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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 1 • Date Feb 1995

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Displaying Results 1 - 19 of 19
  • Semi-empirical high-field conduction model for polyethylene and implications thereof

    Page(s): 97 - 106
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    A semi-empirical model for high-field conduction in polyethylene is developed on the basis of data in the literature for ac conductivity in the range of 30 to 60 kV/mm and `charge injection' from a needle in the range of 160 kV/mm. The model is used to compute high-field conduction-related phenomena by solving Poisson's equation with field-dependent conductivity and time-dependent applied voltage for a highly inhomogeneous field geometry. Parameters computed include field, current density, charge density, power density, and force density. Temperature rise in the dielectric is estimated based on the computed power density View full abstract»

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  • UHF detection of leader discharges in SF6

    Page(s): 143 - 154
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    The ultra-high frequency (UHF) technique of partial discharge detection has been used to study the partial discharges produced by electrode protrusions and surface contamination in a gas insulated substation (GIS). The paper describes the stages of discharge development from inception to breakdown. The work has shown that the UHF technique may be used to identify distinct phases of discharge activity corresponding to discharge inception and a prebreakdown condition. The prebreakdown activity is detected as an increase in the UHF signal magnitude which has been attributed to the formation of leader-type discharges. These discharges form an essential part of the breakdown process, and their detection can be used to identify when the system is close to breakdown. The paper describes a filtering technique that may be employed to optimize the sensitivity of UHF leader detection View full abstract»

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  • Breakdown characteristics in air gaps with artificial floating metals under DC voltage

    Page(s): 155 - 166
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    The DC breakdown characteristics of a sphere-to-sphere gap with conducting objects supported by an insulator rod in atmospheric air are investigated experimentally to provide fundamental parameters determining the breakdown voltage in the presence of floating objects around DC power apparatus by an air insulation. The shape of tested floating objects made of metal is plate, needle or sphere. Experimental results show that the main factors affecting the breakdown voltage are the shape and size of floating objects, the object location and the gap length. If the object is placed at the location giving the lowest breakdown voltage, any object lowers the breakdown voltage below that without the relevant objects View full abstract»

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  • Space charge in polyethylene/ionomer blends

    Page(s): 1 - 11
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    Measurements of thermally stimulated currents were made on ionomer/polyethylene blends. The objective of this work was to study space charge formation in these materials. Two ionomers were compared; both ionomers contained methacrylic acid, neutralized by zinc in one and by sodium in the other. A large current peak centered near 60°C was observed in all blends and associated with the motion of space charges injected into the blends during poling. All blends accumulated large negative space charge concentrations. The approach to the equilibrium space charge distribution (negative and spatially uniform in all blends) is much more rapid in the blends containing the sodium ionomer than in the blends containing the zinc ionomer View full abstract»

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  • Electrical conduction in double-layer P-SiO2/polymer dielectric structures

    Page(s): 62 - 73
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    The dc electrical conduction in MI1M and MI1I2M composite specimens has been studied, where I1 is polyimide (PI) or polyester (PET), and I2 is a thin plasma deposited silicon compound film (oxide, P-SiO2, or nitride, P-SiN). The current-electric field (j(E,T)) characteristics have been measured over a wide range of E values (O⩽E⩽2×10 5 V cm-1) and temperatures (20⩽T⩽170°C). The observed transport characteristics are in broad agreement with data reported in the literature, except for PI based MI1I2M specimens, for which polarity dependent transport behavior occurs for T>60°C and E>2×103 V cm-1. The observed behavior of all specimen types can be explained in terms of protonic hopping conduction; the charge carriers are created by thermal dissociation and ionization of carboxylic acid (-COOH) groups, present in PI as several percent of unimidized polyamic acid, and in PET at chain ends. When the polarity is +MI1I2M-, the relatively abundant free protons in PI flow to the I1/I2 interface, where they can accumulate and give rise to a space charge-induced potential barrier of ~0.2 eV. By modifying Lewis' molecular dipole model so as to include this potential barrier, we derive expressions for j(E,T) which describe the experimentally observed polarity dependence in PI rather well. The effect is not observed in PET based MI1I2M specimens on account of the much lower concentration of free protons in this polymer View full abstract»

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  • On the existence of leader discharges in mineral oil

    Page(s): 167 - 179
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    Time-resolved pictures of discharges with simultaneous recording of predischarge currents have been obtained in different oil gaps under impulse voltage of positive polarity. It has been stated that in certain experimental conditions, a process similar to the leader process in air occurs, with the typical features of a leader channel and a streamer zone View full abstract»

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  • Effect of filler concentration on electrical conductivity and ultralow-frequency dielectric properties

    Page(s): 54 - 61
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    The effect of filler concentration on the dielectric properties in the ultralow-frequency region and on the electrical conductivity was studied for ethylene propylene rubber. First, we investigated the relation between the electrical conductivity and filler concentration: as the volume fraction of fillers qa increased, the conductivity decreased in the low filler concentration region but increased abruptly in the high filler concentration region. The decrease and increase in conductivity can be explained with the action of carrier traps at the interface between EPR and fillers and with the formation of highly conductive paths of filler across the sample, respectively. Secondly, we studied the dielectric properties in the ultralow-frequency region which was obtained from the discharge current. As qa increased, the relaxation time decreased in the low filler concentration region and then rose in the high filler concentration region. The polarization in the high filler concentration region can be explained by two-layer interfacial polarization between filler and rubber View full abstract»

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  • Static electrification and partial discharges induced by oil flow in power transformers

    Page(s): 40 - 45
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    This paper presents the study of the major parameters on static electrification, using full-scale shell type transformers (570 MVA, 405 kV and 100 MVA, 500 kV). The consequences of static electrification on the dielectric strength and the conditions of partial discharge occurrence also are studied with oil flow rate up to 5 times the nominal value. It shows that the leakage currents are strongly influenced by temperature, applied voltage and oil flow rate Q. The latter is the most influential parameter. The winding leakage current I varies as Qn , where 1⩽n⩽3. The maximum value of I is observed at room temperature (20°C) when the transformer is energized View full abstract»

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  • Electroluminescence from polyolefins subjected to a homogeneous ac field

    Page(s): 107 - 113
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    Electroluminescence associated with ac voltage aging of polyolefins has been reported under divergent field configuration. Neither the mechanism of excitation nor the nature of luminescent centers are well understood in these polymers. Homegeneous field experiments are desirable since they are easier to run and more light can be collected, thereby allowing spectral analysis to be made. We describe a suitable experimental configuration for electroluminescence detection from polyethylene and polypropylene films submitted to a homogeneous field and report on surface roughness, field, and time dependence of the light emission. Field and time dependence of the light spectra are also investigated. It is shown that the light originates from the polymer bulk and is controlled by the surface roughness of the films. The light spectrum appears to consist of two components which exhibit different behavior as a function of field and stressing time. A tentative explanation is given in terms of competition between two different excitation processes View full abstract»

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  • Computer-aided design of porcelain insulators under polluted conditions

    Page(s): 121 - 127
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    The development of a user-friendly, interactive personal computer package for designing insulators to be used in polluted conditions is presented. This is accomplished by integrating a dynamic arc model that computes the pollution flashover voltage within the Microsoft Windows application program. Suspension, station post and pin type insulators of numerous shapes have been incorporated into the package. The model is capable of handling AC and DC voltages, in addition to uniform and nonuniform pollution distributions on the insulator surface. The use of the model as a design tool to predict the insulation requirement for a given system voltage, and as a maintenance tool to determine the critical contamination level (in terms of ESDD) that will cause flashover of the insulator at the operating voltage, has been demonstrated. Wherever possible, comparisons of the model predictions with experimental data have been made, and good agreement has been obtained View full abstract»

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  • Influence of space charge buildup on the transition to electrical treeing in PE under ac voltage

    Page(s): 27 - 35
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    Our investigation is focused on the understanding of electrical aging in polymeric materials under 50 Hz ac voltage. In this report, a needle electrode molded into the insulant to simulate defects producing local field enhancement is used. Special emphasis is given to low density polyethylene. The transition between the discharge-free electroluminescent state to micropartial discharge (PD) state (early electrical tree propagation phase) is studied with a sensitivity reaching 0.01 pC. Optical diagnosis is carried out simultaneously. At the moment of tree initiation, electrical discharges from 0.04 to 0.1 pC occur in the positive half cycle. A very small electrical tree of 5 μm was observed. Using the phase angle of the first measurable PD with respect to the applied voltage offers additional information helping to understand tree initiation. Then a correlation between the magnitude of electrical discharges and the characteristic traces of local breakdown (electrical tree) is reported. We proposed an interpretation based on the similarity with grounding tree experiments in which the initiation of a local breakdown is mainly due to a strong Poisson field radiated by a space charge region in the vicinity of the needle tip View full abstract»

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  • Ionic behavior of dc conduction in polyetheretherketone

    Page(s): 74 - 83
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    The conduction process in polyetheretherketone (PEEK) is discussed mainly from the results of dc conduction, thermal pulse current (TPC), thermally stimulated current (TSC), and pulsed electroacoustic (PEA) measurements. It was found that dc conduction currents in PEEK are consistent with the values predicted by a theory based on the conventional thermally activated ionic hopping process. The ionic hopping distance was calculated to change from 2.5 to 3.9 nm when the temperature crosses the glass transition temperature. The results of TPC and PEA measurements show the existence of heterocharges in the vicinity of both electrodes. In TSC spectra, a peak appears at a temperature of ~5 to 20°C above the biasing temperature. The peak position approaches to the constant temperature of 155°C when the biasing temperature is >150°C. The peak height is maximum when the biasing temperature is 150°C. It is considered that this peak is related to the glass transition and caused by the depolarization of ionic space charge. From these results, it is concluded that ionic carriers which may come from byproducts of synthesis play an important role in the dc conduction process in PEEK View full abstract»

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  • Onset voltage for corona pulses in gaseous Ar under high pressure and in liquid Ar

    Page(s): 137 - 142
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    HV, applied to a point cathode (radius of curvature ~1 to ~8 μm) in a nonpolar liquid, brings about a regular current pulse regime above a threshold value Vs. This regime is similar to the Trichel pulse regime for corona in air. We present results of an investigation of this threshold voltage Vs in argon over a wide range of densities. In gaseous argon, the Vs(P) values increase with pressure. In the liquid, Vs is independent of external pressure. The results show that Vs is mainly a function of fluid density N and cathode tip radius τp. The theoretical analysis of the phenomenon is carried out according to the criterion ∫α(x)dx constant, where a(x) is the Townsend ionization coefficient. It is shown that if the dependence of Vs on N for low density gas is extrapolated into the region of liquid densities, it greatly exceeds the experimental values. The dependence of Vs on the fluid number density ~ is derived. Its variation is found to be non-monotonic at very high N values View full abstract»

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  • Quantized thermal depolarization of solid dielectrics

    Page(s): 12 - 26
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    Contrary to the Debye's theory of a distribution of relaxation times, the logarithm of the relaxation current plotted as function of time has several nearly straight sections of decreasing slopes. The slopes formed short series having common geometrical ratios of ~2, 1.5, and 1.2. The ratios represent stepped changes of the probability of charge carrier excitation, generally caused by the stepped decrements of the displacement potential of the polarized charges. As energetic polarization waves deliver activation energies, large groups of charge carriers display mean decrements of energy in steps of single phonons. The model provides new methods of estimating Planck's constant and of the atomic heat capacity. The experiments and the model suggest that all solids obeying the Dulong-Petit law display one value of mean infrared frequency of vibrations. Based on this frequency a novel solid state constant kTln 2 is proposed representing quantum energy steps displayed in most solid state thermal relaxations. The measurements of ratios of time constants creates a new field of spectroscopic studies of the phonon vibration interactions displaying energies of the order of 10-2 eV. The present results may support the hypothesis that the eigenfunctions of Maxwell's equations are represented by a pulse instead of a monochromatic wave View full abstract»

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  • Calibrating the UHF technique of partial discharge detection using a PD simulator

    Page(s): 46 - 53
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    The paper describes a technique for simulating the presence of partial discharges in gas-insulated substation (GIS) equipment. The technique employs a pulse generator to inject current pulses through a suitable protrusion. The current pulses have similar temporal characteristics to those produced by an actual discharge event, with the added advantage that their magnitudes can be controlled accurately. Because the pulses are highly reproducible, they are ideally suited for investigating the UHF technique of partial discharge detection. The paper discusses the relationship between the magnitude of the UHF signal received by a coupler and the discharge magnitude. It also shows that the coupler signal is dependent on the protrusion length and on the spatial relationship between the protrusion and the coupler. The implications for practical discharge measurements in GIS also are discussed View full abstract»

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  • Nanosecond streak photography of discharges on spacer surface in gases

    Page(s): 114 - 120
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    Results of an investigation to clarify the process of surface flashover in gases are reported. Two hemispherical capped rods of 12 mm diameter, separated by a half-cylindrical spacer of 50 mm diameter, formed the samples. The spacer materials were polytetrafluoroethylene, acrylic and nylon. High-speed streak photographs of impulse flashover indicate that the presence of a spacer not only lowers the impulse breakdown voltage but also reduces the breakdown time. The discharge on spacers may be initiated anywhere between the electrodes and develops rapidly into a flashover in ~5 ns for an 18 mm gap View full abstract»

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  • Polluted insulator flashover model for ac voltage

    Page(s): 128 - 136
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    Considerable theoretical and experimental research on polluted insulators has yet to yield a perfect generalized mathematical model which can predict accurately critical flashover voltages and currents. In this paper an ac model has been proposed especially applicable to static arcs. The salient feature of the proposed model is that it takes into account the appropriate selection of arc constants for different chemical nature of the pollutants. The computed results closely follow the experimental curves and can predict rms value of critical flashover voltages with a mean absolute error (MAE) of <4%. The validity of the model was verified by comparing the computed results of rms critical flashover voltages and currents against experimental and theoretical data of other researchers for a number of practical insulators of different geometries and good agreement has been obtained View full abstract»

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  • Electrical conduction at low fields of composites sparsely filled with aluminum particles

    Page(s): 84 - 96
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    The dc and ac electrical conduction properties have been investigated in four composites containing hydroxyl-terminated polybutadiene (HTPB) as binder and different proportions of aluminum particles as filler in a temperature range of -60 to +70°C and a frequency range of 40 Hz to 100 kHz. A change in conduction mechanism at a temperature of 4±3°C was displayed explicitly in de measurements. The transition was shown to be due to crystallization of the HTPB upon cooling. dc conduction is explained in terms of ionic hopping in HTPB. Distinct polarization mechanisms, below and above the transition temperature, have been identified as interfacial polarisation and electrode blocking effect, respectively. The change of conduction behaviors at the crystallization temperature, 4±3°C, was not revealed explicitly in ac measurements. However, using the free volume concept to analyze the frequency shift factor aT, a discontinuity in the fractional free volume curve was displayed. The temperature of the break point, 6.7°C, corresponds to the crystallization temperature of HTPB. The study showed that the crystallization of a polymer is accompanied by an abrupt reduction in free volume, and this structural change affects dc as well as ac conduction behaviors View full abstract»

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  • Measurement of the internal space charge distribution of an anti-electrostatic discharge polymer

    Page(s): 36 - 39
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    Charged dielectrics can cause an electrostatic discharge (ESD) that damages electronic circuits because of the electromagnetic noise associated with the discharge. Newly developed anti-ESD polymers are expected to be `anti-ESD polymer insulating materials', but the mechanism of their action against ESD has not been explained. Since the anti-ESD polymers contain some kind of polymer solid electrolyte, they should be able to be characterized by the behavior of their internal space charge. By measuring the space charge distribution of an anti-ESD polymer using the pulsed electroacoustic method under DC electric field, we found that the polymer was effective in preventing ESD because the internal space charge near the surface partially compensates the electric field of the external charge on the surface. We also found indications that the internal space charge behavior is related to the dispersion of the polymer solid electrolyte View full abstract»

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Aims & Scope

IEEE Transactions on Dielectrics and Electrical Insulation contains topics concerned with dielectric phenomena and measurements with development and characterization of gaseous, vacuum, liquid and solid electrical insulating materials and systems.

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