]. The most commonly used mitigating technique for corrosive sulphur contaminated oil is passivation, normally using Irgamet 39 or 1, 2, 3-benzotriazole (BTA). The passivator is diluted into the oil to a concentration of around 100ppm, where it then reacts with the copper conductors to form a complex layer around the copper, preventing it from interacting with DBDS compounds and forming copper sulphide. This research project will investigate the electrical properties of HV transformers which have tested positive for corrosive sulphur, and the evolution of those properties as the asset degrades due to sulphur corrosion. Parallel to this the long term properties of transformers with passivated insulation oil will be analysed in order to understand the passivator stability and whether it is necessary to keep adding the passivator to sustain its performance. Condition monitoring techniques under investigation will include dielectric spectroscopy, frequency response analysis, recovery voltage method (aka interfacial polarisation) amongst others. Partial discharge techniques will not be investigated, as the voltage between the coil plates is low and therefore it will not contribute significantly to the overall insulation breakdown, in corrosive oil related faults [3]. The goal of this research is to establish key electrical properties in both passivated and non-passivated power transformers that demonstrate detectable changes as the equipment degrades due to the insulation oil being corrosive
Analysis of Meson Exchange and Isobar Currents in (e,e'p) Reactions from O-16
An analysis of the effects of meson exchange and isobar currents in exclusive
(e,e'p) processes from O-16 under quasi-free kinematics is presented. A model
that has probed its feasibility for inclusive quasi-elastic (e,e') processes is
considered. Sensitivity to final state interactions between the outgoing proton
and the residual nucleus is discussed by comparing the results obtained with
phenomenological optical potentials and a continuum nuclear shell-model
calculation. The contribution of the meson-exchange and isobar currents to the
response functions is evaluated and compared to previous calculations, which
differ notably from our results. These two-body contributions cannot solve the
puzzle of the simultaneous description of the different responses
experimentally separated. Copyright 1999 by The American Physical SocietyComment: 5 pages, plus 3 PS figures. To be published in Phys. Rev. C Updated
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Meson Exchange Currents in (e,e'p) recoil polarization observables
A study of the effects of meson-exchange currents and isobar configurations
in reactions is presented. We use a distorted wave
impulse approximation (DWIA) model where final-state interactions are treated
through a phenomenological optical potential. The model includes relativistic
corrections in the kinematics and in the electromagnetic one- and two-body
currents. The full set of polarized response functions is analyzed, as well as
the transferred polarization asymmetry. Results are presented for proton
knock-out from closed-shell nuclei, for moderate to high momentum transfer.Comment: 44 pages, 18 figures. Added physical arguments explaining the
dominance of OB over MEC, and a summary of differences with previous MEC
calculations. To be published in PR
Research Update on Extreme-Mass-Ratio Inspirals
The inspirals of stellar-mass mass compact objects into massive black holes
in the centres of galaxies are one of the most important sources of
gravitational radiation for space-based detectors like LISA or eLISA. These
extreme-mass-ratio inspirals (EMRIs) will enable an ambitious research program
with implications for astrophysics, cosmology, and fundamental physics. This
article is a summary of the talks delivered at the plenary session on EMRIs at
the 10th International LISA Symposium. It contains research updates on the
following topics: astrophysics of EMRIs; EMRI science potential; and EMRI
modeling.Comment: 17 pages, no figures. Proceedings of the LISA Symposium X, to be
published at the Journal of Physic
Drug Allergy with Cutaneous Manifestations. Diagnostic Approach
As reacções adversas a fármacos (RAF) representam um problema frequente na prática clínica. A alergia a fármacos resulta de mecanismos de hipersensibilidade imunológica e representa 6-10% do total de RAF. Clinicamente, as reacções alérgicas a fármacos podem ser classificadas como imediatas (tipo I) ou não-imediatas (com
manifestações clínicas diversas e associadas sobretudo a reacções de tipo IV). Neste artigo são abordados aspectos gerais, nomeadamente os mecanismos imunopatogénicos implicados na alergia a fármacos e reactividade cruzada mas também as manifestações cutâneas mais relevantes, nomeadamente exantemas máculo-papulares, eritema fixo a fármacos (EFF), pustulose exantemática aguda generalizada (PEAG), síndrome de hipersensibilidade a fármacos
(DRESS – drug rash with eosinophilia and systemic symptoms), síndrome de Stevens-Johnson/necrólise epidérmica tóxica (SSJ/NET). O papel dos testes cutâneos (epicutâneos ou intradérmicos de leitura tardia) na abordagem de reacções não-imediatas é também revisto. Os beta-lactâmicos (BL) são o grupo farmacológico mais frequentemente envolvido em reacções de hipersensibilidade imunológica e que mais dificuldades coloca na prática clínica diária, nomeadamente devido aos riscos de reactividade cruzada, pelo que é analisado em maior detalhe ao longo da revisão. A indução de tolerância a fármacos poderá ser considerada em casos selecionados, sobretudo quando na ausência de alternativas terapêuticas igualmente eficazes ou seguras
Effects of Short-Range Correlations in (e,e'p) reactions and nuclear overlap functions
A study of the effects of short-range correlations over the (e,e'p) reaction
for low missing energy in closed shell nuclei is presented. We use correlated,
quasi-hole overlap functions extracted from the asymptotic behavior of the
one-body density matrix, containing central correlations of Jastrow type, up to
first-order in a cluster expansion, and computed in the very high asymptotic
region, up to 100 fm. The method to extract the overlap functions is checked in
a simple shell model, where the exact results are known. We find that the
single-particle wave functions of the valence shells are shifted to the right
due to the short-range repulsion by the nuclear core. The corresponding
spectroscopic factors are reduced only a few percent with respect to the shell
model. However, the (e,e'p) response functions and cross sections are enhanced
in the region of the maximum of the missing momentum distribution due to
short-range correlations.Comment: 45 pages, 15 figure
Parametrization of the angular correlation and degree of linear polarization in two-photon decays of hydrogen-like ions
The two-photon decay in hydrogen-like ions is investigated within the
framework of second order perturbation theory and Dirac's relativistic
equation. Special attention is paid to the angular correlation of the emitted
photons as well as to the degree of linear polarization of one of the two
photons, if the second is just observed under given angles. Expressions for the
angular correlation and the degree of linear polarization are expanded in terms
of -polynomials, whose coefficients depend on the atomic number and
the energy sharing of the emitted photons. The effects of including higher
(electric and magnetic) multipoles upon the emitted photon pairs beyond the
electric-dipole approximation are also discussed. Calculations of the
coefficients are performed for the transitions ,
and , along the
entire hydrogen isoelectronic sequence ()
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