F. Frutos

Selective Dichroic Patterning by Nanosecond Laser Treatment of Ag Nanostripes

Authors thank the Projects FUNCOAT CONSOLIDER-INGENIO CDS2008 – 0023, MAT2007 – 65764, MAT2010-18447, MAT2010-21228, CEN2007 – 2014, P09-TEP-5283, Domingo Martinez Foundation, and Juan de la Cierva Grant No JCI-2009 – 05098 for J.T.

Water treeing in underground power cables: modelling of the trees and calculation of the electric field perturbation

Abstract In order to explain the development of different types of water trees and the related dielectric breakdowns in extruded power cables, it is necessary to analyse the dielectric properties of the corresponding treed regions and their influence on the distribution of electric field. The study presented in this paper is both experimental and theoretical. Experimentally, we performed the laboratory ageing of a power cable for accelerated conditions of applied voltage and frequency: we inspected the different shapes, orientations and dimensions of vented, bow-tie, single-bow-tie and global water trees produced in the polymeric insulation. From a theoretical point of view we proposed adequate models for obtaining the field distribution analytically and using a suitable finite element method. A local electric field amplification is calculated according to the different characteristics of the water trees: length, shape, permittivity and law of variation for permittivity. From our results, a compared analysis of the risk of electric breakdown is performed for the different types of water trees according to their configurations.

The danger of water trees in polymer insulated power cables evaluated from calculations of electric field in the presence of water trees of different shapes and permittivity distributions

Abstract From the assumption that water trees influence the AC electric field distribution in the insulation due to their permittivity higher than that of the surrounding medium, calculations of electric field in the vicinity of model water trees are presented. The electric field is locally amplified and reaches values high enough to be considered as a danger for the cables.

Bending induced self-organized switchable gratings on polymeric substrates

We present a straightforward procedure of self-surface patterning with potential applications as large area gratings, invisible labeling, optomechanical transducers, or smart windows. The methodology is based in the formation of parallel micrometric crack patterns when polydimethylsiloxane foils coated with tilted nanocolumnar SiO2 thin films are manually bent. The SiO2 thin films are grown by glancing angle deposition at room temperature. The results indicate that crack spacing is controlled by the film nanostructure independently of the film thickness and bending curvature. They also show that the in-plane microstructural anisotropy of the SiO2 films due to column association perpendicular to the growth direction determines the anisotropic formation of parallel cracks along two main axes. These self-organized patterned foils are completely transparent and work as customized reversible diffraction gratings under mechanical activation.

A study on the dependence of water tree permittivity with time

During the growth of water trees in the insulation of a cable the distribution of the electric field is modified because of the local change of the dielectric properties of the material. It results in a local enhancement of the electric field which could increase the risk of breakdown. The key factor is the permittivity of the water tree and the aim of this work is to determine its possible values and, particularly, the law of its increase with time during the growth of the trees. The paper presents permittivity measurements in uniform field laboratory models with water trees grown in accelerated conditions. The resulting variations of the electric field in the material are also given.

Annealing effect on the conductivity of XLPE insulation in power cable

Conductivity (Σ) in XLPE insulation of power cables annealed at 90°C at temperatures between 50 and 97°C has been measured. In all cases there is an initial increase in conductivity that develops a maximum and finally decreases for long annealing times. This maximum appears in the sample annealed 20 days when conductivity is measured at 50°C and shifts gradually to higher annealing times up to 40 days when the measurement is performed at 97°C. A linear behavior of ln(Σ) versus T-1/4 is observed, which implies that the transport mechanism is basically via thermally assisted hopping conduction. Infrared spectroscopy indicates that, during annealing, some chemical species diffuse from the semiconducting shields (SC) into the XLPE. Thermally stimulated depolarization currents technique (TSDC) and intensity-current measurements (I-V) point out as well the presence of this diffusion process that becomes less significant after long annealing times. The initial increase in Σ is explained in terms of the increase in traps density due to the diffusion process from the SC shields. Long term decrease in Σ is justified by the observed decrease of diffusion rate for long annealing times.

Anisotropic In-Plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma-Assisted ITO Thin Films e-Beam-Evaporated at Oblique Angles

ITO thin films have been prepared by electron beam evaporation at oblique angles (OA), directly and while assisting their growth with a downstream plasma. The films microstructure, characterized by scanning electron microscopy, atomic force microscopy, and glancing incidence small-angle X-ray scattering, consisted of tilted and separated nanostructures. In the plasma assisted films, the tilting angle decreased and the nanocolumns became associated in the form of bundles along the direction perpendicular to the flux of evaporated material. The annealed films presented different in-depth and sheet resistivity as confirmed by scanning conductivity measurements taken for the individual nanocolumns. In addition, for the plasma-assisted thin films, two different sheet resistance values were determined by measuring along the nanocolumn bundles or the perpendicular to it. This in-plane anisotropy induces the electrochemical deposition of elongated gold nanostructures. The obtained Au-ITO composite thin films were characterized by anisotropic plasmon resonance absorption and a dichroic behavior when examined with linearly polarized light.

Dielectric characterization and conduction modelling of a water tree degraded LDPE

Distribution of electric energy by extruded polymer insulated cables continues to be a subject of outstanding relevance in modern industrialized countries all over the world. Dielectric characterization, conduction modelling and finally diagnostics of polymeric insulations are necessary steps towards the development of reliable and less expensive robust technologies of electric power distribution. This paper is devoted to a detailed experimental/theoretical study of the conductive properties of LDPE affected by different levels of degradation by water trees. Water tree layers of different lengths were grown in accelerated conditions and were characterized by water tree kinetics, time-dependent permittivity and polarization current. The polarization current was found to obey a Curie-von Schweidler law whose parameters were used to characterize the effect of ageing time. A new conduction model that takes into account dipole interactions and was obtained from a two-wells Debye model is presented which allows us to give an interpretation of the effect of ageing. This laboratory study was intended to improve the characterization of service power cables aged by water trees

Numerical simulation of electric currents through insulating materials

A numerical simulation of electric currents is carried out through solid dielectrics from a model based on the local polarization. A degradation of material was considered to be produced from one of the electrodes and we simulated it by admitting modifications in the chemical structure of the material, which implies an alteration in the value of the corresponding dipole moments that characterize the material, as well as the concentration of the latest. Different simulations are achieved according to the ratio of the degraded thickness and the temperature.

Comparative study of conductivity in mid-voltage cable XLPE insulation

It is well established that space charge conditions insulation lifetime. Because of this, space charge formation and relaxation processes characterization has become a very active research field, not only from an basic science point of view, but also in order to improve insulation performance in cable and other devices. In the case of mid-voltage power cable, crosslinked polyethylene is widely used as insulator. Service temperature of such cables is around 900/spl deg/C, which in the case of the cable insulation studied is in the melting temperature range. In previous works it has been determined that at this temperature conductivity plays a relevant role in the space charge relaxation process, so that the characterization of conductive processes at service temperature may become important in order to enhance cable insulation performance. The aim of this work is to contribute to a better knowledge of the conductive properties of XLPE in both time and frequency domains.