In the framework of developing defect-based life models, in which breakdown is explicitly associated with partial discharge (PD)-induced damage growth from a defect, ageing tests and PD measurements were carried out in the laboratory on polyethylene (PE) layered specimens containing artificial cavities. PD activity was monitored continuously during aging. A quasi-deterministic series of stages can be observed in the behavior of the main PD parameters (i.e. discharge repetition rate and amplitude). The evolution of the parameters reflects the physicalchemical changes taking place at the dielectric/cavity interface during the aging process. PD activity shows similar time behavior under constant cavity gas volume and constant cavity gas pressure conditions, suggesting that the variation of PD parameters may not be attributed to the variation of the gas pressure. It is speculated that the change of PD activity is related to the composition of the cavity gas, as well as to the properties of dielectric/cavity interface.

Organically-modified nanofiller clays can have significantly different aspect ratios as well as accumulate a relatively large amount of water in the composite bulk due to the contribution of the filler itself and the interaction between filler and polymer matrix. This paper investigates the effect of water absorption in a nanostructured thermoplastic polymer, namely ethylene-vinyl-acetate (EVA), on electrical property modifications considering the contribution of aspect ratio. The change of electrical properties (particularly space charge accumulation, electric strength, bulk conductivity and permittivity/losses) is studied as a function of water content absorbed by nanofillers having different aspect ratio, i.e. fluorohectorite and bohemite. An increase of the space charge build up and of the conductivity (and decrease of the electric strength) is observed as a function of the water content for specimens containing layered silicates as nanofillers (fluorohectorite), while the electric properties of bohemite specimens do not show significant variations with the absorbed water content. This behavior can be associated with the different aspect ratio of the nanofillers. A filler having higher aspect ratio, in fact, seems to be more effective in worsening the electrical properties of the final nanocomposite. This could be explained considering that the higher the aspect ratio of the nanoparticles, the larger the percolation probability. To quantify how water content may affect electric properties of the final nanocomposite a simplified model providing the percolation probability as a function of water content and geometry of the particles is developed in this paper.

This paper deals with aging phenomena in polymers under electric stress. In particular, we focus our efforts on the development of a novel theoretical method accounting for the discharge process (partial discharge) in well known defects present in polymers, which are essentially tiny air gaps embedded in a polymeric matrix. Such defects are believed to act as trigger points for the partial discharges and their induced aging process. The model accounts for the amplitude as well as the energy distribution of the electrons during their motion, particularly at the time in which they impact on the polymer surface. The knowledge of the number of generated electrons and of their energy distributions is fundamental to evaluate the amount of damage caused by an avalanche on the polymer-void interface and get novel insights of the basic phenomena underlying the relevant aging processes. The calculation of such quantities would require generally the combined solution of the Boltzmann equation in the energy and space/time domains. The proposed method simplifies the problem, taking into account only the main phenomena involved in the process and provides a partial discharge (PD) model virtually free of adjustable parameters. This model is validated by an accurate experimental procedure aimed at reproducing the same conditions of the simulations and regarding air gaps embedded in polymeric dielectrics. The experimental results confirm the validity and accuracy of the proposed approach.

The research work reported in this paper has the aim to contribute in the development of a new physical aging model for damage inception and growth from micro-cavities in polymeric insulating materials subjected to partial discharges (PD) under AC voltage. Aging tests were performed on polyethylene (PE) layered specimens with cylindrical micro-cavities. The PD activity tak ing place inside the micro-cavities was monitored during the aging process. PD patterns, discharge repetition rates and discharge amplitudes were recorded. Common features can be extracted in the changing trend of PD repetition rate and discharge amplitude. Furthermore, damage growth rate, as well as degradation severity, are inferred through simple statistics.

Recent works indicate that polypropylene (PP) and ethylene-vinylacetate (EVA) filled by nanosilicates may present low content of space charge and high electric strength. It has been proved that the dispersion of an inorganic phase in the polymeric matrix may improve voltage endurance, dielectric strength, thermal stability and mechanical properties in relation to particle size and arrangement on nanometric scale. Two polymeric materials, widely employed as electrical insulation for apparatus involved in energy transport, that is, ethylene vinylacetate (EVA) and isotactic polypropylene (PP) are examined. The nanofiller consists of an organophilic layered silicate (MEE), the purified MEE will be named MEE-w.Dielectric spectroscopy in time and frequency domain constitutes a useful tool for electrical insulation evaluation and diagnosis, and it can contribute to the understanding of electrical behaviour of complex solid polymer systems. In this paper, the results of dielectric spectroscopy analysis in time and frequency domain, for purified and unpurified filler nanocomposites, in a wide temperature range, are presented.Frequency-domain dielectric measurements were performed in order to appreciate the relaxation processes taking place at lower temperatures (-20 to 20 °C) over the 10 2 -10 6 Hz frequency range.The charging-discharging current measurements, through Fourier time to frequency data transformation, gave information about the slowest polarisation processes, that are usually hidden by the conduction current contribution. Space charge measurement data are also considered in order to understand the effect of nanostructuration and purification on charge carriers. While the relaxation process of EVA, α process, associated with glass transition of the material amorphous phase, results unchanged from base to nanostructured material (i.e., the nanofiller particles do not affect the chain mobility of the polymer), nanocomposites EVA and PP have shown the rise of a new process at higher temperatures respect to the typical host material processes, as well as a different distribution of relaxation processes. In Fig. 1, that displays the imaginary permittivity of EVA+MEE-w nanocomposite, the loss peaks, due to the α and α' process, can be clearly seen. The α' process can be ascribed to the interaction between the polymer matrix and MEE nanolayers.The temperature location of the process, which approaches the melting region of EVA, and its absence in the pure polymer let us argue that the α' relaxation process can be ascribed to a charge polarization of the Maxwell-Wagner-Sillars type, which is due likely to charge trapping at the interfaces between the nanofiller particles and the polymer. All the presented relaxation processes are thermally-activated and their activation energies, reported in Table 1, were estimated reporting the loss peak frequency and temperature of the isothermal measurements in the Arrhenius plot.The EVA nanocomposite obtained by an unpurified filler has shown a huge increment of real and imaginary...