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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 6 • Date Dec 2000

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Displaying Results 1 - 24 of 24
  • Ozone production using pulsed dielectric barrier discharge in oxygen

    Page(s): 849 - 854
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    The production of ozone was investigated using a dielectric barrier discharge in oxygen, and employing short-duration pulsed power. The dependence of the ozone concentration (parts per million, ppm) and ozone production yield (g(O3)/kWh) on the peak pulsed voltage (17.5 to 57.9 kV) and the pulse repetition rate (25 to 400 pulses/s, pps) were investigated. In the present study, the following parameters were kept constant: a pressure of 1.01×105 Pa, a temperature of 26±4°C a gas flow rate of 3.0 1/min and a gaseous gap length of 11 mm. A concentric coaxial cylindrical reactor was used. A spiral copper wire (1 mm in diameter) was wound on a polyvinylchloride (PVC) cylindrical configuration (26 mm in diameter) and placed centrally in a concentric coaxial electrode system with 4 mm thick PVC dielectric layer adjacent to a copper outer electrode of 58 mm in internal diameter. HV and current pulses were provided by a magnetic pulse compressor power source View full abstract»

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  • Conductivity of carbon black-PE composites as a function of temperature and UV aging

    Page(s): 855 - 859
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    We performed a study of mechanisms related to ac electrical conductivity of virgin and aged low-density polyethylene (LDPE) loaded with different concentrations of carbon black (CB). Experimental ac conductivity measurements were carried out at different temperatures on samples with different CB concentrations. Scanning calorimetry (DSC) and thermal expansion measurements were used as auxiliary techniques in this study. We also investigated the aging effect of UV radiation on the electrical properties of the samples. Based on the distribution of the CB in the polyethylene matrix, the ac conductivity is explained in terms of the hopping mechanism considering the material as a system where the energy potential barriers are distributed randomly in the bulk of the material View full abstract»

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  • An easy parameter estimation by the EM algorithm in the new up-and-down method

    Page(s): 838 - 842
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    Flashover voltage estimation has been carried out conventionally with the up-and-down method by Dixon and Mood. Recently, Hirose and Kato proposed a new version of the method to change the way of analyzing data. Although the method has better properties than the conventional one, it requires solving maximum likelihood equations. In this paper we reduce the troublesome task (e.g. implementation and the selection of a proper initial value) attendant on the requirement by using the expectation-maximization (EM) algorithm that gives a useful iterative formula to solve the equations. The iterative formula almost always can give the solutions of the likelihood equations because of its excellent global convergence of the EM algorithm View full abstract»

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  • Delay characteristics of surface charging on a cylindrical insulator in vacuum

    Page(s): 812 - 817
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    This paper describes the temporal characteristics of surface charging on a cylindrical insulator made of polymethylmethacrylate (PMMA) resin or alumina ceramic. The insulator is subjected to a dc voltage step or to a trapezoidal voltage in vacuum. The charging is observed by using an electrostatic probe located at the surface of the cathode close to the triple junction where the insulator, cathode and vacuum meet. The probe signal shows a step increase associated with the charging. The interval between the voltage rise and the charging, which is defined as the delay time of charging in this paper, ranges from 10 -5 s to 10 s depending on the material and the applied voltage. It also depends on the condition of the insulator-cathode interface, the electrical prestress and the surface roughness of the insulator. Among these factors, the surface roughness has the largest effect on the delay time. Two-dimensional Monte Carlo simulation based on the secondary emission electron avalanche model has been performed to analyze the charging progress. During the delay period, positive charge accumulates on the surface of the insulator near the cathode, which further accelerates the charging View full abstract»

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  • A quantative approach to estimate the life expectancy of motor insulation systems

    Page(s): 790 - 796
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    Electric motors represent the largest single load in most power systems. With continuing increase in nonlinear loads, the effects of harmonic contents of power systems on various components is becoming more important. This paper is concerned with the effects of power system harmonics and voltage unbalance on the useful life of electric motors. A simple equivalent circuit is used to calculate the additional heat losses caused by supply distortions. A lumped-parameter thermal network is proposed to calculate the resulting heat distribution within the machine. An Arrhenius chart is then used to estimate motor insulation remaining life. The proposed approach is used to determine the effect of supply harmonics and voltage unbalance on the useful life of a three phase induction motor and the results are verified experimentally View full abstract»

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  • Corona onset as a design optimization criterion for high voltage hardware

    Page(s): 744 - 751
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    Hardware for use on HV systems is designed to be corona free. Recent hardware design activity has been centered around attempts to increase the corona onset voltage by optimizing the electric field distribution on its surface. Corona onset, however, is not only a function of the surface electric field, but also its rate of decay away from the surface, and the temperature and pressure of the gas in which it is immersed. While corona onset is a better criterion, formulas for predicting it have been validated only for simple geometries. The first goal of this work, then, is to validate corona onset conditions for more practical electrode geometries. The second goal is to determine whether the use of corona onset rather than surface electric field as an optimization criterion can result in hardware with a measurably higher corona onset voltage. To test this idea, two electrodes were designed, one using electric field optimization and the second using corona onset optimization. The corona onset voltage of each electrode was then measured and the results compared with predicted values. It was found that the measured results compared favorably with the predicted values and that the use of corona onset optimization can result in a modest but measurable increase in corona onset voltage View full abstract»

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  • Investigating dc polyethylene threshold by space charge. Current and electroluminescence measurements

    Page(s): 797 - 803
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    It has been proposed recently that the lowest level of dc field, above which electrical degradation may occur, is associated with the formation of space charges in insulation. Therefore, inference of field levels above which space charge accumulates in a given material may provide fundamental information for material characterization and system design. Threshold field values were estimated through evidence of space charge accumulation derived from conduction current, space charge amplitude and electroluminescence (EL) counts vs, electrical field characteristics. Press-molded plates of polyethylene-based materials were used for this investigation. In order to be able to detect even by EL experiments the presence of the space charge accumulated at low fields, measurements were carried out during both polarization and short-circuit of specimens. It is shown that the threshold corresponding to the onset of EL upon grounding the specimen is close to the space charge accumulation threshold derived from space charge and conduction current measurements. This earlier value is considerably lower than the one previously detected from constant dc-voltage EL measurements, thus confirming that even at low fields and in the presence of voltage reversals or transients, space charges can trigger luminescence phenomena. The methodology developed in this paper enlarges the applicability of the EL technique to accommodate a wider electrical field range in view of insulation characterization View full abstract»

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  • Partial discharge characteristics and tree inception in artificial simulated tree channels

    Page(s): 843 - 848
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    In attempt to clarify the mechanism of electrical treeing degradation and tree propagation, partial discharge (PD) characteristics in an artificially-simulated tree channel in a low-density polyethylene (LDPE) were investigated. The PD pattern and PD pulse sequence showed that the PD in artificial channel with diameter of 10 μm was useful for clarification of PD in actual tree. The PD formed a PD pulse train whose magnitude changed with time. In the samples where tree propagation was seen at the tip of the artificial channel, pulse trains with large PD magnitude sometimes occurred and the region of light emission reached the vicinity of the channel tip before tree propagation. In the samples where tree propagation was not seen, the region of light emission due to PD did not reach the channel tip. These results suggest that tree propagation takes place when the PD with large magnitude and long luminous region occur in an artificial channel and damage the channel tip View full abstract»

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  • Statistical inferences for breakdown voltages

    Page(s): 869 - 871
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    This paper examines some analysis of breakdown voltages, and gives diagrams and arguments to support re-analysis of the relevant data sets, again using the three parameter Weibull distribution. The paper shows that graphical means provide a useful method for examining the log-likelihood of data, particularly when careful examination of this function is required. The paper also considers the agreement between the sample data and parameter estimates, and between the empirical distributions of data and fitted counterparts View full abstract»

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  • Evaluation of materials for WTR-insulated power cables

    Page(s): 818 - 824
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    In order to improve the resistance to water treeing of standard crosslinked polyethylene (XLPE) insulation used in medium voltage power cables, a number of materials were selected and studied that represent different water tree growth prevention strategies: (a) using materials with crystallinity and morphology different from that of XLPE, or (b) using additives modifying the interaction between water and materials. Two laboratory procedures were adopted that assess the water tree resistance performance of the materials through different criteria: one based on divergent field geometry (CNRS) and another based on uniform field geometry (Pirelli). Both tests are shown to be complementary in their prediction of the performance of the selected water tree resistant materials. The work reveals a specific additive that greatly increases the water tree resistance performance of the insulation in different polymeric matrices View full abstract»

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  • PD characteristics and mechanisms in artificial air-filled voids at room and liquid nitrogen temperatures [comment and reply]

    Page(s): 875 - 876
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    For original paper see Tsuru et al, ibid., vol.6, p.43 (1999). Tsuru et al. studied discharge mechanisms in cavities in some specific test samples at room and liquid nitrogen temperatures. They claimed that the sensitivity of their partial discharge (PD) measurements was 3 pC, while that of the phase-resolved measurement was 10 pC. Danikas thinks that such sensitivities are rather low if one wants to investigate in detail the mechanisms of PD in air-filled cavities. Tsuru et al reply to the comments View full abstract»

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  • Theory and application of dynamic aging for life estimation in machine insulation

    Page(s): 773 - 782
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    After a brief discussion on possible on-line indicators of aging, this paper presents a concept of dynamic aging theory (DAT). Based on the philosophy of a DAT, a dynamic multistress aging model is proposed. Necessary DAT formulation and software have been developed for testing the model against laboratory aging data. In this study, dynamic stagnation voltage (DSV), a derived partial discharge parameter, and the groundwall insulation response to a traveling acoustic wave have been found to be the most appropriate parameters for assessment of remaining insulation machine life. This approach is shown to be a meaningful and practical solution to a very complex problem. Although this study is in its initial stage, it does show significant promise as an on-line tool for dynamically predicting remaining life based on DAT and other appropriate feedback measures View full abstract»

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  • Testing of low-voltage motor turn insulation intended for pulse-width modulated applications

    Page(s): 783 - 789
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    The problem of premature failure of low voltage rotating machines fed by inverters could have serious economic impact if it is not addressed. Indeed, even though much already has been written on the different topics related to the use of such power supplies, there is a great need for a better understanding of the physical mechanisms involved, particularly with regards to aging mechanisms of insulating materials. The aim of this paper is to highlight the effect of adjustable speed drives (ASD) on the stator finding insulating materials. Pulse-like electrical stresses have been applied to three types of samples (thin film, twisted pair, and layer to layer). Physical characterization and/or lifetime measurements have been performed. The results are presented and discussed according to the nature of the interaction existing between the electrical stress main characteristics (magnitude, frequency, dV/dt) and the material properties. According to these results, the development of a standardized insulation quality control test, based on a physical understanding of involved stresses and material behavior, appears now to be necessary View full abstract»

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  • The effect of water treeing on the electric field distribution of XLPE. Consequences for the dielectric strength

    Page(s): 860 - 868
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    This paper describes the influence of water trees on dielectric properties of flat samples cut from a HV crosslinked polyethylene (XLPE) cable insulation. In order to obtain experimental results in a short period of time, we used accelerated aging conditions of voltage and frequency as well as different techniques for producing a multitude of artificial water tree inception points. The average value of the water tree permittivity and a law for the time-dependence of permittivity inside the treed degraded areas were deduced from measurements of capacitance and water tree lengths. On the basis of the experimental knowledge of permittivity and water tree length, we determined with analytical numerical methods, the distribution of the electric field for different shapes of treed regions. Because of the increase in length and permittivity of the water tree, the electric field in front of the degraded area is amplified, which is an important factor of risk leading to the electric breakdown View full abstract»

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  • Performance of a microwave sensor for the precise measurement of water vapor in gases

    Page(s): 825 - 831
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    A microwave sensor designed for the measurement of humidity in sulfur hexafluoride (SF6) has been developed for possible in-line use in the electrical utilities industry. This technique could be used to measure efficiently the humidity level to prevent excess moisture and to keep the electrical integrity of sensitive electrical equipment. The measuring system is based on the dielectric properties of the gas mixture at atmospheric pressure and ambient temperature. The voltage difference coming from a measuring cavity resonator and a reference one is related to the amount of water vapor in SF6. The setup is sensitive enough to detect moisture levels down to 3 ppm View full abstract»

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  • Energy efficiency improvement of electrical sterilization using oscillatory waveforms from a RLC discharging circuit

    Page(s): 872 - 874
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    This paper describes effects of pulsed HV waveforms on biological cell inactivation. A simple RLC circuit with various inductance values was employed to investigate the waveform effects on the electrical sterilization of Escherichia Coli. It has been found that the minimum value of cell survivability can be realized by choosing an appropriate inductance value, even when the initial energy stored in a capacitor is constant. In order to realize higher energy efficiency than a conventional RC discharging network, circuit parameters should be determined so that amplitude and width of each pulse becomes larger than the critical values 15 kV/cm and 50 μs, respectively View full abstract»

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  • The nature of divergent-field surge-voltage endurance in epoxy resin

    Page(s): 764 - 771
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    An experimental study has been conducted to quantify the voltage endurance of electrical grade epoxy resin insulation when subjected to pulse voltages in a divergent field. Voltage endurance has been quantified as a function of pulse width, repetition rate, and polarity. Supporting measurements of free radical generation have been made by electron paramagnetic resonance (EPR), and the progress of the aging mechanism tracked by sensitive photonic emission measurements. The results suggest (at least for positive polarity) that estimates of endurance under pulse voltage conditions can be made from conventional power frequency data, However, it is shown that a correction is necessary to allow for the effects of waveform, and a theory is advanced for that purpose View full abstract»

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  • Self-consistent modeling of electrical tree propagation and PD activity

    Page(s): 725 - 733
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    The self-consistent model of electrical tree propagation and partial discharge (PD) activity within growing tree channels is presented. The local electric field and the damage accumulation in the dielectric material surrounding the channels govern the tree growth. The damage increment is proportional to the energy release in the channels due to PD. The electric field distribution is determined by the charge deposition within the tree structure and the electrode geometry. The charge distribution changes within the channels during PD. PD starts when the electric field along the channels exceeds threshold inception value and stops when the field falls below the threshold quenching value. The numerical three-dimensional realization of the model has been used for simulation of electrical treeing with sinusoidal and triangular voltages in a needle-plane geometry. The spatial-temporal dynamics of the tree growth and phase-resolved characteristics of the PD have been studied for various magnitudes of the applied voltage. The simulation results have been compared with experimental data given in the literature View full abstract»

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  • Electrical treeing in mechanically prestressed insulation

    Page(s): 721 - 724
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    For several years it has been apparent that the mechanical characteristics of insulating materials have a pronounced influence on their electrical breakdown by treeing. As a consequence of this discovery, it is possible to look again from a different perspective at the treeing behavior in materials containing fillers and barriers. The effect of temperature on treeing can be interpreted in terms of the effect of temperature on the various mechanical properties of the material, and of any particulate or solid barrier inclusions. The internal mechanical stresses produced in resin samples containing cast-in metallic electrodes can affect treeing behavior both positively and negatively, depending on whether this stress in the material is tensile or compressive. In the light of this experience it has been found possible to enhance the treeing resistance of electrical insulation by using these mechanical effects in the most advantageous way, viz. by mechanically prestressing the material by the inclusion of fibers in tension; a method which is akin to that employed in the prestressing of concrete View full abstract»

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  • Charge and voltage characteristics of corona discharges in a coaxial geometry

    Page(s): 734 - 743
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    An engineering model is proposed to evaluate the variation of corona charge and corona current as a function of applied voltage for coaxial geometry (q-v curves). The two main assumptions of the model are the following: for streamer discharges to propagate continuously, the background field must be at, or just above, a critical level; and the streamers maintain a constant voltage gradient in the corona sheath. The model simulations are compared with the experimental q-v curves and a reasonable agreement is obtained between them View full abstract»

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  • An incremental analysis of spark paths in air using 3-dimensional image processing

    Page(s): 758 - 763
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    A video camera has been used to record the path of spark breakdowns across a positive point/plane air-gap. By using a prism as a mirror, two images were obtained from which the path could be determined in three dimensions by subsequent computer processing and analysis. Images were analyzed for each of five angular positions of the point electrode's axis, the gap remaining constant. The fractal dimension of each path was determined and found to increase slightly with the inclination of the pointed electrode. Each spark appeared to have propagated semi-randomly in a direction which included a `memory' factor, that is, it tended to follow the previous direction while moving monotonically towards the opposite electrode. This was confirmed by an analysis of 400 spark paths which showed that each section has a specific relationship to the previous section, but that there is a wide variation around this mean which leads to the characteristic shape of a spark path. Simulations based on these results yield spark tracks which look similar to those measured and have similar fractal dimensions. These analyses strongly suggest that the development of spark paths in air is by successive steps and in a direction suggested by an angular probability distribution which is related to the field at the tip of the propagating leader View full abstract»

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  • Negative corona in air using a point/cup electrode system

    Page(s): 752 - 757
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    Calculations and simulations of potential and charged-particle distributions have been made by various workers for single pulses along the axis of point/plane electrode systems but not for the overall current. The difficult geometry makes this inevitable. In this paper the authors describe measurements of corona currents in air using a high-symmetry `point/cup' electrode system comprising a hemispherical-tipped point which is concentric with a concave hemispherical electrode; and simulations based on a spherically symmetric geometry. The simulations assumed avalanches to originate from the random release of electrons in the inter-electrode gap. The predicted onset voltages and the shape of the experimental curves for corona current vs. Voltage were in good agreement with the experimental results over a range of point-tip radii (0.5 to 3 mm) and pressures (0.02 to 0.10 MPa). Space charge effects were not included in the simulation so it follows that the accumulation of space charge is negligible at the onset voltage in air, although it is likely that it will have an increasing effect as the voltage increases View full abstract»

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  • Dynamism of interconnected channels in water-treed polyethylene

    Page(s): 832 - 837
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    Loss current through non-penetrated water treed polyethylene increases with exposure time at 1000 V, 500 Hz. The increment is discussed by taking the capacitance of the water treed sheet sample to be the Maxwell-Wagner type capacitor composed of two layers: the capacitance both for the non-treed region and the unfilled channel region is connected in series to the parallel combination of the capacitance and the conductance for the water-filled channel region. By assuming a non-ohmic dependence of the conductance, the fundamental and 3rd harmonic components of the loss current have been calculated numerically, based on the equivalent circuit. The numerical results give a good agreement with the observed changes of the magnitudes of fundamental and 3rd harmonic components I1 and I3, and their phase angles θ1 and θ3, respectively, for the exposure time. It has been concluded that the water-filled channel region in the water treed region grows with exposure time. It is also concluded that the conductivity of the water-filled channel region increases slightly with its growth View full abstract»

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  • Fault arc tests in cable bundles under microgravity conditions

    Page(s): 804 - 811
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    This paper discusses some forms of wiring degradation and failure modes which can occur when the wiring is in operation on spacecraft. Some of the environmental conditions which will be encountered by a manned spacecraft will be introduced. In the past numerous experiments on cable bundles for space applications have been performed under different stress and environmental conditions. As part of the extensive testing, many variables were investigated such as wire gauge size, applied current and environment. Some results have been published at previous meetings as well as in publications. One major parameter, however, remained to be investigated and that is the effect of gravity on the mechanism of arcing failure. One method by which this can be studied is by making use of the opportunities offered using a parabolic flight where periods of microgravity (μg) of some 20 to 25 s can be achieved. In a μg-night campaign the effects related to this essential parameter were studied in detail. The μg-flight campaign conducted in an Airbus A300 took place in Bordeaux, France, September 1997. This paper presents results based on the performance of arcing experiments under vacuum conditions. These systematic series of tests were the first ones covering the μg-parameter under vacuum conditions to the authors knowledge 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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Reuben Hackam