4,886 research outputs found
3D Simulation of Partial Discharge in High Voltage Power Networks
Open accessPartial discharge (PD) events arise inside power cables due to defects of cableâs insulation material, characterized by a lower electrical breakdown strength than the surrounding dielectric material. These electrical discharges cause signals to propagate along the cable, manifesting as noise phenomena. More significantly, they contribute to insulation degradation and can produce a disruptive effect with a consequent interruption of power network operation. PD events are, therefore, one of the best âearly warningâ indicators of insulation degradation and, for this reason, the modeling and studying of such phenomena, together with the development of on-line PDs location methods, are important topics for network integrity assessment, and to define methods to improve the power networksâ Electricity Security. This paper presents a 3D model of PD events inside a void in epoxy-resin insulation cables for High Voltage (HV) power networks. The 3D model has been developed using the High Frequency (HF) Solver of CST Studio SuiteÂź software. PD events of a few ”s duration have been modelled and analyzed. The PD behavior has been investigated using varying electrical stress. A first study of the PD signal propagation in a power network is described
Early warning system for the prevention and control of unauthorized accesses to air navigation services infrastructures
Early warning systems are fundamental instruments for the management of
critical situations since they are able to signal in advance any anomaly with
respect to ordinary situations.
The purpose of this paper is to present an early warning system, based on
artificial neural networks, for the prevention and control of unauthorized
accesses to the air navigation services infrastructure in Italy
Chemical composition and origin of nebulae around Luminous Blue Variables
We use the analysis of the heavy element abundances (C, N, O, S) in
circumstellar nebulae around Luminous Blue Variables to infer the evolutionary
phase in which the material has been ejected.
(1) We discuss the different effects that may have changed the gas
composition of the nebula since it was ejected
(2) We calculate the expected abundance changes at the stellar surface due to
envelope convection in the red supergiant phase. If the observed LBV nebulae
are ejected during the RSG phase, the abundances of the LBV nebulae require a
significantly smaller amount of mass to be lost than assumed in evolutionary
models.
(3) We calculate the changes in the surface composition during the main
sequence phase by rotation induced mixing. If the nebulae are ejected at the
end of the MS-phase, the abundances in LBV nebulae are compatible with mixing
times between 5 x 10^6 and 1 x 10^7 years. The existence of ON stars supports
this scenario.
(4) The predicted He/H ratio in the nebulae are significantly smaller than
the current observed photospheric values of their central stars.
Combining various arguments we show that the LBV nebulae are ejected during
the blue SG phase and that the stars have not gone through a RSG phase. The
chemical enhancements are due to rotation induced mixing, and the ejection is
possibly triggered by near-critical rotation. During the ejection, the outflow
was optically thick, which resulted in a large effective radius and a low
effective temperature. This also explains the observed properties of LBV dust.Comment: 18 pages, 4 figures, to be published in The Astrophysical Journal,
April 20, 200
Steady and ranging sets in graph persistence
Generalised persistence functions (gp-functions) are defined on -indexed diagrams in a given category. A sufficient condition for
stability is also introduced. In the category of graphs, a standard way of
producing gp-functions is proposed: steady and ranging sets for a given
feature. The example of steady and ranging hubs is studied in depth; their
meaning is investigated in three concrete networks
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