Showing papers in "IEEE Transactions on Electrical Insulation in 1987"
TL;DR: In this article, the authors measured the amount and approximate distribution profile of charges in electron-beam irradiated polymethyl methacrylate (PMMA) samples and obtained the height of the first acoustic signals produced in response to a HV impulse applied to the sample.
Abstract: The distribution profile and quantity of charges in insulators have been studied by various methods, some of them utilizing thermal, mechanical, and electrical stimulations. A new method, incorporating pulse electric stimulations and pulse acoustic detection, is reported here along with preliminary results of the measurement of the amount and approximate distribution profile of charges in electron-beam irradiated polymethyl methacrylate (PMMA) samples. The amount and the mean penetration depth of charges in the sample are obtained from the height of the first acoustic signals produced in response to a HV impulse applied to the sample. A 5 mm thick PMMA plate irradiated with a 1 MeV electrons at 0.5 to 3.0 Mrad was found to trap charges of approximately -0.3 ?C/cm2 at a mean depth of about 2 mm.
124 citations
TL;DR: In this paper, the well known space-charge-limited-current model is generalized in several steps to field-dependent mobility, negative differential resistance and impact ionization, and a fractal model for breakdown is introduced.
Abstract: In this review we concentrate on simple phenomenological models which lead to testable predictions. We discuss inhomogeneous field situations since in practice breakdown and aging starts as points of local field enhancement. The well known space-charge-limited-current model is generalized in several steps to field-dependent mobility, negative differential resistance and impact ionization. A fractal model for breakdown is introduced. The model contains a small number of physically transparent parameters.
119 citations
TL;DR: A review of very recent electret work can be found in this paper, where a history-oriented introduction refers to important early studies of charge storage in insulators; a number of relevant surveys are listed for further information on previous electret research.
Abstract: The present review of very recent electret work starts with a history-oriented introduction which refers to important early studies of charge storage in insulators; a number of relevant surveys are listed for further information on previous electret research. This paper concentrates on work published during the last few years and especially on papers not covered in earlier reviews. New electret materials and novel experimental techniques are briefly discussed; here, inorganic substances and methods for the nondestructive determination of charge or polarization profiles are given special consideration. Recent developments on the subject of charge storage and transport, and of radiation effects in electrets, are reported. A major topic is the ongoing investigation of piezo-, pyro-, and ferroelectricity in polymer electrets where many new results were obtained during the last few years, The paper concludes with a discussion of a small number of interesting electret applications which may serve as examples for the large amount of device applications suggested in the literature.
82 citations
TL;DR: In this paper, the authors examined the insulation from six 5 kV power cables, which has been in service underground for 6 to 8 years, by infrared (IR) spectroscopy and oxidation induction time (OIT) analysis.
Abstract: The insulation from six 5 kV power cables, which has been in service underground for 6 to 8 years, was examined by infrared (IR) spectroscopy and oxidation induction time (OIT) analysis. Sections of insulation containing water trees were found to contain high levels of ionic contaminants. All insulation samples showed evidence of oxidative degradation in service and frequently there was a higher than average level of oxidation in the treed regions of the insulation. Sections of the insulation containing water trees had appreciably shorter OIT's than untreed regions, indicating that they were more prone to subsequent oxidative degradation. A model for water tree formation and electrical breakdown of the insulation is described where oxidative degradation during prolonged service reduces the ability of the insulation to withstand stress concentrations at defects, and water trees are initiated. Some localized oxidation may accompany the tree propagation step. Extensive localized oxidation then takes place in the treed regions, catalyzed by ionic contaminants, and insulation failure occurs.
77 citations
TL;DR: In this paper, a critical review is presented on the various electrical methods available for partial discharge detection and measurement in terms of their chronological development and application on cable, capacitor, transformer, and machine insulation specimens.
Abstract: A critical review is presented on the various electrical methods available for partial discharge detection and measurement in terms of their chronological development and application on cable, capacitor, transformer, and machine insulation specimens. A substantial portion of the effort is devoted to corona pulse detection techniques, because these constitute the most popular forms of corona measurement. It is pointed out, however, that under some conditions discharges may assume a pulseless or pseudo-glow type character and that in such circumstances pulse detection procedures become ineffective and must be replaced by appropriate bridge methods.
75 citations
TL;DR: In this paper, the electric properties of polymers are reviewed from a viewpoint of electronic conduction, and the effects of space charge effects on electrical conduction and breakdown phenomena are discussed.
Abstract: Electric properties of polymer materials are reviewed especially from a viewpoint of electronic conduction. Carrier in jection is a major electronic carrier source in insulating polymers. The barrier height for injection is generally as high as 2 to 3 eV and the surface states affect the interface between polymer and metal. Space charges often accumulate in polymers or at the interface of polymer-metal and polymer polymer. They modify the internal electric field so strongly that space charge effects are significant not only in the electrical conduction and breakdown phenomena, but also in the piezoelectricity of polymers.
73 citations
TL;DR: In this article, a new statistical method of lifetime estimation of a solid electrical insulation using data from an accelerated aging test is described, which simultaneously uses the maximum likelihood estimation and the Newton-Raphson methods.
Abstract: This paper describes a new statistical method of lifetime estimation of a solid electrical insulation using data from an accelerated aging test. The method simultaneously uses the maximum likelihood estimation and the Newton-Raphson methods. The novel aspect of this work is the introduction of a new threshold term in addition to well-known estimation techniques based on the inverse power model combined with the Weibull distribution. Thus, we can estimate the threshold value with a confidence interval. In order to reduce time and costs of voltage-lifetime test5, an accelerated testing procedure with type I censoring is considered.
66 citations
TL;DR: In this article, the degradation of XLPE resulting from its exposure to partial discharges, leads to the formation of solid, liquid and gaseous products that result directly from the discharge synthesis reactions taking place within the intervening electrode gap space.
Abstract: It is shown that the degradation of XLPE, resulting from its exposure to partial discharges, leads to the formation of solid, liquid and gaseous products that result directly from the discharge synthesis reactions taking place within the intervening electrode gap space. The prime reagents in the discharge synthesis process are found to be CO2, CO, H20 and acetophenone. In order that these synthesis reactions occur without any impediment, a non-vented test cell is required; this condition is complied within most physically real cavities occluded within the insulation systems of XLPE cables.
56 citations
TL;DR: In this paper, Kerr electro-optic field mapping measurements have been performed in electron-beam-irradiated polymethylmethacrylate to understand electron-caused discharges along insulating solid surfaces on spacecraft as well as in solids.
Abstract: To understand electron-caused discharges along insulating solid surfaces on spacecraft as well as in solids, Kerr electro-optic field mapping measurements have been performed in electron-beam-irradiated polymethylmethacrylate. The accumulated trapped charge results in large self-electric fields of the order of 1 to 3.5 MV/cm. Measurements were done at electron-beam energies up to 2.6 MeV at low (20 nA/cm/sup 2/) and high (300 nA/cm/sup 2/) current densities and for sample thickness of 0.635, 0.95, 1.27 and 2.54 cm. It was found that volume breakdown through the electron charged region may occur spontaneously when accumulated charge levels are high. Typically at breakdown the net charge density magnitude exceeded 1 mu C/cm/sup 2/ and internal fields exceeded 1.8 MV/cm. Charge dynamics during electron-beam irradiation was also analyzed on the basis of a simple model consisting of the continuity and Poisson's equations including charge generation and radiation-induced conductivity. >
56 citations
TL;DR: In this article, a good correlation was found between the amount of gas generated from insulating papers in insulating oil and the retention of tensile strength and degree of polymerization.
Abstract: Insulating papers and pressboards must have high dielectric strength and high tensile strength. However, these materials are gradually degraded due to thermal stress, oxygen, and moisture. Because the tensile strength decreases owing to degradation, the degradation of insulating material may affect the life of transformers. In our laboratory, through accelerated tests using models of oil-impregnated insulating systems, changes in characteristics of insulating cellulose materials have been investigated. These data were compared with data obtained from insulating papers of transformers with long service life. From these investigations, good correlation was found between the amount of gas generated from insulating papers in insulating oil and the retention of tensile strength and of degree of polymerization. Using this correlation, the degree of degradation of insulating papers in transformers may be known from the amount of gas. The average characteristic curves of insulating papers from transformers coincides with the degradation curve of 90 ° C ob tained in experiments. And if the life of insulating papers is considered to be equal to the life of transformers, the life of transformers is between 20 and 40 years.
56 citations
TL;DR: In this paper, the insulation strength of solid dielectric spacers, sand wiched between electrodes and stressed by dc voltage, can be influenced by surface charges building up during voltage application.
Abstract: The insulation strength of solid dielectric spacers, sand wiched between electrodes and stressed by dc voltage, can be influenced by surface charges building up during voltage application. The measurement of these trapped charges is diffi cult during the charge accumulation process, but may be car ried out by means of well-defined probes while interrupting the voltage application. The field can be calculated numerical ly, starting from the trapped charge and employing suitable boundary conditions during voltage application. A comparison with measured quantities can contribute to an explanation of the physical charging mechanism. The surface charge simulation method which was used in this analysis is quite obviously an essential tool for the treatment of this and related problems.
TL;DR: In this paper, a dependence of the electric strength on thickness, area and volume of dielectric is demonstrated for polypropylene film, and it is shown that an increase of the volume caused by increasing the thickness of the polyethylene is much more significant than the same increase caused by an increase in area.
Abstract: A dependence of the electric strength on thickness, area and volume of dielectric is demonstrated for polypropylene film. It is found that the electric strength of theoly polymer film decreases with volume. It is shown that theunc function of electric strength is not simply dependent on the volume as obtained from theory of extreme values, but that an increase of the volume by increasing the thickness of the dielectric is much more significant than the same increase of the dielectric volume caused by an increase in area.
TL;DR: In this paper, the concept of the kinetic compensation effect, known from the literature of chemical reaction kinetics, has been extended to the field of insulation aging, and it is proposed for the interpretation of life data and further study of this phenomenon.
Abstract: The concept of the kinetic compensation effect, known from the literature of chemical reaction kinetics, has been extended to the field of insulation aging. Observations on data available in the electrotechnical literature are reported concerning the linear correlation (aging compensation effect: ACE) between the regression coefficients ? (ordinate intercepts) and b (slopes) of life lines derived by measuring different properties, or by different measurements of the same material. It is proposed here to introduce ACE for the interpretation of life data and the further study of this phenomenon.
Abstract: A method is described for optimizing the field stress on HV insulators by modifying their profile, seeking a uniform distribution of the tangential field among the insulator surface. This results in an increase of the onset voltage for surface flashover and in a significant saving of the space of the HV installation. The optimization process was achieved by an algorithm developed for calculating the tangential field Component and mathematical expressions of the profile to be corrected through an iterative procedure. The algorithm was based on a modified charge-simulation technique to satisfy a better matching of the boundary conditions to the electrode and insulator surfaces involved in the HV installation. The algorithm is expanded to study the effect of contamination on the tangential field distribution. It is found that the higher the conductivity of the contamination layer, the higher is the field uniformity along the insulator surface.
TL;DR: The literature relevant to the use of insulators and dielectrics in a space environment is reviewed and discussed in terms of specific aspects of the space environment such as temperature, pressure, radiation, corrosion and meteoroid flux.
Abstract: The literature relevant to the use of insulators and dielectrics in a space environment is reviewed and discussed in terms of specific aspects of the space environment such as temperature, pressure, radiation, corrosion and meteoroid flux. Whereever possible, data are presented from space experiments or simulators designed to duplicate as closely as possible the space environment. Data from high energy physics, nuclear power and electric utility communities are included where relevant. The need for accelerated aging methodology for a multistimulus/multifactor stressed environment is also discussed.
TL;DR: In this paper, the degradation of spacer materials exposed to either direct arcing conditions or arc by-products was investigated to correlate their surface molecular features with changes in electrical performance.
Abstract: SF6 gas-insulated switchgear is a sealed system, but low- level contamination can accumulate and affect spacer performance. The degradation of a number of spacer materials exposed to either direct arcing conditions or arc by-products was investigated to correlate their surface molecular features with changes in electrical performance. Electron Spectroscopy for Chemical Analysis, Scanning Electron Microscopy, Infrared and Thermal Analysis were used to probe the surface of the dielectrics. In the immediate vicinity of the spark, spacer surfaces undergo irreversible chemical, morphological, and physical changes with extensive pyrolysis and surface erosion. There is clear evidence of chemical reactions with high concentrations of fluorine bonded to the organic resin. Exposure of spacer materials to SF6 arcing by-products results in significant reduction in electrical properties with profound changes in the chemical structure of both epoxy moieties and/ or filler particles. This paper will describe the surface characterization techniques as applied to degraded spacer surfaces and discuss the possible degradation mechanisms.
TL;DR: In this paper, it was shown that the activation enthalpy and entropy of each relaxation are linearly related -the compensation law -and that the energy parameter typical of a given relaxation is the Gibbs free energy change?G.
Abstract: Polyethylene relaxation results are shown to be described very well by the rate theory associated with the name of Eyring. In agreement with this theory the oa, B, and ? relaxations have constant AH and AS values as long as the results are plotted properly, i. e. as log(?/T) vs 1/T. Since AS values are far from being negligible this means that the energy parameter typical of a given relaxation is the Gibbs free energy change ?G. It is shown that the ?G values of the ? relaxation are linearly related with crystallinity and not with crystallite thickness. In agreement with our theoretical model, the activation enthalpy and entropy of each relaxation are linearly related - the compensation law. It is concluded that important information on polymer relaxations can be gained from the ?S-?H plots.
TL;DR: In this article, the initiation of pre-breakdown streamers at a point cathode in liquid hydrocarbons is described. Butler et al. used a computer implementation of the method of images, and computed the electric field for selected geometries to confirm that the field strengths in the vicinity of these streamers are probably high enough so that electron multiplication processes can occur.
Abstract: Measurements are presented of the initiation of prebreakdown streamers at a point cathode in liquid hydrocarbons. Using a computer implementation of the method of images, the electric field is computed for selected geometries to confirm that the field strengths in the vicinity of these streamers are probably high enough so that electron multiplication processes can occur. High magnification photographs of streamer initiation show that the initial streamer velocity is (2.8±0.4) x 104 cm/s in toluene.
TL;DR: In this article, a multichannel analyzer was used to study the statistical variations of corona pulse amplitudes for AC corona in point-plane air gaps, and the results for corona under AC voltages were compared with previously obtained results for DC corona.
Abstract: Using a multichannel analyzer, statistical variations of corona pulse amplitudes are studied for AC corona in point-plane air gaps. It is shown that, for each half cycle, corona pulse amplitudes generally follow a Gaussian type of probability distribution when the applied voltage is near the corona inception level. However, as the applied voltage is increased, the distributions show deviations from this behavior. Typical pulse amplitude parameters such as the average value, the standard deviation, and the coefficient of variation are measured for different experimental conditions, and the results for corona under AC voltages are compared with previously obtained results for DC corona. >
TL;DR: In this paper, the electrical conductivity and dielectric loss of polyether ether ketone (PEEK) films were investigated over a wide range of frequencies and electric fields in the temperature range of 40 to 140°C.
Abstract: The electrical conductivity and dielectric loss of films of polyether ether ketone have been investigated over a wide range of frequencies and electric fields in the temperature range of 40 to 140°C. The volume resistivity of PEEK was found to be 1014 ?m at temperatures 80°C but decreased significantly above this temperature. The dielectric loss factor was found to be 0.01 in the frequency range 100 Hz to 100 kHz, but two lowfrequency loss processes were observed by thermally stimulated discharge measurements. The excellent high-temperature mechnical properties suggest that PEEK may be useful as the insulating material on low voltage communication cables in hostile environments, but not for power or HV distribution cables over long distances where higher resistivities may be required.
TL;DR: In this article, the secondary emission yield of dielectrics was determined for Teflon, Aclar, Kapton, and Mylar with a single beam energy.
Abstract: A new method for the determination of the secondary emission yield of dielectrics is discussed. Results are given for Teflon® , Aclar, Kapton® , and Mylar® . Surfaces of dielectrics irradiated with electrons of energies above 2 keV become negatively charged. For lower energies the number of backscattered primaries plus backward emitted secondaries eventually exceeds that of the incoming primaries. Then the surface can be positively charged. It was recognized that this effect might lead to a simple method for the determination of the total backscatter plus secondary emission yield. This effect is used to determine the total emission yield curve in one run with a single beam energy.
TL;DR: In this article, the authors used thermally and optically stimulated exoelectron emission to detect hot-electron-induced chemical degradation of hydrocarbon films and found that radiation-induced electron traps are found if the films are irradiated with hot electrons above a threshold energy of 3.5 to 4 eV.
Abstract: Charge trapping in thin dielectric films on metal substrates is investigated by thermally and optically stimulated exoelectron emission. The techniques are used to detect hot-electron-induced chemical degradation of hydrocarbon films. Radiation-induced electron traps are found if the films are irradiated with hot electrons above a threshold energy of 3.5 to 4 eV.
TL;DR: In this paper, a model in which space charge flow across water-polymer interfaces causes electrochemical reactions such as partial oxydation was proposed, and the morphology of the water tree was determined by microphase phase separation of the partially oxydized material.
Abstract: We demonstrate that the electrostatic stress in model structures representative for water trees can never reach the yield stress of polyethylene. We propose a model in which space charge flow across water-polymer interfaces causes electrochemical reactions such as partial oxydation. The morphology of the water tree is determined by microphase phase separation of the partially oxydized material.
TL;DR: In this article, a simple model based on the drift of charges in the material and the sign reversal of the Coulomb force acting on them is presented for the return voltage due to electrons injected at known depths in polymer sheets by an electron gun.
Abstract: Charged capacitors which have been temporarily short-circuited display a "return voltage" buildup which may cause severe shocks. This old phenomenon has been reinvestigated on various materials (polyethylene, polyester, Kapton®, etc.) poled by different techniques (HV, corona, plasma, etc.) after different poling times. Return voltage buildup is observed on all insulating (polar and non polar) materials. A simple model based on the drift of charges in the material and the sign reversal of the Coulomb force acting on them is presented. Expressions for the total charge density, the depth of the charge centroid and the mobility of charges in the sample are derived in terms of measured quantities such as the initial rate of buildup and the maximum return potential. A confirmation of the model is obtained for the return voltage due to electrons injected at known depths in polymer sheets by an electron gun. As a consequence, the same technique might be used with electrode-poled samples, to assess the injecting character of the electrodes, and possible modifications produced by contamination, surface treatments, etc.
TL;DR: In this paper, the dielectric strength of insulator-bridged vacuum gaps has been investigated under dc stresses, and it was shown that negative-angled conical frustrums withstand high er conditioned withstand voltages than positive frustrums and cylindrical insulators.
Abstract: The dielectric strength of insulator-bridged vacuum gaps has been investigated under dc stresses. Contrary to many previous investigations, careful study undertaken here reveals that negative-angled conical frustrums withstand high er conditioned withstand voltages than positive frustrums and cylindrical insulators. The superior voltage perform ance of negative-angled conical frustrums over positive angled ones stems from negative charging of the former which supresses electron emission from the cathode triple junction X-ray emission within the bridged vacuum gap, and electron interaction with the insulator surface, due to emission from the cathode triple junction, are both shown to lower the voltage hold-off significantly. Novel insulator profiles were tested which exhibited significantly superior voltage hold-off over conventional cylinders and conical frustrums. The design strategy for such designs relied on: (1) providing stress relief at the cathode triple junction by facilitating the self-generation of negative charge on the insulator region near the critical junction, (2) minimizing X-ray production from the anode due to the impact of elec trons, and (3) minimizing electron interaction with the in sulator surface by moving the cathode triple junction away from the main body of the insulator.
TL;DR: In this article, the breakdown strength of paper-oil insulation samples was measured under dc, ac and pulsating voltages and the results showed that the presence of moisture showed a pronounced effect upon the break strength.
Abstract: Electrical breakdown strength of paper-oil insulation samples was meas ured under dc, ac and pulsating voltages. The latter was obtained by superimposing ac upon dc voltage and provides an attractive method for a simultaneous testing and assessment of the state of insulation of the various parts of HV apparatus in service. The measurements were carried out over a wide range of the pulsation ratio defined as p=Eac/Edc. The results obtained under pulsating voltages follow closely an expression which relates the breakdown strength to the sum of arctangent and arccotangent function of the parameter p. The study was carried out using dry paper as well as paper containing various degrees of moisture. The presence of moisture showed a pronounced effect upon the breakdown strength which varied with the pulsation parameter p.
TL;DR: In this paper, the authors derive the continuum version of the Hamiltonian introduced by Su, Schrieffer and Heeger, and the equations that result therefrom from these equations the wavefunctions of the solitons and polarons and their creation energies are obtained The description of the excit lions is completed with a phenomenological dispersion relation that describes the variation of their masses with velocity.
Abstract: The first highly conducting polymer, polyacetylene with up to ~500 ?-1 cm-1, was reported ten years ago Since then, polyacetylene and other polymers in which comparable conductivities have been achieved have been studied intensively This review is devoted to polyacetylene It starts with a description of its bonding, band structure, crystal structure and morphology Basic to the nature of conduction in polymers is the fact, predicted by theory and verified by experiments, that addition of an electron or hole to a polymer chain leads to a characteristic pattern of bond relaxation The characteristic pattern, which may also be viewed as an excitation of the chain, in polyacetylene may be either a soliton or a polaron, each with an extent of 10 to 20 atoms As a preliminary to the mathematical description of these excitations, we derive the continuum version of the Hamiltonian introduced by Su, Schrieffer and Heeger, and the equations that result therefrom From these equations the wavefunctions of the solitons and polarons and their creation energies are obtained The description of the excit lions is completed with a phenomenological dispersion relation that describes the variation of their masses with velocity This formalism is then used to derive the diffusion coefficient for neutral solitons These solitons, present as defects in most pristine, ie, not deliberately doped, trans-polyacetylene samples, have been shown by nuclear magnetic resonance and other experiments to diffuse fairly rapidly along the chains
TL;DR: In this paper, the authors measured the pre-breakdown currents in polyimide (PI) films under the application of dc ramp voltage (30 V/s) and the breakdown mechanism was discussed on the basis of the results.
Abstract: Pre-breakdown currents in polyimide (PI) films were measured under the application of dc ramp voltage (30 V/s) and the breakdown mechanism is discussed on the basis of the results. A steep current increase was observed from about 10-3 before the breakdown and was ascribed to a thermal runway which results in a filamentary thermal breakdown path. An analytical solution of the pre-breakdown current was obtained on the basis of this filamentary thermal breakdown with a constant filamentary path; it deviated from the experimental data, suggesting a decrease in the effective radius of the filamentary path with time. A numerical calculation of the pre-breakdown current has been carried out for the filamentary thermal breakdown with a temperature distribution in the filament. The calculated pre-breakdown current agrees well with experimental results, and the effective radius of the filament also decreases as observed. From these results, we conclude that filamentary thermal breakdown occurs in PI at high temperatures.
TL;DR: In this article, the authors combined effort of three different laboratories to investigate water treeing behavior of several polymers by complementary techniques, such as continuous optical monitoring, fatigue environmental stress cracking, tensile drawing behavior, and transmission electron microscopy.
Abstract: Water treeing is one of the main causes of premature breakdown of the polymeric insulation of HV power cables. The work reported in this paper is the combined effort of three different laboratories to investigate water treeing behavior of several polymers by complementary techniques. These include determination of water tree growth kinetics by continuous optical monitoring, fatigue environmental stress cracking, tensile drawing behavior, and transmission electron microscopy. Two kinds of sample geometry have been used for the water electrodes so as to enlarge signi- ficance of results and experiments have been conducted at 20°C and 65°C. The structural and mechanical evidence presented shows that water tree growth can be modified by several mechanisms and points out the considerable influence of polymer morphology on the propagation of water trees.
TL;DR: In this article, a model of water-tree formation was postulated, which considered water treeing essentially as an oxidative degradation with electrochemical initiation at defects and at water/polymer interfaces.
Abstract: Phenomenological investigations of water trees showed clearly that their formation can be inhibited by additives. Stabilizers for concentrated solutions of H2O2 were found to be particularly effective. Based on this observation, a model of water-tree formation was postulated, which considers water treeing essentially as an oxidative degradation with electrochemical initiation at defects and at water/polymer interfaces. Based on this model numerous additives inhibiting water trees have been found, amongst them barbituric acid. This substance is amenable to numerous chemical modifications, and in this contribution the influence of the structural variation of barbituric acid derivatives upon water treeing is described. Depending upon the chemical structure, their effectiveness varies from complete inhibition of water trees on the one side to a complete lack of retardation on the other.