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We introduce and develop two bipolar transport models which are based on appreciably different physical assumptions regarding the distribution function in the energy levels of trap states. In the first model, conduction is described by an effective mobility of the carriers and the accumulation of stored space charge is taken into account through a single trapping level. In the second model the hypothesis of an exponential distribution function of trap depth is made, with conduction taking place via a hopping process from site to site. The results of simulations of the two models are compared with experimental data for the external current and the space-time evolution of the electrical space charge distribution. The two descriptions are evaluated in a critical way, and the prospects for these models to adequately describe real systems are given.

An interpretation of the anomalous low-frequency dispersion process is presented which is based on a cluster description of the structural ordering and fluctuation in carrier-dominated dielectrics. It is shown that this form of response occurs for systems of low spatial dimensionality and generates a sample-size-dependent conductivity. The relationship of the mechanism to that of power-law noise in electrical systems is identified and its structural interpretation explored. Particular features of hydrogen-bonded systems are described in which the dispersion is likely to be important in a biological context.

Dielectric studies are described aimed at providing an understanding of the charge storage and transport of an epoxy resin containing TiO 2 nanoparticles. Comparative results for conventionally filled composites are given, and the results discussed in terms of the underlying physics. It is shown that nanometric fillers mitigate the interfacial polarization characteristic of conventional materials with a reduction in the internal field accumulations. Background and Vision

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The results of space charge evolution in cross-linked polyethylene power cables under dc electrical field at a uniform temperature and during external voltage polarity reversal are presented in the paper. A mirror image charge distribution was observed in the steady state, but the pre-existing field altered the way in which the steady state charge distribution was formed from that obtaining when the cable was first polarized. Polarity reversing charge was generated in the middle of the insulation and moved towards the appropriate electrodes under the influence of a field in excess of the maximum applied field. Our results show that the mirror effect is a steady state effect that is due to crossinterface currents that depend only on the interface field and not its polarity. Measurements on cable sections with an elevated mean temperature and temperature gradient show that the interface currents are temperature dependent, and that differences between the activation energies of the interface and bulk currents can eliminate and possibly even invert the polarity of the space charge distribution.Index Terms -Space charge, PEA, XLPE insulated power cables, voltage polarity reversal, temperature gradient, "mirror image effect".

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