L.A. Dissado

The role of trapped space charges in the electrical aging of insulating materials

An investigation of the effect of trapped space charges on the aging of polymeric insulating materials subjected to thermo-electrical stress is reported in this paper. Possible scenarios of degradation mechanisms, thermally activated, but accelerated by the presence of space charges, are examined. The model which derives from this approach has some interesting features: it is characterized by electrical and thermal thresholds, its parameters have a physical background, it can be cast into a probabilistic framework. Acceleration of aging due to space charges is attributed to a reduction of the free-energy barrier to degradation, seen as a local partially-reversible reaction, which is caused by energy stored in space-charge centers. The validity of the model is limited to dc voltage, and to the time of formation of microcavity-crazes, rather than to breakdown times, since other mechanisms will occur under electrical field once large enough cavities are formed in the insulation. The model is applied to the results of thermo-electrical life tests performed on PET, showing very good fitting, as well as interesting relationships between parameter estimates and insulation morphology. It is shown that the model can also fit well to ac life data, where it takes on a phenomenological meaning.

Towards an understanding of nanometric dielectrics

Dielectric studies are described aimed at providing an understanding of the charge storage and transport of an epoxy resin containing TiO/sub 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.

The incorporation of space charge degradation in the life model for electrical insulating materials

The time function of the Eyring reaction rate theory of insulation life is modified to demonstrate a physical origin for temperature threshold. Various mechanisms by which trapped charges may be involved in degrading the polymer are examined, and incorporated into the life model through an alteration to the free energies of the reacting system. The corresponding life functions are shown to possess the field-dependent threshold form previously obtained phenomenologically for insulating materials. The physical interpretation of the model parameters is discussed.

Propagation of electrical tree structures in solid polymeric insulation

Two alternative theoretical approaches to electrical tree propagation exist. Stochastic models attribute tree structures to random probabilistic factors, whereas in the discharge-avalanche model mechanism-driven field fluctuations are responsible. Here we review the predictions of these approaches in the light of the available experimental evidence. It is shown that both models give the fractal structures and the form of structure distribution observed experimentally. The width of the distribution functions predicted are, however, less than those found experimentally. The quantitative formulation available to the physical model also enables it to reproduce several other features of tree propagation such as voltage dependence, growth laws, and discharge behavior patterns. This is not possible in the stochastic approach without mechanistic assumptions which are difficult to relate to the stochastic process. The connection between the discharge-avalanche model and deterministic chaos is explored. Experimental evidence is presented supporting the contention that the electrical treeing phenomenon is the result of a deterministic breakdown mechanism operating in a chaotic regime at fields lower than those required for runaway breakdown. Space-charge deposition and re-arrangement is proposed as the physical origin of the chaotic field fluctuations. Tree shapes are shown to be related to the variation in the fluctuation range available as the tree grows in accord with the predictions of the discharge-avalanche model.

Weibull Statistics in Dielectric Breakdown; Theoretical Basis, Applications and Implications

A physical justification for the use of Weibull statistics in the assessment of dieelctric breakdown is presented in terms of a theoretical model of structural fluctuations in non-crystalline materials which has previously been applied to relaxation dynamics in such materials. A number of breakdown mechanisms have been considered and the relationship of the Weibull parameter to experimentally measurable relaxation data is outlined. The equivalence between the statistics of dynamic and static tests is explored and the implications of the Weibull statistics in extrapolating to working conditions noted. It is stressed that an important application of Weibull statistics is that the ability to construct a viable proof test is determined by the value of the time parameter.

Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface

From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.

The fractal nature of the cluster model dielectric response functions

Calculable fractal circuit models are used to show that the cluster model response functions result from the combination of two types of self‐similarity. The analysis is extended to the molecular scale where the cluster model is seen to be based on sequential relaxation processes. An outline is given of the physical origin for such behavior, and the self‐similar processes are identified with the basic concepts of (i) an efficient (compact) exploration of a fractal lattice and (ii) self‐similarity in the contacts between internally connected regions (clusters). The relationship of the cluster model parameters n and m to system dimensionalities are derived for a number of cases.

MODELS OF BIPOLAR CHARGE TRANSPORT IN POLYETHYLENE

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.

A space-charge life model for ac electrical aging of polymers

The dc space-charge model, previously developed by the authors, here is modified to account for the contribution to electrical degradation provided by ac fields. First, the dc model is applied to both dc and ac multistress life test results relevant to a given material. The variations of model parameters from dc to ac data fitting provide indications about the modifications in the degradation mechanisms when passing from a dc to ac regime. Then, a description of aging under ac is achieved through proper assumptions about the space-charge buildup, injection mechanism, and dynamic condition of the polymer lattice. This approach enables the effect of frequency to be accounted for, in a framework where ac aging presents significant analogies with mechanical fatigue. The validity of the ac space-charge model is checked by applying it to the results of accelerated life tests performed on various insulating materials, at different values of voltage, temperature and frequency, on the whole finding very good agreement with experimental data.

Space charge formation and its modified electric field under applied voltage reversal and temperature gradient in XLPE cable

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 cross-interface 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.