Leonard A. Dissado

Anomalous low-frequency dispersion. Near direct current conductivity in disordered low-dimensional materials

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.

Fast and slow charge packets in polymeric materials under DC stress

The presence of slow space charge packets crossing the insulation thickness from one electrode to the other and causing significant electrical field distortion has been reported already in several papers. They are activated in general by very high dc fields or, in highly polluted materials, by relatively low fields and constitute an important ageing factor, concerning dc electrical stress. It has been observed, in fact, that such packets can cause accelerated breakdown of insulation. The development of fast systems for space charge measurements has allowed the presence of almost instant heterocharge to be observed close to electrodes in certain field and temperature conditions, especially in cable models. This has been explained often by the separation of ionic charge populations, even though such heterocharge appears also in materials, such as Polyethylene or crosslinked Polyethylene that represent the best extra-clean technologies. The measurements reported here use a high speed technique to investigate the build up of heterocharge in model cables that have been treated to remove volatile chemical species. They show that in fact the heterocharge is built up by many very small and very fast charge packets (i.e. charge packets having a high mobility), which are injected from both electrodes and cross the insulation in less than one second. Because the packet charge is unable to exit the counter-electrode at the same rate at which it arrives, hetero-charge is built up within just a few seconds from the beginning of the polarization. The mobility of these charges, depending significantly on temperature, is estimated through observation of charge packets as a function of time, and compared with that of the already-known slow packets, generally occurring at higher fields with respect to fast packets. The basis for the interpretation and modelling of such phenomena is discussed.

Effect of long-time electrical and thermal stresses upon the endurance capability of cable insulation material

This paper presents the results of endurance tests that have been carried out on cross-linked polyethylene (XLPE) cable peelings. The peelings were taken from cables that were manufactured from a single batch of XLPE and subjected to electrical (up to 28 kV/mm), thermal T = 363 K (90degC) and electro-thermal stressing for at least 5000 hours. The endurance tests of the peelings (thickness 150 mum) were carried out at the same temperature of T = 363 K as the thermally stressed cable, but at two different AC electrical fields of 55 and 70 kV/mm. The resulting life data for the different sample sets are compared to one another and to that of peelings taken from unaged cables. Weibull analysis of the failures shows that only peelings from cables that had experienced a thermal stress component during their time of stressing as a cable, exhibited a statistically significant reduction in endurance capability. Possible reasons for this reduction of life are discussed.

Debye and non-Debye relaxation

Debye relaxation behaviour is unique in the sense that all memory of excitation is instantaneously lost. It is shown that the presence of higher-frequency relaxations for components of the same dipole degrades this property by extending the memory into the timescale of relaxation, thus giving a nonDebye behaviour which can be expected to occur in all practical dielectric materials. In this case it is not possible, as has been suggested, that knowledge of the polarisation or of the depolarisation current at a single time is sufficient, alone, to define the relaxation behaviour of the system for all subsequent times. Furthermore the Guo and Guo experiment (1983) is shown to be incapable of discriminating between this picture of sequential relaxation and the alternative possibility of an ensemble of independent relaxations.

Relaxation in elastic and viscoelastic materials

The mechanical compliance and modulus retardation/relaxation functions are examined in terms of a general behaviour which contains more than one process. An analytical approach to the transformation in the anelastic response between the compliance and the modulus is derived and applied to a cooperative model of relaxation behaviour. In particular it is shown that mechanical viscoelasticity is equivalent to the anomalous low frequency dispersion process that has been observed in dielectrics containing quasifree charges. Comparison with published experimental data over a wide range of solid materials shows the validity of the cooperative model to mechanical relaxation.

The low-frequency dielectric properties of leaves

The dielectric properties of a range of plant foliage have been examined in the frequency range between 10−2 Hz and 104 Hz. The results are reported and analyzed in terms of circuit combinations of dispersive dielectric elements. In all cases some type of charge transport contributed a component to the combination, but apart from some of the evergreen samples this component was not that of a frequency-independent de conductivity. For both the annual and deciduous plant leaves the principal transport process was found to have the frequency dependence characteristic of ‘quasi-dc transport’, and this process dominated the observed dielectric disperison in these samples. The observed behaviors are discussed in terms of the known properties and structure of leaves and it is shown that the dielectric technique can be a useful and informative investigative technique in these complex systems.

Effect of electrical and thermal stressing on charge traps in XLPE cable insulation

Insulation peelings were taken from a single 90 kV ac transmission cable whose segments had been stressed for between ~8000 and 10 000 h either at elevated temperature (363 K) or field 19.5 kV/mm (rms) (T= 293 K). Control peelings were taken from a cable segment that had experienced no stressing. The various peelings were subjected to an endurance test at 70 kV/mm (rms) and T = 363 K, which demonstrated that only those peelings that had experienced thermal stressing had a reduced lifetime compared to the set of control peelings. Space charge measurements made prior to the endurance test showed only negative space charge in all types of peelings and an analysis of the decay dynamics in the control set showed that it resided in two trap distributions at 0.8 to 0.9 eV and 1 to ~1.5 eV. Peelings from a second endurance test (55 kV/mm (rms), T = 363 K) were suspended when the test was terminated at 6088 h. Space charge measurements showed that these samples exhibited positive as well as negative space charge with the positive charge being ~60% of the total. The space charge decay dynamics also showed two trap distributions, ~0.65eV to 0.85 eV and 1eV to ~1.3 eV, with both positive and negative charge in each energy range. Detailed differences in the trap energy range and density in both distributions were found between peelings with different cable histories, and those differences that correlate with a reduction of endurance life are identified. A tentative explanation in terms of anti-oxidant consumption and trap generation by chemical reaction is proffered.

Influence of adsorbed water on the dielectric response of a ceramic material

The dielectric response of a porous ceramic has been measured in the frequency range 10–2–105 Hz and in the temperature range 290–900 K when both dry and exposed to water. These measurements have allowed the frequency dependence of the susceptibility and the temperature dependence of its magnitude and characteristic frequency to be determined. In all cases an anomalous low-frequency dispersion has been observed and substantial differences have been found between the response of the sample when dry and when containing adsorbed water. The quantitative dependence upon water content has been determined, and a tentative explanation of the behaviour is presented in terms of imperfect transport within an adsorbate system. The applicability of this model to other types of dielectrically active system which contain adsorbed species is outlined.

Space charge accumulation due to ultra-fast charge packets in XLPE insulated cables: the effect of temperature and field

Small but repetitive charge packets injected from both electrodes, that cross insulation very rapidly and accumulate at the counter electrodes, are observed in polyethylene-based insulating materials. The results presented in this paper show that the charge packet amplitude remains almost independent of field and temperature, while mobility and repetition rate are thermally activated quantities. This indicates that a minimum quantity of charge is needed for the packet to be injected and transported through the insulation, while at higher temperatures the number of packets per unit time and the mobility increases.

Study of the factors that suppress space charge accumulation in LDPE nanocomposites

Two mechanisms have been proposed to explain the suppression of space charge in polyethylene by the addition of nano-fillers, i.e. an interface change that reduces charge injection and a bulk modification that affects charge migration and recombination. The relative importance of each mechanism in Low Density Polyethylene (LDPE) nanocomposites is investigated by the measurement of space charge in samples of double-layer and triple-layer structures. The experimental results show that the nano-fillers reduce charge injection, but do not entirely eliminate the space charge. Bulk changes also play an important role in space charge suppression. The space charge near the interface between the unfilled LDPE and nanocomposites shows a surface charge at 20 °C, but at 60 °C a bipolar blocking phenomenon is observed in some circumstances. It is shown that these space charge distributions can be reproduced using a band theory approach with the assumption that the introduction of deep traps by nano-fillers raises the Fermi level of the nanocomposite towards the conduction level.