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 90°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

Dielectric study of the glass transition: correlation with calorimetric data

The glass transition in amorphous poly(ethylene terephthalate) is studied by thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC). The ability of TSDC to decompose a distributed relaxation, as the glass transition, into its elementary components is demonstrated. Two polarization techniques, windows polarization (WP) and non-isothermal windows polarization (NIW), are employed to assess the influence of thermal history in the results. The Tool-Narayanaswami-Moynihan (TNM) model has been used to fit the TSDC spectra. The most important contributions to the relaxation comes from modes with non-linearity (x) around 0.7. Activation energies yield by this model are located around 1eV for polarization temperature (Tp) below 50C and they raise up to values higher than 8eV as Tp increases (up to 80C). There are few differences between results obtained with WP and NIW but, nonetheless, these are discussed. The obtained kinetic parameters are tested against DSC results in several conditions. Calculated DSC curves at several cooling and heating rates can reproduce qualitatively experimental DSC results. These results also demonstrate that modelization of the non-equilibrium kinetics involved in TSDC spectroscopy is a useful experimental tool for glass transition studies in polar polymers.Comment: 13 pages, 2 tables, 10 figures; minor change

Annealing Effect on the Conductivity of XLPE Insulation in Power Cable

Conductivity () in XLPE insulation of power cables annealed at 90 ºC at temperatures between 50 and 97 ºC 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 ºC and shifts gradually to higher annealing times up to 40 days when the measurement is performed at 97 ºC. A linear behavior of ln() 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 is explained in terms of the increase in traps density due to the diffusion process from the SC shields. Long term decrease in is justified by the observed decrease of diffusion rate for long annealing times

ZnO:Co Diluted Magnetic Semiconductor or Hybrid Nanostructure for Spintronics?

We have studied the influence of intrinsic and extrinsic defects in the magnetic and electrical transport properties of Co-doped ZnO thin films. X ray absorption measurements show that Co substitute Zn in the ZnO structure and it is in the 2+ oxidation state. Magnetization (M) measurements show that doped samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found that the values of the orbital L and spin S numbers are between 1 and 1.3 for L and S = 3/2, in agreement with the representative values for isolated Co 2+. The obtained negative values of the Curie-Weiss temperatures indicate the existence of antiferromagnetic interactions between transition metal atoms.Comment: To be published in Journal of Materials Scienc

Double glass transition in polyethylene naphthalate structural relaxation by MDSC, BDS and TSDC

We present the experimental study of the primary, $\alpha$, and secondary, $\beta^*$, relaxations of the glassy polymer polyethylene naphthalate (PEN), by Modulated Differential Scanning Calorimetry (MDSC), Thermally Stimulated Discharge Currents (TSDC) and Broadband Dielectric Spectroscopy (BDS). Results show how the $\alpha$ and $\beta^*$ relaxations can be considered part of a very broad and distributed relaxation. The $\beta^*$ relaxation is composed of a main contribution ($\beta_3^*$) and two additional ones ($\beta_1^*$ and $\beta_2^*$) and each elementary mode of the relaxation has its own glass transition temperature. This scenario gives rise to an extended glass transition mainly centered in $T_{g\beta^*} \sim 305$ K and $T_{g\alpha} \sim 387$ K

Effect of annealing on conductivity in XLPE mid-voltage cable insulation

A new study of the electrical conductivity of crosslinked polyethylene (XLPE) mid-voltage (MV) cable insulation is presented. Its main objective is to show the effect of annealing treatments on MV cables under actual service conditions. Complementary time domain (absorption/resorption currents) and frequency domain (dynamic electrical analysis) techniques are applied on different laboratory samples containing XLPE insulations: sections of XLPE insulated cable (with and without semiconducting screens) in the case of absorption/resorption currents and, in the case of dynamic electrical analysis, a thin ribbon obtained from the cable insulation by mechanical procedures. For annealing temperatures below a certain critical temperature, conductivity decreases both for XLPE cylinders (cable sections from which inner and outer semiconducting screens have been removed) and for real cables (sections of cable with semiconducting screens) but its value is smaller in the case of cables. If the annealing temperature is higher than the critical temperature, the behaviour of conductivity is more complex. In XLPE, cylinders conductivity initially decreases with the annealing time but after some annealing time it begins to increase, it passes over a maximum and eventually it decreases monotonically. In the case of real cable sections, conductivity grows, tending to a saturation value, which is noticeably higher than the corresponding value of the maximum obtained for XLPE cylinders. The experimental results are explained satisfactorily by means of the Mott equation that takes into account hopping conduction assisted both by temperature and electric field