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Electrical Insulation, IEEE Transactions on

Issue 3 • Date Jun 1988

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Displaying Results 1 - 21 of 21
  • A study of the up-and-down method for non-normal distribution functions

    Page(s): 357 - 364
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    The assessment of breakdown probabilities is examined by the up-and-down method. The exact maximum-likelihood estimates for a number of response patterns are calculated for three different distribution functions and are compared with the estimates corresponding to the normal distribution. Estimates of the 50% probability breakdown voltage, and of the scale parameter of the breakdown probability functions, are investigated View full abstract»

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  • Progressively-censored aging tests on XLPE-insulated cable models

    Page(s): 365 - 372
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    The problem of evaluating the time-to-failure percentiles in progressively-censored tests on solid insulating materials is addressed. Statistical methods to estimate the parameters of the Weibull distribution (and their confidence limits) are examined on the basis of the results of aging with combined thermal-electrical stresses carried out on XLPE insulated cable models. These tests are performed at the same stresses on samples more than 1 m long and subjected to progressive censoring of aging times, or on short specimens about 20 cm long and subjected to complete, or singly-censored, life tests. This procedure allows the effectiveness of progressively-censored tests in estimating life percentiles to be verified, and the accuracy of the methods to be compared View full abstract»

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  • An analysis of field-dependent water tree growth models

    Page(s): 345 - 356
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    The directly observed growth of individual water trees has been used to examine analytically the applicability of a number of field-dependent growth models. Aqueous electrodes with different radii of curvature have been used to investigate the influence of the inception geometry upon tree growth. Measurements of growth rate have been made over a range of frequencies and the rate constants and compared with the results of dynamic dielectric and mechanical responses, in an attempt to elucidate the material factors that are involved in treeing. It is shown that the frequency dependence of the growth rate cannot be attributed to a modification of the dielectric by the tree, causing a frequency-dependent change of field strength for large enough trees. The rate constants obtained are frequency dependent and the analytical results are consistent with a mechanism of electrical origin, which causes microvoids and crazes to nucleate and grow. In general, the propagation rate of the damage seems to be rate limiting, but in a few cases generally characterized by a sharp inception geometry and thus possibly mechanical strain, it is the nucleation rate that is the slowest View full abstract»

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  • Impulse breakdown of covered cylinders in SF6 and SF6-gas mixtures

    Page(s): 467 - 473
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    The effect of the following parameters on the breakdown field strength of coated cylinders in air, nitrogen, SF6, a mixture of 50% SF6/50% air, and a 50% SF6/50% nitrogen mixture were studied: (1) voltage waveform and polarity and (2) insulating material and thickness. The results are presented graphically and in a table View full abstract»

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  • The generalized integration method for predicting impulse volt-time characteristics for non-standard wave shapes-a theoretical basis

    Page(s): 373 - 381
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    A theoretical basis for the integration method of predicting the strength of insulation subjected to impulses of nonstandard wave shapes is presented. Two theoretical approaches are developed for a general physical model of impulse breakdown applicable to gaseous, liquid, and solid insulation, and from an energy-balance model applied to an incipient breakdown channel in a general dielectric. It is therefore not surprising to find that the integration method is reasonably successful as a tool for predicting nonstandard wave-shape insulation strengths, using as input the data derived from tests with standard-wave-shape impulses. Numerical results are presented that provide an understanding of the shape of volt-time curves. An energy balance model has also been used to derive the resistance of the channel after breakdown has occurred View full abstract»

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  • Effect of temperature on the breakdown voltage of paper-covered conductor impregnated with various liquids

    Page(s): 475 - 483
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    Lengths of flat copper conductor insulated with aromatic polyamid (aramid) tape or cellulose tape were bent into rings and secured around the edge of discs. Two discs were placed edge-to-edge in one plane and breakdown voltages were measured between the conductors. The conductor assemblies were dried under vacuum, impregnated and immersed in several low-flammability liquids at temperatures up to 180°C under air. Comparisons were made with mineral oil which was tested up to 130°C under nitrogen. With the discs set to provide a 1-mm liquid gap between the insulated conductors, the variation of breakdown voltage with temperature is limited to a few percent and values tend to their highest at 80°C for both AC and lightning impulse. With the conductors in contact, alternating breakdown voltages vary little, whereas impulse voltages fall appreciably with temperature rise, especially for cellulose View full abstract»

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  • An expression of the electric field distribution in rod-plane gaps

    Page(s): 493 - 494
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    A closed-form expression is derived to represent the electric field distribution along the axis of rod plane gaps in the ranges 0.1-1 cm for the electrode radius and 2-12 cm for the gap spacing. The expression is valuable in modeling discharge development in SF6 gaps View full abstract»

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  • Effects of agglomeration of carbon particles in the semiconducting material on the dielectric strength of XLPE insulation

    Page(s): 335 - 344
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    The relationship between the breakdown strength and the material structure of the order of a few micrometers near the interface between the polyethylene insulation and the semiconducting material is discussed. It was found that the agglomeration of the carbon particles in the semiconducting material ranges from about 100-300 nm. The autocorrelation function A(D) of the transmission electron microscope (TEM) photographs can represent some aspects of the agglomeration of the carbon particles. The characteristic distance D 0 in the autocorrelation function ranges from about 100-300 nm, and corresponds closely to the size of the agglomeration of the carbon particles. It was found that the highest breakdown strength in terms of the Weibull 10% breakdown strength may be obtained when D0 is around 200 nm. The thermal breakdown may explain the relationship between the breakdown strength and the agglomeration of the carbon particles View full abstract»

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  • Investigation of space charge in low-density polyethylene using a field probe technique

    Page(s): 441 - 445
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    A test method that uses a capacitive field probe to investigate the space charge distribution in low-density polyethylene (LDPE) is described. Specimens of 7-mm thickness were stressed under 100 kV DC at room temperature and for different time periods. The results indicate that the LDPE insulation layer between electrodes is occupied by positive and negative homocharges. The dependence of space charge distribution on the stressing time is also evident View full abstract»

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  • Anisotropy in electric properties of fabrics containing new conductive fibers

    Page(s): 383 - 386
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    Electric properties of 2/1 warp-faced twill fabric containing 10% conductive fiber were studied by measuring its resistivity. Surface resistivity measurements in different directions exposed an anisotropy effect ascribed to the sample weave pattern. Bulk conductivity, evaluated on a basis of an electrical network model, has been found to be in very good agreement with the results. Stretching the fabric in the direction of current flow increases the conductivity View full abstract»

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  • The measurement and modeling of the dielectric response of molecules during curing of epoxy resin

    Page(s): 409 - 417
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    Results of dielectric measurements as a function of frequency are presented for the curing of an epoxy resin. By considering the change in the time occurrence of the peak in magnitude of the dissipation factor, and also by comparing this with the results for the cured resin, it is concluded that such peaks are not the result of either gelation or an α-relation process. The results were interpreted on the basis of a model considering the growing polymer dipoles to be in solution, the solvent being the unreacted monomer and hardener. If simplifying assumptions are made, it proved possible to estimate the activation energy for the curing process. The value of 41 kJ/mol (0.42 eV) compared favorably with the results obtained by other workers for similar systems View full abstract»

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  • Measurement of the charge of a single or of a series of streamers in liquids subjected to a voltage step

    Page(s): 427 - 431
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    An experimental technique using an integrator to measure the charge of streamers in dielectric liquids is presented. As an application of this device, the correlation between the measured charge and streamer propagation is discussed. Using a computer, the total value of streamer charge is stored and then correlated to the gases produced by the steamers. In this way, an attempt is made to discover and analyze the events occurring during breakdown View full abstract»

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  • Contact and surface impedances on humid mica and glass

    Page(s): 397 - 407
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    Impedance measurements in the frequency range 10-2-10 4 Hz are reported on humid surfaces of ruby mica and glass, with the relative humidity (RH) of the ambient air as a variable parameter. Both the interfacial impedance of the one-dimensional contacts to the surface conducting layer and the surface impedance proper are found to be strongly dispersive, contrary to the usual assumption that surface conduction or surface leakage entails ohmic direct current phenomena. The glass samples were specially shaped to enhance the surface contribution in comparison with the contact impedance. Equivalent circuits are derived, and their physical significance is discussed. The amplitude of the rate processes is approximately exponential with RH View full abstract»

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  • Optical detection of surface discharges

    Page(s): 447 - 449
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    The use of an optical detection system for measuring surface discharges is described. A photomultiplier tube (PM) transforms the photon pulses emitted by the discharge process into an electrical signal. To suppress the noise inherent to PM operation, filters were built in the amplitude- and time-domains. Results of discharge measurements showed that in some situations, the sensitivity of the optical system exceeds that of electrical methods View full abstract»

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  • Electrical conduction of polyethylene below and above its melting point

    Page(s): 325 - 333
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    The electrical conduction of low-density polyethylene (LDPE) has been investigated between 20°C and 150°C and up to 1 MV/cm. Special molded specimens and a self-compensating test cell were used; particular care has been taken in the control of experimental conditions. Absorption currents show broad maxima, while resorption currents display polarity reversal. Steady-state currents increase with a power law of the electrical field, and they tend to saturate at high fields and temperatures. Results are discussed in the framework of space-charge-limited conduction and dispersive hopping transport models View full abstract»

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  • Application of a new geometrical approach to determination of combined stress endurance of insulating materials

    Page(s): 489 - 492
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    The determination of combined-stress endurance requires that tests at single and combined stresses are performed. Such tests are shortened, their planning facilitated and interpretation of the results simplified and made more accurate by computer-aided manufacturing (CAM), the method of combined analysis of the data, based on a geometrical approach to combined stress life. An index is proposed to evaluate material endurance in combined-stress conditions, which can easily be determined by the E-T curves for long times to failure View full abstract»

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  • Study of the decomposition of SF6 in the presence of water, subjected to gamma irradiation or corona discharges

    Page(s): 451 - 465
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    The influence of water on the degradation of SF6 was studied by subjecting mixtures of SF6 and H2O either to irradiation by 60Co γ-rays or to corona discharges (point-plane electrodes; voltage: DC negative or positive polarity). The stable-reaction products formed were analyzed and assayed with various analytical techniques including gas-phase chromatography, gas chromatography-mass spectrometry, specific electrode, colorimetry, atomic absorption, and EDAX (energy-dispersive analysis by X-rays). The influence of the energy input, the water content, the pressure of SF6 and, for the corona discharges, the value of the discharge current and the polarity of the applied voltage on the formation of SOF2 and SO2F2 which were the only two stable gaseous compounds detected, were all studied. The results obtained were compared with data from the literature and with the reaction schemes proposed to date, to explain the decomposition of SF6 under the effect of similar or different input energies View full abstract»

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  • On testing methodology of thermosetting dielectrics subjected to partial discharges: effect of temperature

    Page(s): 419 - 426
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    Following the idea of performing tests in environmental conditions as similar as possible to the real working ones, many sets of partial discharge tests carried out on an epoxy resin within a broad environmental temperature range are reported on. Life tests were carried out on specimens with a calibrated, enclosed cavity and with embedded electrodes. In the tests performed at temperatures higher than ambient, an interesting strong decrease of data scattering resulted and emphasis on the subject is given. Actually, a strong and unexpected effect was found of the residual mechanical casting stress inside the resin on time-to-breakdown, depending on the difference between the material glass transition temperature and the temperature used in the test. Further data such as discharge amplitude distributions, discharge repetition rate and inception and extinction voltages at the investigated temperatures are also reported and commented upon View full abstract»

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  • The influence of successive electrical breakdowns on the breakdown voltage of n-hexane

    Page(s): 485 - 487
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    The breakdown voltage of n-hexane measured in a point-plant test cell was found to increase with the number of breakdown events if the point had positive polarity. With a negative point, no such variation was observed. Cationic formation of polymer and deposition of polymer on the cathode is though to be the cause for this increase in breakdown voltage View full abstract»

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  • Measurement of spatial charge distribution in thick dielectrics using the pulsed electroacoustic method

    Page(s): 433 - 439
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    Measurement techniques of volume charge distribution within insulating materials are developed using a pulsed electroacoustic method. The fundamental equation of the relation between the charge distribution in the insulating material and the signal voltage from the electroacoustic transducer is presented along with the typical measured charge profiles in polymethylmethacrylate (PMMA) during and after electron-beam irradiation View full abstract»

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  • The layered-capacitor method for bridge measurements of conductive dielectrics

    Page(s): 387 - 396
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    Conductive dielectrics (ice, rocks) exhibit both dielectric relaxation and DC conductance. To suppress electrode polarization effects in bridge measurements, ice samples are sandwiched between layers of polytetrafluorethylene (Teflon). A previous approach fitted up to four discrete relaxation ranges to the effective response functions of the layered dielectric and in a final step computed relaxation parameters corresponding to ice. New algorithms, based on a generalized equivalent circuit, first invert the data to extract ice response from that of the layered capacitor. Polarization strengths and relaxation times are then fitted to the ice response by a least-square procedure. DC conductivity is obtained separately at each measurement frequency. Experimental and computational methods have been carefully verified and documented. Methodology developed by the authors provide a basis for systematic investigation of contaminant effects on the electrical properties of ice. Intrinsic processes can be measured free from carrier injection effects. Future directions for computational strategies are suggested 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