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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 6 • Date Dec 1999

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Displaying Results 1 - 23 of 23
  • A space-charge life model for ac electrical aging of polymers

    Page(s): 864 - 875
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    The dc space-charge model, previously developed by the authors, here is modified to account for the contribution to electrical degradation provided by ac fields. First, the dc model is applied to both dc and ac multistress life test results relevant to a given material. The variations of model parameters from dc to ac data fitting provide indications about the modifications in the degradation mechanisms when passing from a dc to ac regime. Then, a description of aging under ac is achieved through proper assumptions about the space-charge buildup, injection mechanism, and dynamic condition of the polymer lattice. This approach enables the effect of frequency to be accounted for, in a framework where ac aging presents significant analogies with mechanical fatigue. The validity of the ac space-charge model is checked by applying it to the results of accelerated life tests performed on various insulating materials, at different values of voltage, temperature and frequency, on the whole finding very good agreement with experimental data View full abstract»

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  • Remote sensing of ESD through optical and magnetic radiation fields

    Page(s): 896 - 899
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    The optical radiation and the magnetic field generated by electrostatic discharges are measured, together with their current signatures. The observations show that during the initial growth, the temporal variation of the optical pulse is similar to that of the current. The two waveforms deviate from each other during the decaying part of the discharge, the optical signal having a longer duration. The peak current is linearly correlated with a correlation coefficient of 0.9, to the peak optical radiation. The measured currents were used to calculate the magnetic fields and a good agreement was observed between the measurements and calculations. The results show that both the optical radiation and the magnetic field can be used to sense remotely the currents in electrostatic discharges View full abstract»

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  • Influence of aging on electrical properties of polyethylene naphthalate-2,6 dicarboxylate films

    Page(s): 792 - 797
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    Poly(ethylene naphthalate 2,6 dicarboxylate), thanks to its heat resistance, may be used as insulating material in motors and laminated chip capacitors. In these specific applications the occurrence of partial or corona discharges leads to insulation aging and possible breakdown. An experimental investigation has been carried out to determine the changes in dielectric properties as function of the aging conditions, e.g. the temperature of irradiation. Dielectric loss values have been determined in the 102 to 106 Hz frequency range from 150 to 200°C. Three relaxation peaks were found α,β*,β, in the order of decreasing temperature View full abstract»

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  • Preparing and polarizing stability of Teflon AF nonlinear optical polymer electret double layer film system

    Page(s): 831 - 834
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    Nonlinear optical activity of an nonlinear optical (NLO) polymer electret originates from the preferential orientation of dipoles doped or chemically attached to the polymer matrix. In this paper, the idea of fabricating a Teflon AF/nonlinear optical polymer electret double layer film system is proposed to improve the orientation stability of the dipole in the polymer electret. The polar stability has been investigated by means of measuring the open-circuit thermally stimulated depolarization (TSD) spectrum, the electro-optical coefficient, and the isothermal surface potential decay. The results indicate that the double layer film system may markedly increase the orientation stability of the dipole in the NLO polymer layer. Because the Teflon AF layer has high charge storage stability, the dipole orientation can be bound by the electric field generated by the space charge in Teflon AF layer View full abstract»

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  • Fractal phenomena. Dependence of hydrophobicity on surface appearance and structural features of SIR insulators

    Page(s): 781 - 791
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    This paper reports the study of the dependence of hydrophobicity on the surface appearances and inherent chemical structure of the high-temperature vulcanized silicone rubber (HTVSR) insulator based on the fractal geometric principle. The fractal phenomena link the surface roughness with the hydrophobicity characteristics of silicone rubber (SIR) insulators. The rough surfaces of the insulators, no matter whether due to salt-fog aging or roughening by sandpaper, show the property of a fractal surface. According to the amount of hydrophobic material on the surface, surface roughness exerts different effects on the hydrophobicity of SIR insulators. Due to the behavior of fractal surfaces, the hydrophobicity of the appropriately roughened insulator can be enhanced, but depressed if dipped in hexane. Outdoor polymer insulating materials could be aged by environmental stresses and contaminants so that their surfaces become rough. The permanent change of surface roughness could be an indicator of aging state of the insulation material. The hydrophobicity of SIR insulators aged in salt fog in a fog chamber are investigated. During this aging, the inorganic filler and the hydrophobic groups in SIR are worn away so that the insulator surface becomes rough. The surface of the aged insulator becomes hydrophilic. After a subsequent rest, the hydrophobic polymer liquid in bulk diffuses and migrates to the insulator surface, which becomes more hydrophobic. Such a unique dependency of hydrophobicity on the surface appearance and chemical structure of SIR material is available for SIR insulator to maintain good contamination performances in outdoor applications View full abstract»

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  • Surface degradation properties of ultraviolet treated epoxy/glass fiber

    Page(s): 907 - 912
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    In order to analyze the degradation process of epoxy/glass fiber in outdoor conditions, FRP laminate was exposed to different wavelengths of ultraviolet (UV) rays and evaluated by comparing contact angle, surface resistivity, surface potential decay, and XPS. As irradiation energy increased, the surface properties deteriorated remarkably at the region of 300 nm. But the measured values within 400 to 440 nm showed a slightly abnormal increase compared with the untreated FRP laminate. Also, the results of XPS confirmed that a great deal of oxygen groups appeared on the spot which showed the maximum decrease of surface properties, particularly the existence of ether groups on the surface of the discoloured spot. It was concluded that the surface degradation of UV-treated epoxy composites is dominated by the abrupt increase of ester and carboxyl groups as hydrocarbon chains are oxidized. This degradation process occurs continuously through the reorientation of oxygen groups by thermodynamic reaction and the formation of an electric double layer by electrostatic force View full abstract»

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  • TSC measurement of space charge in low-density PE under a needle-plane electrode system

    Page(s): 858 - 863
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    We have investigated the thermally stimulated current (TSC) in low-density polyethylene (LDPE) film and surface potentials built in the LDPE films due to excess charges injected from a needle electrode, using 10, 15 and 20 needle-plane electrode systems. The amount of charge injected and its depth were determined from TSC and thermally stimulated surface potential (TSSP) measurements View full abstract»

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  • The influence of electrode material on delayed discharges in vacuum interrupters

    Page(s): 913 - 915
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    In this paper, the influence of contact materials on late discharges in vacuum switching devices was investigated. Experimental results show that the HV withstand capability of contact materials has a strong influence on the frequency of late discharges in vacuum switching devices. The addition of a small amount of W and Co into the CuCr alloy increases the voltage withstand significantly, resulting in a reduction of the frequency of late discharges by as much as 50%. Therefore, an efficient approach to decrease the frequency of late discharges is to increase the HV withstand of the contact materials View full abstract»

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  • Studies of elastic waves caused by corona discharges in oil

    Page(s): 825 - 830
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    In order to diagnose the deterioration of insulation and evaluate its location in oil-filled pole transformers, measurements of elastic waves brought by corona discharges in oil were performed, based on an acoustic emission (AE) technique. The detected signals in these measurements were analyzed by means of a fast Fourier transform (FFT) and their properties were discussed theoretically. It was found that the AE signals due to the Lamb waves of symmetric mode caused in a thin steel tank could be recognized easily by the FFT analysis of initial stages of the detected AE signals. This suggests that the accuracy on location of the AE sources in the transformer would be improved. Also, it was shown that a low pass filter was effective in improving the location accuracy View full abstract»

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  • Simulation of transformer PD pulse propagation and monitoring for a 500 kV substation

    Page(s): 803 - 813
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    This paper discusses the development of a simulation model to study the propagation of partial discharge (PD) pulses of transformers in a 500 kV substation. The results will be used to design an on-line PD monitoring system for transformers. The impedance matrix of a 500 kV, single-phase transformer is computed based on geometry data and its external coupling network (often referred to as coupling impedance) is calculated with electromagnetic transients program (EMTP). It was found that the frequency characteristic of the coupling network is complex and that the traditional lumped coupling capacitance representation is only feasible at <60 kHz. The entire substation network was included to calculate the PD propagation of the transformer. PD responses are obtained at the tank grounding and 500 kV bushing tap grounding with a simulated PD pulse injected at the 500 kV terminal and 220 kV terminal of the transformer, respectively. Simulation shows that, using the resonant frequency of the substation network as monitoring frequency, one effectively can increase the monitoring sensitivity. The off-line and on-line PD calibration can also be made based on the developed simulation model View full abstract»

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  • Charge storage in double layers of thermally grown silicon dioxide and APCVD silicon nitride

    Page(s): 852 - 857
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    Experimental results on charge storage and discharge in double layers of silicon dioxide and silicon nitride will be reported and discussed. SiO2 with a thickness of 300 nm was thermally grown on silicon wafers, while cover layers of Si3N4 with thicknesses of 50, 100, and 150 nm were deposited chemically at atmospheric pressure. The samples were charged by the point-to-grid corona method. At room temperature, the measured surface potential V was stable during a period of almost three years. Isothermal measurements under different environmental conditions showed an improved charge retention compared to a single layer grown silicon dioxide. After ~3 h at 300°C, the observed voltage drop was <10% for the double layers and ~60% for bare SiO2. Similar results were obtained under a humid condition of 95%RH and 60°C. Besides, thermally stimulated current (TSC) was measured in setup with a temperature ramp of 200°C/h. For the double layers, a current peak with a maximum temperature at ~500°C was observed. The measured current in the range of 300 to 400°C, the location of current maxima observed in thermally grown silicon dioxide or APCVD silicon nitride, was negligible. In addition to improved electret properties the internal stress in the investigated double layers can be adjusted by a proper thickness ratio of oxide layer to nitride layer. Therefore double layers of silicon dioxide and nitride seem to be promising materials for integrated sensors and actuators based on the electret effect View full abstract»

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  • Comparison of SF6/N2 and SF6/CO 2 gas mixtures as alternatives to SF6 gas

    Page(s): 892 - 895
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    The interest in SF6 gas mixtures has been re-ignited in recent years by the issue of the greenhouse effect of the SF6 gas, and most research work is now focused on a SF6/N2 gas mixture, which is suitable for application in electrical apparatus with slightly non-uniform fields. This paper presents a comparison of SF6/N2 and SF6/CO2 gas mixtures with a viewpoint of their possible applications to gas-insulated transformers, where both highly non-uniform field problems and partial discharges in gas/film insulation are inevitable. It is shown that in this case the dielectric strength of SF6/CO2 is superior to that of SF6/N 2 with a minor disadvantage related to the gas decomposition in SF6/CO2. However, this may not be a problem for the SF6/CO2 gas mixture to be used in gas-insulated transformers, where internal breakdown is not allowed View full abstract»

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  • Influence of radiation environments on space charge formation in γ-irradiated LDPE

    Page(s): 882 - 886
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    In the present paper the formation of space charge in irradiated low-density polyethylene (LDPE) was investigated using the pulsed electroacoustic (PEA) technique. Specimens of LDPE, ~100 μm thick, were irradiated to various doses at room temperature in a 60Co γ source in one of three different environments: ambient room air, oxygen-free nitrogen and vacuum. The results obtained indicate that the space charge distribution is dependent on the irradiation environment. There is a large concentration of positive charge evident in the sample irradiated in air compared to very little for material irradiated in either vacuum or nitrogen under similar conditions. This indicates that the presence of oxygen during irradiation, and thereby oxidation itself, has a major influence on the mechanism by which space charge evolves. Thus the monitoring of space charge may in itself be an indicator of the degree of oxidation and of aging in a material. The evolution of space charge and its decay are discussed View full abstract»

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  • Modeling of pressure effects in HVDC cables

    Page(s): 845 - 851
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    A model is developed for the prediction of pressure effects in HVDC mass impregnated cables as a result of temperature changes. To test the model assumptions, experiments were performed in cable like geometries. It is concluded that the model may predict the formation of gas cavities View full abstract»

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  • Influence of fog parameters on the aging of HTV silicone rubber

    Page(s): 835 - 844
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    Testing of outdoor insulators in a fog chamber is universally used. Many factors affect the generation of fog. Fog parameters such as the pressure of the compressed air and the water flow rate feeding the nozzles to form the fog, as well as the size and the speed of the fog droplets impinging on the surface of the insulators, play a major role in the outcome of electrical performance tests. In order to be able to compare test results from different laboratories, the conditions of fog generation should be defined unambiguously and clearly. In the present paper relationships between air pressure, water flow rate and air speed which are related to the distribution and the speed of the fog droplets, are determined as a function of distance from the IEEE-IEC standard nozzles forming the fog. The distribution of different fog droplet sizes is measured. The influence of the fog parameters on the aging characteristics of surface resistance, surface roughness, leakage current and surface hydrophobicity of high temperature vulcanized (HTV) silicone rubber are presented View full abstract»

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  • A Galerkin model to study the field distribution in electrical components employing nonlinear stress grading materials

    Page(s): 765 - 773
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    An effective model is presented for the evaluation of the electric field dynamics inside electrical components, using nonlinear stress grading materials. The model, implemented in a numerical procedure, permits the solution of the Laplace equation and the diffusion equation by adopting the Galerkin method. Two-dimensional domains, even of very complex shapes and the finite thickness of the grading materials are properly taken into account, allowing an accurate evaluation of the electric field distribution and a sound understanding of the influence of the different types of nonlinearities on the stress grading efficiency. The proposed technique has been applied to study the field distributions inside a cable termination equipped with a stress control tube and in a suspension cap-and-pin glass insulator covered with an anti-corona layer. Numerical results for sinusoidal power frequency and standard impulse voltages elucidate the different role of resistive and capacitive contributions in determining the overall potential maps View full abstract»

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  • Flashover of discontinuous pollution layer on HV insulators

    Page(s): 900 - 906
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    This paper deals with discontinuous pollution layers deposited on the insulator surface. Two zones are known to form: a clean (dry) zone and a polluted (wetted) zone. The influence of the length and position of the dry zone on the leakage current and the arc length are investigated. These were made on a plane experimental model under ac voltage. Two configurations were studied: pollution deposited near the HV electrode, and pollution deposited near the ground electrode. This study was made using pollution layers with different conductivities. Based on the experimental results, a theoretical model allowing us to calculate the gap impedance, the transferred (gap) voltage and the leakage current was established. This model enables to predict the pollution severity based on the sample's equivalent impedance View full abstract»

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  • Numerical performance analysis of semiconductor coatings for corona suppression

    Page(s): 774 - 780
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    Corona is initiated in high field regions. To suppress corona or to increase the corona inception voltage in HV insulators or bushings, suitable semiconductor coatings, able to relieve high electric fields, are commonly used. The choice of appropriate materials to be used for the most effective corona suppression is typically made on the basis of expensive and time consuming trial and error tests or on the basis of approximate theoretical analysis. In this paper a guideline, based on the adoption of a suitable numerical method, is provided in order to lead to the optimal choice of the electrical characteristics of semiconductor materials for the most effective corona suppression View full abstract»

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  • The effect of nonsinusoidal voltage on intrinsic aging of cable and capacitor insulating materials

    Page(s): 798 - 802
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    This paper investigates the effect of nonsinusoidal voltage, i.e. voltage affected by the presence of harmonics, on intrinsic aging of cable and capacitor insulating materials, i.e. cross-linked polyethylene (XLPE) and polypropylene (PP). The results of life tests under sinusoidal and distorted voltage, the latter obtained superimposing one or more harmonics on the 50 Hz component, are processed by statistical techniques derived from design-of-experiment procedures. It is shown that the factor predominant on aging acceleration due to the voltage distortion is voltage-peak increase, but also waveform slope and voltage rms value have statistical significance. Life models are obtained for XLPE and PP insulating materials, which show the endurance of the two materials to aging under nonsinusoidal conditions View full abstract»

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  • Experimental investigation on the cause of harmfulness of the blue water tree to XLPE cable insulation

    Page(s): 887 - 891
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    Through investigations made on crosslinked polyethylene (XLPE) insulated power cables that had experienced failures, blue-colored water trees were observed in addition to ordinary white water trees. The blue water tree was found to be much more harmful than the white water tree, since more electrical trees started at the blue trees. Elemental analysis revealed that iron and sulfur exist in the blue tree. In the case of this blue water tree, space charge exists abundantly but only at the tree tip, while no charge signal is observed on the electrode from which the tree starts. This indicates that the conductivity of the blue tree is very high. This is considered to be the main reason of the harmfulness of this tree View full abstract»

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  • Discrimination between internal PD and other pulses using directional coupling sensors on HV cable systems

    Page(s): 814 - 824
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    On-site partial discharge (PD) measurement is required to ensure proper installation of extra high voltage (EHV) cable systems accessories. To achieve high sensitivity and good localization, two problems have to be overcome. First, the strong high frequency in long XLPE cables requires that the sensors be located along the cable, preferably directly at the accessories. Secondly, the detection system must be able to distinguish internal PD from other pulses. This paper describes a solution based on directional coupling sensors and a data visualization system, which displays phase-amplitude diagrams for individual PD sources which are identified by the direction of pulse propagation. It has been applied to on-site measurements, type and routine testing of HV cable joints and stress cones. Due to the reliable discrimination between internal PD from the accessory measured and from other pulses, testing can be done in unshielded rooms even using terminations with internal PD and corona. The method works independently well on line voltage, resonance sources, oscillating voltages and 0.1 Hz cosine-square voltage. It has been used to verify the cable accessories installed in the 6.3 km long 380 kV cable system in Berlin, Germany View full abstract»

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  • Influence of additives on the dielectric strength of high-density polyethylene

    Page(s): 876 - 881
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    In this study, we present the results of the influence of chemical additives (antioxidant and UV stabilizer) and pigments (titanium dioxide and carbon black) on the short-term dielectric breakdown test of high-density polyethylene (HDPE). These additives and pigments are commonly added to polyolefins, which are used as insulating material for medium voltage cables. The incorporation was performed in a single screw extruder and thin films specimens were obtained by hot compression from extruded materials. For the dielectric breakdown test, an automated system has been used. A voltage ramp of 500 V/s was applied to specimens immersed in a silicon oil bath at room temperature. The degree of crystallinity and chemical modification of the formulations were evaluated by X-ray diffraction and Fourier transform infrared (FTIR), respectively. The dielectric breakdown results have been analyzed by a Weibull distribution. The shape and scale parameters of this distribution have been obtained by a graphic and maximum likelihood method. These results showed that the carbon black is the component that affects the dielectric strength, that the β shape parameter from the graphic method can be used to evaluate additive mixing conditions, and that the weakest point for formation of the rupture channel is on the carbon black agglomerate 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