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Electrical Insulation, IEEE Transactions on

Issue 4 • Date Aug. 1990

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Displaying Results 1 - 21 of 21
  • Comments, with reply, on "Dependence of the electric strength on thickness, area and volume of polypropylene" by S. Cygan and J. R. Laghari

    Page(s): 762 - 763
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    The commenter notes that the conclusion reached by authors of the above-titled paper (see ibid., vol.22, p.835-7, Dec.1987), that there is a much stronger dependence of electric strength on thickness than on area, was also reached by M.G. Danikas (see Conf. Dielectric Materials, Measurements, and Applications, p.9-12, 1988) and by W.R. Bell and M.G. Danikas (see IEEE Int. Symp. Electrical Insulation, p.264-7, 1982) for transformer oil. He discusses some possible factors causing this result. The authors of the original paper add some further comments.<> View full abstract»

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  • Air-gap effect and the importance of the electrodes on the sample in the lumped-capacitance method for dielectric measurements

    Page(s): 638 - 641
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    A theoretical analysis is presented for the effect of an air gap in the lumped-capacitance method of dielectric measurements in the microwave frequency region. It is shown that when the material to be measured has a large permittivity, a very small equivalent air gap arising from poor contact between the sample surface and the end of the inner conductor (or the short-circuit) can cause a large error if neglected. The importance of a metal layer on the sample surface is emphasized, in order to eliminate the equivalent air gap and thus reduce the measuring error. Experimental results are given which confirm the analysis View full abstract»

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  • Measurement of dynamic potential distribution during the propagation of a local arc along a polluted surface

    Page(s): 757 - 761
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    A new method for measuring the dynamic potential distribution during the propagation of a local arc along a polluted surface is described. In order to study the role of electric stress during the pollution flashover processes, a flat polluted glass plate model was designed. The measuring electrodes made with a strip of aluminum foil were stuck on the bottom of the plate, opposite to the side on which the local arc propagated. The capacitance between the polluted surface and the measuring probe, which is in series with an additional capacitor, was used to construct a potential divider. By means of the dividers and a magnetic tape data recorder, the dynamic potential distribution, the position of local arc, and its propagation velocity were measured. The results show that the velocity of arc propagation along a polluted surface is 0.2 to 1 m/s and that the maximum average potential gradient in the pollution layer is ≈0.5 kV/cm View full abstract»

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  • Calculation of DC and AC flashover voltage of polluted insulators

    Page(s): 723 - 729
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    The phenomena and the propagation processes of DC and AC arcs on the surface of polluted insulators were observed and investigated by means of a high-speed camera. Time-resolved photographs indicate that there is an obvious distinction between DC and AC flashover processes. Based on the alternating character of an AC arc on the polluted surface, the relationship between its reignition and recovery conditions is analyzed. The most important factor is the recovery rather than the reignition condition. A formula expressing the resistance of the pollution layer on the insulator is derived. From this, the flashover voltage of a polluted insulator of complex shape can be calculated. A computer program has been developed to calculate the flashover under both DC and AC. Some arc phenomena, such as arc bridging between the sheds or ribs of insulators, the drift of the arc from the insulator surface, and the number of arcs in series before flashover, are taken into account. Much experimentation has been done to determine the flashover voltages of polluted insulators with complex shapes under both DC and AC. Experimental data are in good agreement with the calculated results View full abstract»

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  • Dielectric strength of oil-immersed transformer insulation with superimposed AC and lightning impulse voltage

    Page(s): 683 - 687
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    The 50-Hz AC partial discharge (PD) triggered by a lightning impulse voltage of 1.2/50 μs and breakdown characteristics under superimposed voltage conditions are studied for several modes of oil-immersed transformer insulation, representing typical combinations of mineral oil and oil-impregnated paper. It is concluded that (1) no AC PD phenomena are triggered by lightning impulse voltage when the preapplied 50-Hz voltage is less than 50% of the RMS value of the AC PD inception voltage for all combinations of polarities, and (2) peak breakdown values of the superimposed voltage are reduced by 10 to 19% for a single applied lightning impulse with a low solid-to-liquid dielectric packing fraction (SLPF) for oil-impregnated paper in the intercoil model, but the difference in breakdown voltage between the superimposed voltage and a single lightning impulse is very small for a high SLPF for oil-impregnated paper, i.e. above about 30% View full abstract»

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  • Backscattered electron imaging and energy-dispersive X-ray studies of water-treed polymeric insulation

    Page(s): 730 - 736
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    The backscattered-electron imaging mode in a scanning electron microscope has been used to detect and image water trees in electrically stressed cross-linked polyethylene (XLPE) insulation. The location and distribution of inorganic contaminants, such as Cu or Cl, within the water-treed regions of the insulation were revealed using backscattered-electron imaging. Energy-dispersive X-ray (EDX) spectroscopy and mapping were subsequently used to ascertain the chemical nature of the contaminants. Contaminants were only detected by EDX inside the treed areas delineated by the backscattered-electron images. A careful comparison of backscattered-electron and optical images of thin microtomed XLPE samples did not show any contaminant penetration beyond the visible-treed region. Results from a variety of service-aged and laboratory-stressed samples are presented, as well as a preliminary attempt to extend backscattered-electron imaging to optically opaque ethylene-propylene-rubber insulation View full abstract»

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  • Dielectric response of polar materials

    Page(s): 622 - 629
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    The proposed approach is based on the unique property that the ratio of the real to the imaginary parts of the susceptibility or of the polarization decrement are independent of frequency in the respective frequency ranges. The resulting energy criterion is discussed, and a screened polarization model and mechanical relaxation model are developed and applied to dipolar spectra both below and above the loss peak frequency. A discussion is presented of the implications for the understanding of the physical nature of loss in dipolar materials within a framework which has applicability to a wide range of polarization processes. Given the inevitable connection between the energy criterion and the fractional power laws, the question of which is the primary cause and which the consequence is discussed View full abstract»

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  • Pulse-count distribution as a possible diagnostic tool for assessing the level of degradation of rotating machine insulation

    Page(s): 747 - 756
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    A method is presented by which one can qualitatively assess the condition of epoxy mica insulation systems used in large rotating machines as a function of time. Experimental techniques for acquisition and analysis of failure data are presented. Experiments were conducted on actual rotating machine coils subjected to combined electrical and thermal stress, under simulated conditions, so as to reflect the service environment as far as possible. Diagnostic measurements such as incremental loss tangent, capacitance change, partial discharge pulse count/magnitude, and PD energy were performed along with destructive measurements (dielectric strength). There appeared to be an excellent correlation between pulse count/magnitude and dielectric strength. This fact can be used in the assessment of the degree of degeneration of the insulation View full abstract»

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  • Environmental aging of insulating materials

    Page(s): 667 - 671
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    Predicting the useful life of organic materials has been made possible by the use of Arrhenius plots. Recently, data have become available to show the combined effects of the humidity and temperature on the aging performance of moisture-sensitive materials. A method is developed to use such information to calculate the predicted life of a material operating in enclosed (warm) electrical equipment in a room of known temperature and humidity. Although several assumptions must be made in this development of predicted life, guidelines for selecting reasonable assumptions are indicated. First, an explicit assumption is made that the period selected for measurement of room temperature and humidity is typical of the room conditions over the life of the equipment under consideration. Second, the assumed temperatures for the iso-aging curve calculations should span the room temperature and the equipment operating temperature. Finally, the assumed relative humidity for translation of the iso-aging curve should be somewhat above the humidity at the calculated equivalent temperature in order to allow a conservative prediction View full abstract»

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  • Detection of Joule heating before dielectric breakdown in polyethylene films

    Page(s): 715 - 722
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    Local Joule heating in polyethylene films was detected at room temperature before dielectric breakdown under DC field. When a high-temperature point appeared in a film, the conduction current increased with time. The final breakdown occurred at the point of the highest temperature. This concurrence of the breakdown point and the Joule heating point suggests that a thermal process takes part in the DC electrical breakdown of polyethylene films at room temperature and that there are weak points leading to the breakdown in the film where the current density is above the average. Almost the same results were observed for ethylene-vinyl acetate copolymer films, although the local Joule heating and the electrical breakdown occurred at lower electric fields than those for polyethylene films View full abstract»

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  • Evaluation of internal electric field of PE/EVA composite systems by TSC-separation methods

    Page(s): 702 - 706
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    For the purpose of internal field evaluation of an insulating composite, the Maxwell-Wagner model is usually applied. However, it is constrained by the possibility of ignoring the influence of interfaces. In order to evaluate internal field under any conditions, the thermally stimulated current (TSC) separation method was developed. This method is useful for analysis of a composite if it is dipolar. Details are provided of the TSC separation method and the experimental results on polyethylene and an ethylene-vinyl acetate copolymer composite. It proved that this method is not only useful for internal field analysis but could also be used to identify the space charge location inside the composite View full abstract»

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  • A model for the simulation of relaxation phenomena in dielectrics

    Page(s): 617 - 621
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    A simulation of the behavior of the complex permittivity in a polymeric material as a function of frequency is presented. Starting from experimental data and taking into account the occurrence of a distribution function in the relaxation times, an electrical equivalent circuit for each material can be established. Then, the model is used to analyze some insulating systems. The curves are calculated for a stratified insulation and for a single material subjected to chemical degradation. An experimental study on ad hoc polypropylene films purposely processed with different concentrations of a particular polar additive is presented and discussed on the basis of the model. Several possible applications are considered View full abstract»

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  • Dielectric spectroscopy for characterization of the microstructure of copolymers

    Page(s): 630 - 637
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    Thermally stimulated current spectroscopy has been applied to the characterization of amorphous phases in multiphasic polymers. A preliminary study of the homopolymer has shown that the compensation diagram can be used for defining the structure of an amorphous phase. In synthetic copolymers, the compensation diagram gives phase segregation. Data obtained on a series of latexes are reported as an example. The same methodology can be used in natural copolymers. In the collagen investigated, despite its complex chemical structure, simple behavior of the biphasic copolymers has been found View full abstract»

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  • Mechanism of prebreakdown spacer charging in non-uniform fields under unidirectional voltages

    Page(s): 642 - 654
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    Charge densities on cylindrical spacers placed in a rod-plane gap geometry were measured using a capacitive probe in an SF6 environment. The corona discharge occurring at the tip of the rod electrode was found to be a main source of charge accumulation for DC and lightning impulse voltages. Further, microdischarges occurring at asperities or other locations with high electric fields on the plane electrode may explain the nature of spacer charge accumulation under positive polarity voltage application. For DC voltages that are lower than the corona inception, the charge accumulation may be explained by the drift of charge carriers present in the gas; this mechanism is not effective under impulse voltages View full abstract»

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  • Degradation of electrical insulating paper monitored with high performance liquid chromatography

    Page(s): 737 - 746
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    The degradation of electrical kraft paper under carefully controlled laboratory experiments is being examined in order to find correlations between the change in dielectric and mechanical properties and the increase in concentration of furan compounds in the oil. These correlations could be valuable for assessing the extent of degradation of paper insulation in operating HV equipment. Preliminary investigation has shown that the concentration of furan compounds in mineral oil can be determined relatively easily using high-performance liquid chromatography (HPLC). While some of these compounds are themselves unstable, three have been found which are worth monitoring to correlate with the tensile and dielectric properties of the paper. Useful trends have been obtained, and further investigation is taking place to obtain clear cut correlation of practical value for monitoring the condition of actual transformers View full abstract»

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  • The statistics of electrical tree inception

    Page(s): 660 - 666
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    A theoretical model is proposed to explain the statistics of electrical tree inception based on the geometry of the initiating electrode and the field variation around its surface. Numerical calculations show that the model is sufficient to describe qualitatively the observed features of both the time to inception distribution and the distribution of inception voltages (fields) obtained in ramp experiments. The implications of the model for the analysis of such experiments are discussed View full abstract»

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  • The use of ferroelectrics for acceleration of charged particles

    Page(s): 655 - 659
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    Several mechanisms are described which lead to fast polarization changes in ferroelectric materials. The resulting surface charges may ultimately generate electric field spikes on the order of 1 GV/m. The fast response of ferroelectric materials to phase transitions under the influence of external electrical and/or mechanical stress allows the design of structures which emit and accelerate charged-particle beams View full abstract»

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  • The influence of morphology on electrical tree initiation in polyethylene under AC and impulse voltages

    Page(s): 707 - 714
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    AC and impulse voltages were applied to needle electrode specimens modeling the interface of the semiconducting layer and insulation of XLPE (cross-linked polyethylene) cables. TEM (transmission electron microscope) photographs of minute trees (<10 μm) immediately after their initiation were examined in detail. It is shown that AC trees are hollow tubes of ≈100 nm in diameter in the initial stage. The tree starts from a minute projection of the electrode due to cohesion of carbon particles. Lamellae are radially concentrated at the tree starting point. The tree extends along lamellae and has corrugated constrictions at a pitch of about 50 to 100 nm. AC tree initiation is supposed to be a local breakdown due to concentration of electric current at a minute region, on the order of 100 nm. Minute impulse trees are ≈100 nm in diameter, and the lamellae concentration around the tree starting point is not clear. The impulse tree is more parallel to the lamallae than the AC tree and contains no corrugated constrictions. Impulse tree initiation is concluded to be a filamentary breakdown over several micrometers View full abstract»

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  • The effect of pressure on streamer inception and propagation in liquid hydrocarbons

    Page(s): 672 - 682
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    High-speed electrooptical techniques are used to study prebreakdown behavior in n-hexane and toluene at pressures between 0.1 and 5 MPa. The results show that pressure inhibits the formation of low-density regions and increases prebreakdown inception voltage. The observed pressure effects suggest that the low-density region is the result of liquid vaporization near the electrode. Results obtained with a chopped voltage pulse indicate that the shape of the applied voltage is not critical to streamer growth. Streamers were observed growing after the voltage pulse had been removed, which suggests that the streamer growth process is: (1) to some extent locally controlled by the presence of injected charge, or (2) controlled by thermal expansion of the low-density region. It was also observed that the streamer would initiate, grow, and then disappear under impulse voltage conditions View full abstract»

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  • Modification of XLPE exposed to partial discharges at elevated temperature

    Page(s): 688 - 692
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    Extensions are presented of research on partial-discharge-induced surface modifications of XLPE (crosslinked polyethylene) wherein the XLPE sample surface during corona exposure is maintained at an elevated temperature ranging up to 160°C. As for the ambient temperature case, circular degradation patterns are found, but their appearance and composition depend strongly on temperature. Furthermore, oxidation within a shallow (several micrometers) surface layer is found to increase sharply with rising temperature, an effect attributed to inward diffusion of corona-produced oxidizing species, particularly ozone and nitrogen oxides View full abstract»

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  • Influence of physical aging processes on electrical properties of amorphous polymers

    Page(s): 693 - 701
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    The influence of sub-Tg (the glass transition temperature) physical aging processes on the electrical properties of amorphous polymethylmethacrylate (PMMA) and polydiancarbonate was investigated by means of the thermally stimulated depolarization currents, isothermal transient charging currents, and AC low-frequency dielectric methods. A marked decrease in polarization and loss factor corresponding to the main α relaxation (glass transition) was observed with increasing aging times, especially for aging temperatures ranging from 10 to 30°C below Tg. This behavior is compatible with the free-volume concept and can be explained by a gradual decrease in segmental mobility following a shift of the relaxation time spectrum of molecular motions to longer time scales. In PMMA, the secondary β relaxation is also significantly affected, which is probably an indication of cooperation of the main chain. The experimental evidence suggests that proper consideration should be given to thermal history and aging affects in standard electrical testing of polymer materials View full abstract»

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Aims & Scope

This Transactions ceased production in 1993. The current retitled publication is  IEEE Transactions on Dielectrics and Electrical Insulation.

Full Aims & Scope