John C. Fothergill

Internal Charge Behaviour of Nanocomposites.

The incorporation of 23 nm titanium dioxide nanoparticles into an epoxy matrix to form a nanocomposite structure is described. It is shown that the use of nanometric particles results in a substantial change in the behaviour of the composite, which can be traced to the mitigation of internal charge when a comparison is made with conventional TiO2 fillers. A variety of diagnostic techniques (including dielectric spectroscopy, electroluminescence, thermally stimulated current, photoluminescence) have been used to augment pulsed electro-acoustic space charge measurement to provide a basis for understanding the underlying physics of the phenomenon. It would appear that, when the size of the inclusions becomes small enough, they act co-operatively with the host structure and cease to exhibit interfacial properties leading to Maxwell-Wagner polarization. It is postulated that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through nano-filled dielectrics. The practical consequences of this have also been explored in terms of the electric strength exhibited. It would appear that there was a window in which real advantages accrue from the nano-formulated material. An optimum loading of about 10% (by weight) is indicated.

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.

Estimating the cumulative probability of failure data points to be plotted on Weibull and other probability paper

Using both an exact technique and Monte Carlo simulations to estimate the cumulative probability of failure of the ith of n identically stressed samples, it is shown that the approximation F(i,n)=(i-0.3)/(n+0.4) is considerably more accurate than other commonly used approximations. The significance of this in the interpretation of electrical breakdown tests is demonstrated. >

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.

Filamentary electromechanical breakdown

A breakdown mechanism is proposed in which a filamentary-shaped crack propagates through a dielectric, releasing both electrostatic energy and electromechanical strain energy stored in the material due to the applied electric field. The mechanism predicts a breakdown strength which is proportional to the fourth root of Youngs modulus and a time to breakdown comparable with that taken for a sound wave to cross the dielectric ( approximately=0.1 mu s in 25 mu m thick polyethene films). Analysis of previously unexplained results of M. Hikita et al. (1985) on thin films of polyethylene yields good agreement with the proposed mechanism and a filament radius which is comparable to the gross morphological features of the dielectric. >

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.

Charge distribution and electroluminescence in cross-linked polyethylene under dc field

The intent of this paper is to cross-correlate the information obtained by space charge distribution analysis and electroluminescence (EL) detection in cross-linked polyethylene samples submitted to dc fields, with the objective to make a link between space charge phenomena and energy release as revealed by the detection of visible photons. Space charge measurements carried out at different field levels by the pulsed electro-acoustic method show the presence of a low-field threshold, close to 15-20 kV mm-1, above which considerable space charge begins to accumulate in the insulation. Charges are seen to cross the insulation thickness through a packet-like behaviour at higher fields, starting at about 60-70 kV mm-1. EL measurements show the existence of two distinct thresholds, one related to the continuous excitation of EL under voltage, the other being transient EL detected upon specimen short circuit. The former occurs at values of field corresponding to charge packet formation and the latter to the onset of space charge accumulation. The correspondence between pertinent values of the electric field obtained through space charge and EL analyses provides support for the existence of degradation thresholds in insulating materials. Special emphasis is given to the relationship between charge packet formation and propagation, and EL. Although the two phenomena are observed in the same field range, it is found that the onset of continuous EL follows the formation at the electrodes of positive and negative space charge regions that extend into the bulk prior to the propagation of charge packets. Charge recombination appears to be the excitation process of EL since oppositely charged domains meet in the material bulk. To gain an insight into specific light-excitation processes associated with charge packet propagation, EL has been recorded for several hours under fields at which charge packet dynamics were evidenced. It is shown that current and luminescence oscillations are detected during charge packet propagation, and that they are in phase. The mechanisms underlying EL and charge packets are further considered on the basis of these results.

Electric field criteria for charge packet formation and movement in XLPE

The formation of space charge packets in crosslinked polyethylene (XLPE) tapes from un-aged cable insulation has been studied utilizing the pulsed electroacoustic (PEA) technique. The 150 /spl mu/m thick sheets were studied under constant applied DC field of 120 kV/mm at a temperature of 20/spl deg/C for a period of 48 h. After an inception period of /spl sim/3.5 h, during which heterocharge accumulates at the anode and increases the local field there, a sequence of positive charge packets was observed to transit the sample starting from near the anode. Calculation of the internal field showed that the packets required a field of /spl ges/140 kV/mm for their initiation. Reduction of the applied field step-wise from 120 to 80 kV/mm indicated that the charge packet would keep moving as long as the local field at its front was >100 kv/mm, but with a reducing magnitude. A return to an applied field of 120 kV/mm confirmed that the local field required to initiate a new packet was >135 kV/mm. The results are discussed in terms of current theories of charge packet formation. The first packet appears to be a moving front of field ionization. The generation of subsequent packets is governed by the field at the anode and the balance of charge injection and extraction process, which occur there. The nature of the negative charges produced at the ionization front is not clear, but they are unlikely to be electrons.

Ageing, Space Charge and Nanodielectrics: Ten Things We Don't Know About Dielectrics

Three areas of considerable progress are identified in the area of polymeric insulation since the mid 1990s (i.e. since the inception of the Eric O. Forster Memorial Lectures): the understanding of ageing, the measurement of space charge, and the development of nanodielectrics. The paper proposes a typology to distinguish and define electrical ageing, degradation and breakdown. With reference to the principal theories of therm o-ageing, it is concluded that there is little direct evidence for ageing even though it is likely to exist, at least above a critical field. The capability to measure space charge has improved considerably and this shows considerable potential as a diagnostic tool. Space charge behaviour is extremely difficult to analyse and general predictions may be impossible in practice. It is, however, necessary to control space charge accumulation in HVDC systems. Nanodielectrics continue to offer promise and understanding is improving in this area, particularly in the development of models of interaction zones around nanoparticles.

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps