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Conduction and Breakdown in Solid Dielectrics, 1989., Proceedings of the 3rd International Conference on

Date 3-6 July 1989

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Displaying Results 1 - 25 of 119
  • On the electrodynamics of partial discharges in voids in solid dielectrics

    Page(s): 107 - 116
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    It is suggested that a correct interpretation of partial-discharge transients can be obtained only through the concept of induced charge. The application of this concept enables a partial-discharge theory to be developed by means of which the influence of relevant void parameters can be assessed in a correct way. It is argued that the widely adopted abc-capacitance model does not allow this insight to be achieved. In addition, it is claimed that the abc-model is based on an erroneous application of the concept of capacitance, and, although this simple approach can be a useful tool when discussing measuring techniques, it may lead to quite incorrect conclusions. The relationship between the transients in the applied voltage and current pulses leading to partial breakdown and the induced charge is found.<> View full abstract»

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  • Electrical properties of some cobalt phthalocyanines

    Page(s): 163 - 167
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    The conductivity and dielectric properties of monomeric and polymeric cobalt phthalocyanine (CoPc) prepared under different conditions are discussed. All CoPc derivatives studied show very high specific conductivity as compared to microscopic-grade CoPc. The dielectric constants are frequency dependent, with the highest response occurring at the lower frequencies of the applied electric field, indicative of nomadic polarization. The dielectric constant increases strongly with temperature.<> View full abstract»

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  • Proceedings of the 3rd International Conference on Conduction and Breakdown in Solid Dielectrics (Cat. No.89CH2726-8)

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    Freely Available from IEEE
  • Fundamental principles and design of a digital PD-measurement system

    Page(s): 244 - 248
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    The advantages that digital signal processing offers for PD (partial discharge) measurement are discussed. A microprocessor-based system for the digital acquisition, storage and processing of PD pulses is described. Particular emphasis is placed on the apparent-charge measurement technique. It is shown that the results obtained are better than those provided by the usual PD measurement technique. A digital system can offer the following advantages over existing instrumentation: high noise suppression, high sensitivity, high accuracy, and high pulse repetition frequency View full abstract»

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  • Time resolved studies of electronic transport in polymeric insulators

    Page(s): 1 - 9
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    Transport in a wide variety of glassy polymeric insulators has been found to be characterized by a convoluted yet familiar pattern of electric field and temperature dependence below Tg, the glass transition temperature, and by a relatively abrupt change in temperature dependence near Tg. Though diverse, all of these glassy dielectrics share the feature that electronic transport is mediated by field-assisted thermally stimulated emission from localized states, a process which remains incompletely understood. By proper `molecular engineering' it has become possible to design dielectric polymers which exhibit efficient and completely trap-free transport, a requisite feature for practical applications to electrophotography. The importance of the mechanistic understanding derived from analysis of time-resolved injection in the historical development of viable organic-polymer-based photoconductors is discussed. Three materials systems are described: (1) poly(N-vinylcarbazole), PVK; (2) molecularly doped polymers; and (3) polysilanes and polygermanes View full abstract»

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  • Digital computer system for measurement of partial discharges in insulation structures

    Page(s): 582 - 586
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    A digital discharge analyzer has been built for continuous and multiparameter registration of partial discharges. The measurements indicate that there are significant differences in the behavior of different discharge parameters for different voids. The registration of phase-position quantities is shown to give more information than just the registration of discharge magnitude versus time. Statistical analysis is shown to be a good tool for describing changes of the phase-position quantities. It is also demonstrated that skewness and kurtosis can discriminate among flat, narrow, and square cylindrical voids View full abstract»

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  • On the influence of storage time on the long-term breakdown characteristics of solid synthetic materials

    Page(s): 102 - 106
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    The author presents results of accelerated voltage tests performed with two model arrangements reflecting conditions of degradation typical for cast epoxy and extruded low-density polyethylene insulations, i.e. internal PD (partial discharge) in regular voids and ET (electrical treeing) from sharp points. As the intervals from the time at which the specimens had been manufactured until the execution of the tests varied from 2 days up to 10 years, the results reflected the influence of storage time on the life of the specimens. No significant variations of inception voltage with storage time was observed for the void arrangement; however they are clearly visible for the point arrangement. Due to the low sensitivity of the method of measurement used in the latter case, differences relevant to sample preparation remained undetected View full abstract»

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  • Tunneling transport in insulators

    Page(s): 10 - 14
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    A model for tunneling transport in insulators that includes the internal field in the tunneling effects is proposed. The model is verified by comparing the values of the estimated parameters in the low-field and high-field cases with experimental results. The relationship between the hopping conduction in terms of internal field and the usual approach of phonon-assisted tunneling is examined. It is noted that quite different situations, such as the Poole-Frenkel effect and tunneling phenomena, call for a unifying picture of all the bulk-limited electronic transport processes in insulators in terms of the internal electric field View full abstract»

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  • Electrical conduction of cellulose under DC field

    Page(s): 182 - 186
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    The temperature gradient of crystal lattice spacings and X-ray reflection intensities has been found to change discontinuously at 150°C for cellulose I (Cell I) and at 100°C for cellulose II (Cell II). These breaks reflect a transition related to the onset of the backbone motion in the crystalline phase. The authors have attempted to confirm this conclusion by measuring the DC conductivity from room temperature to about 230°C for Cell I and Cell II both parallel and perpendicular to the fiber direction. The conduction mechanism is discussed in relation to the absorption current of Cell I under a DC field in the temperature range from 140°C to 230°C, where the DC conductivity becomes relatively high. The results indicate that the increase of carrier mobility above the transition temperature of crystallites arises from the backbone motion in the disordered crystalline phase (mesophase) and that proton transfer is an elementary mechanism for both absorption and steady currents View full abstract»

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  • Partial discharge mechanism in a series insulating structure metal-gas gap-solid dielectric

    Page(s): 407 - 411
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    The author presents test results on partial discharge (PD) intensity in a metal-gas-gap-solid dielectric arrangement at 101.3 kPa and 50.6 kPa. The test was carried out for AC and step voltages. The measurements reveal that the PD generation process in this arrangement is of streamer-plus-surface-discharge character View full abstract»

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  • Discharge energy/material damage considerations in a solid/liquid insulating system

    Page(s): 331 - 335
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    The correlation between the discharge parameters and the corresponding material damage was investigated for small insulating systems (arranged in butt gap formations) of UOPE- (uniaxially oriented polyethylene-) tape/oil systems under both electrical and thermal stresses. These systems were investigated with the aid of a point/plane electrode arrangement. The tests showed that there is a strong correlation between the total surface area damage of the UOPE tapes and the energy dissipated by the discharges. The coefficient of correlation between the total surface area damage and the total energy dissipated in the discharges was between 0.71 and 0.97. This suggests that the measurement of total discharge energy is a very useful technique in evaluating electrical insulation and that it may prove possible to use the technique as a diagnostic for predicting the remaining life of electrical insulation View full abstract»

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  • Fractal processes and Weibull statistics

    Page(s): 538 - 532
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    The use of Weibull statistics to study dielectric failure is discussed. Fractal processes involved in breakdown are considered, with particular emphasis on the stochastic progress of a filamentary discharge or conducting path. Here fluctuations in the length at a given time will lead to a Weibull statistic in time. Electrical trees are used to illustrate some of the points discussed View full abstract»

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  • Examination of the breakdown site in new and aged polyethylene insulated cables

    Page(s): 61 - 65
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    Discharges have been monitored and located in short lengths of cable tested to breakdown. These tests have been used to detect prebreakdown deterioration. The discharge location system is novel and has enabled limited energy breakdown sites to be located. Not all the breakdowns were initiated at the region of the inner screen boundary. In short-term tests to breakdown on new and dry aged cables at a maximum field of 30 MV/m there was no gradual increase in the discharge level above the detection limit of 1 pC. It appears possible that ageing with applied voltage at ambient temperature can improve the breakdown strength View full abstract»

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  • Further results from the modeling of the dielectric response of materials containing mobile charge

    Page(s): 187 - 191
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    Two models, a modified Sterne-Weaver model and the hopping model, have been developed to study the dielectric response of materials containing mobile charge. The first model is the same as the basic Sterne-Weaver model except that both species of charge carrier are free to move. It is shown that by regarding the movement of the charge carrier as a hopping process across a potential barrier of specified height and width, the effects of the applied electric field on the movement of the carriers can be calculated by considering the modifications it causes to the successful-jump probability. The two models behave in a similar fashion under steady-state conditions. This contrasts with the behavior described previously, when lower values of activation energy (higher mobilities) were used. Under those conditions an order of magnitude change was observed in the polarization response of the two models coupled with a considerable difference in the phase relationship of the polarization to the applied field. In addition, a transient component of polarization is observed in the present study that was not observed previously View full abstract»

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  • Water treeing: interaction between test temperature and other test parameters

    Page(s): 500 - 506
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    The authors discuss complementary studies on the effect of temperature on water tree growth, conducted at two different laboratories, using the same LDPE (low-density polyethylene) reference product at 23°C and 65°C, in a point-to-plane geometry. These very similar tests yielded totally contradictory conclusions on the effect of temperature on water tree growth. In one test the water trees propagated more favourably at 23°C than at 65°C. In the other test the water trees grew larger at 65°C than at 23°C. The latter result has been observed exclusively in the case of a wet grounding environment. As this was the only fundamental difference between the tests, the authors hypothesize that the enhanced water tree growth at 65°C is to be due to an enrichment in water at the tip of the growing water tree resulting from water diffusion by dielectrophoresis followed by water condensation under the effect of the electric field View full abstract»

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  • A model of the electrical breakdown process due to electrical treeing growth

    Page(s): 412 - 416
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    The authors develop a mathematical model that can describe the electrical breakdown due to electrical treeing growth in point-plane electrode specimens of thermoplastic polymers subjected to AC voltage ramps. The damage due to the tree growth is modeled as a conductive sphere, allowing an easy computation of the geometrical field in front of it. The breakdown voltage VBD, the radius of the sphere Rb, and the electric field on the sphere AEb are related by means of the simple equation VBD(D)=AEb×R b(D), where D is the interelectrode gap. The model has been verified experimentally View full abstract»

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  • New exact computer analysis of the TSC curve by the asymptotic estimation method and an approach to the electrical conduction of organic crystals

    Page(s): 587 - 591
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    Experimental results were obtained for a naphthacene-doped anthracene single crystal excited by Q-switched ruby laser pulses. Two representative TSC (thermally stimulated current) curves were obtained. The curves were analyzed by an asymptotic estimation method, involving the autoseparation of the TSC curve and the exact estimation of the energy depth of the carrier trap from the separated curve View full abstract»

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  • Electrical ageing of polyethylene during constant AC stress

    Page(s): 50 - 54
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    The authors investigated the ageing of polyethylene under inhomogeneous electric fields well below the intrinsic breakdown field strength (>570 kV/mm RMS) but above a local critical field strength at which charge injection takes place. They observed partial discharge and the treeing behavior under different conditions of AC and DC stressing. Additional experiments with samples impregnated with an electronegative gas (SF6) yielded further information on the process of polymer degradation. The results show that there is no deterioration during DC ageing; the polarity reversal that occurs under AC stressing is necessary for the ageing process. The observation of brittle walls of the tree structure indicates that the rate of impingement of hot electrons on the polymer is relatively low. Therefore the heat produced may be easily dissipated without appreciable rise of the temperature within the treed region. The experiments with the impregnated samples are in agreement with the assumption of a hot electron process. The SF6 captures hot electrons and therefore reduces the probability of damage; in the present case no deterioration is observable up to 600 h View full abstract»

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  • DC characteristics of cable insulating materials

    Page(s): 66 - 71
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    The insulation performance of PE (polyethylene) under DC voltage was investigated using sheet samples and miniature cables. Pressed sheets of various kinds of PE, which differed in material density and polymerization process, were prepared, and their thermally stimulated current and breakdown strength were measured. To confirm that insulating properties obtained in the experiments using sheet samples can also be obtained in thicker samples, experiments were conducted using miniature cables with 3.5-mm insulation thickness and a conductor cross-sectional area of 60 mm2 to evaluate breakdown strength and to measure the thermally stimulated current. Both sets of results showed that HDPE (high-density PE) has excellent insulating properties under DC voltage. The breakdown strength is high and the space charge accumulation is small. These properties of HDPE are considered to be due to its molecular structure, with its short lengths of molecular branches View full abstract»

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  • Diagnostics of the volume distribution of conductivity in solids by electroacoustic methods

    Page(s): 565 - 568
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    The principles underlying the use of the acoustic probe and electrostriction methods for the nondestructive measurement of the volume distribution of the electric field intensity and charge density in solids are discussed. Acoustic probing consists of recording an electric signal generated by a charge or polarized sample under the action of a mechanical pulse causing a compression wave. The electrostriction method is based on recording an acoustic signal in the surface of an electrified sample which is affected by an electric pulse. Analysis shows that the two methods are equivalent as regards their physical basis and information provided. The fields of application of these two methods are shown to be somewhat different. Acoustic probing is more correct and technically simple when high-ohmic materials with a long charge relaxation time are under investigation. As an example, the study of diffusion in the nitric-acid-polyethylene system is considered View full abstract»

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  • Review of statistical testing of solid insulating materials

    Page(s): 574 - 581
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    Different statistical methods used for the determination of the insulation characteristics of solid insulating materials are reviewed. Experimental results reported in the literature indicate that the breakdown probability of the test voltages is a function of the test method, of their parameters, and of the nature of the breakdown probability function. The application of statistical methods to establish the insulation strength and lifetime is described. Techniques for determining withstand voltages of electrical insulation and for determining the low probability of breakdown of solid insulation from critical flashover data are discussed. The application of the Weibull distribution for the description of breakdown data is examined. It is noted that great care must be used in interpreting statistical test results, due to the flexibility of low-breakdown-probability test methods. Example test results are presented for three groups of epoxy specimens View full abstract»

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  • Influence of physical aging processes on electrical properties of polymers

    Page(s): 197 - 201
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    The influence of aging temperature and time on the electrical properties of amorphous polydiancarbonate (PC) and poly(methyl methacrylate) (PMMA) was investigated using the techniques of thermally stimulated depolarization currents, transient currents, and AC dielectric losses. It is shown that in the temperature range located between the α relaxation (glass transition) and the secondary β relaxation, the polarization and loss properties of amorphous polymers undergo pronounced changes during physical aging. A progressive decrease in molecular mobility (and corresponding increase in relaxation times) leads to a marked decrease in polarization and dielectric losses as a function of aging time, showing that physical aging is a general phenomenon which should not be ignored in any testing of electrical properties of plastics View full abstract»

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  • Development of TSC measurement and data analysis system

    Page(s): 254 - 258
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    A computerized TSC (thermally stimulated current) measurement and data analysis system is described. The computer not only eliminates the X-Y recorder, but also allows data averaging, data storage, comparison with other data, and data processing for a variety of purposes. Computerization makes the data as reproducible as possible, speeds up data processing, provides accurate analysis, and allows mass data filing. System software and hardware are discussed, and data display and analysis are described View full abstract»

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  • Determination of charge carrier mobility in doped low density polyethylene using DC transients

    Page(s): 192 - 196
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    Charge carrier mobility was determined for plain and doped low-density polyethylene (LDPE) using DC transient currents. Barium titanate was used as a strongly polar dopant and titanium dioxide as a semiconductor dopant. The values of the mobility obtained were on the order of 10-10 cm2 v-1 s-1. Results indicate that the inclusion of 1% by weight of BaTiO3 and TiO2 has a considerable effect on the conduction properties of the polymer. BaTiO3 increased the charge carrier mobility by a factor of three and also increased the conductivity of the polymer. TiO2 increased the charge carrier mobility by a factor of five. Charge trapping and space charge formation were modified by the introduction of titanium dioxide View full abstract»

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  • An experimental investigation into field enhanced ionic migration

    Page(s): 507 - 511
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    Experiments have been carried out to measure the effects of field and field gradient on ionic penetration in a practical cable insulation material, namely, cross-linked polyethylene (XLPE). Application of an alternating electric field is shown to increase the flux of ions into XLPE. Flux enhancement is dependent on the field gradients present in the system. The flux also increases as the temperature is raised. The experimental results are in a qualitative agreement with the mechanism of field- and field-gradient-assisted ion migration proposed by R.A. Fouracre et al. (1986) View full abstract»

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