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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 3 • Date Jun 1995

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Displaying Results 1 - 21 of 21
  • Can fractal features be used for recognizing 3-d partial discharge patterns

    Page(s): 352 - 359
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    Fractals have been used extensively to provide a description and to model mathematically many of the naturally occurring sampler shapes, such as coastlines, mountain ranges, clouds, etc., and have also received increased attention in the field of image processing, for purposes of segmentation and recognition of regions and objects present in natural scenes. Among the numerous fractal features that could be defined and used for an image surface, fractal dimension and lacunarity have been found to be useful for recognition purposes. Partial discharges (PD) occurring in all HV insulation systems is a very complex phenomenon, and more so are the shapes of the various 3-d patterns obtained during routine tests and measurements. It has been fairly well established that these pattern shapes and underlying defects causing PD have a 1:1 correspondence, and therefore methods to describe and quantify these pattern shapes must be explored, before recognition systems based on them could be developed. This contribution reports preliminary results of such a study, wherein the 3-d PD pattern surface was considered to be a fractal, and the computed fractal features (fractal dimension and lacunarity) were analyzed and found to possess fairly reasonable pattern discriminating abilities. This new approach appears promising, and further research is essential before any long-term predictions can be made View full abstract»

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  • Effect of charge accumulation on ac breakdown through a crack between two insulating walls

    Page(s): 492 - 498
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    Breakdown in air through a crack between two insulating plane walls was investigated, using a knife edge, plane electrode configuration. A crack of width s was formed by two plane walls of PE blocks. The minimum crack width studied was 15 μm. The maximum electrode distance studied was 80 mm. The experiments were performed in air controlled at 20 to 25°C and 40 to 50%RH. In the region of the crack width 15>s>100 μm (A region), ac breakdown voltage (BDV) decreased with the decrease in s. In the region 0.1>s>1 mm (B region), BDV slightly increased with a decrease in the width. BDV decreased at the width of the crack where the magnitude of positive charge due to ac partial discharges (PD) was large. The magnitude of the negative charge due to PD was almost unchanged in the B region, but it increased in the A region. The surface voltage distribution on the wall suggested that positive charge accumulation on the wall and its neutralization play an important role of charge quantity of PD on the crack width View full abstract»

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  • Dielectric relaxation properties of filled ethylene propylene rubber

    Page(s): 394 - 408
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    Thermally stimulated discharge currents and time domain dielectric spectroscopy were employed to characterize the behavior of clay-filled ethylene propylene rubber. Measurements were made on samples with different clay concentrations and particle sizes. The main effect of the clay filler on the electrical properties is on interfacial polarization which occurs at the clay polymer interface. The experimental results are sensitive to the shape of the clay particles. A few results indicate that other mechanisms also affect the electrical behavior of this material View full abstract»

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  • Characteristics of preflashover light emission from dielectric surfaces in vacuum

    Page(s): 483 - 491
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    Luminescence from dielectric surfaces during preflashover in vacuum was observed under pulse excitation. By preflashover luminescence, we mean light emission under no flashover conditions. Study of light emission with respect to electrode configurations and spacing between the two electrodes shows that preflashover light emission is related to electron emission from the cathode and trapping/detrapping processes on the dielectric surface layer. Spectroscopic observations of the light emission indicate that the preflashover spectrum changes from one voltage shot to another voltage shot of the same amplitude and that the spectral distribution of the light emission depends on the dielectric material used. We believe that the light emission from preflashover is electroluminescence which may be attributed to de-excitation of excited species in the surface layers of the dielectric material View full abstract»

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  • An analysis of 2,6-di-tert-butyl-p-cresol in insulating oils by high-performance liquid chromatography

    Page(s): 413 - 417
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    The presence of antioxidant is the key factor in controlling the oxidation of an insulating oil. Any monitoring program set up to keep track of its concentration could result in substantial savings by prolonging the oil service life and slowing down the transformer aging process. This paper describes a method for measuring the amount of the antioxidant 2,6-di-tert-butyl-para-cresol (DBPC) in new or service-aged insulating oils by high-performance liquid chromatography (HPLC) using a Hamilton PRP-1 column (10 μm) and an ultraviolet detector (283 nm). The mobile phase is a mixture of acetonitrile-water (97.5/2.5, v/v) with a flow rate of 1 cm3/min. The byproducts from DBPC, oil oxidation and oil components, whose retention time in HPLC is identical to that of DBPC, were eliminated by filtering the oil on a Sep-Pak Classic silica cartridge. The proposed method has a detection limit of 2.0 ppm and a precision better than 1.7% View full abstract»

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  • DC fields and analytical image solutions for a radially anisotropic spherical conductor

    Page(s): 360 - 367
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    Static problems involving a conducting sphere with radial anisotropy (radial and tangential conductivity, denoted α and η) are considered. The internal and external fields and the dipole moment are derived for a radially anisotropic sphere, and a condition relating α, η and the external isotropic conductivity α 0 is given, for which the dipole moment vanishes, implying that the sphere becomes transparent in an homogeneous current flow. Also, problems involving external and internal point sources of current are examined, and simple interpretations in terms of image sources are presented for cases where α≫η and α≈η. Image expressions for radial and tangential dipole sources are elaborated View full abstract»

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  • Polarization behavior of ferroelectric Bi4Ti3O12 ceramics

    Page(s): 409 - 412
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    The effect of oxidation and reduction atmospheres on the ferroelectric behavior of bismuth titanate was investigated. The effective surface density σeff of free charges in samples polarized by applying an electric field Ep=1 MV m-1 at a temperature of 100°C for 90 min was determined by compensation voltage measurements. The thermally treated samples in oxidation atmospheres exhibit a significant decrease of σeff while those treated in a reducing atmosphere exhibit an increase accompanied by an improvement in the stability of the electret charge. Such observations indicate that the formation as well as the transformation of the electret state of Bi4Ti3O 12 ceramics may be related to the enhancement and diminishing of oxygen vacancies. In all cases homocharge with a time independent sign was obtained View full abstract»

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  • Loss and recovery of hydrophobicity on RTV coating surfaces. Loss of hydrophobicity [comment and reply]

    Page(s): 506 - 507
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    For original paper see ibid., vol. 1, no. 4, p. 601-4 (1994). Discusses the optimal contamination layer to be allowed on an RTV coated insulator. Is there at all such an optimal quantity? Does such an optimal contamination layer depend on the thickness of the RTV coating? The question of differences in silicone rubber properties to embed surface pollutants as well as to recover hydrophobicity has been analysed at length. Do the original authors have data which indicate the diffusion of the low molecular weight (LMW) fraction from thinner and thicker RTV coatings? A reply by the authors is appended View full abstract»

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  • Dielectrometry measurements of moisture dynamics in oil-impregnated pressboard

    Page(s): 329 - 351
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    In oil-impregnated pressboard of medium and low humidity the complex dielectric spectrum's shape and amplitude do not change with variations in temperature and moisture content, but only shift in frequency. Thus it is possible to create a `universal' curve, with appropriate temperature correction factors, which can be used to extract information about the moisture dynamics of solid transformer insulation from dielectrometry measurements. To create the universal curve, measurements are taken with the material placed in a parallel-plate structure whose complex impedance is measured. To measure spatial distributions of dielectric properties, imposed ω-k (frequency-wavenumber) dielectrometry is used, where a set of interdigitated electrodes on one surface of the material cause the electric field to have a limited depth of penetration into the dielectric, proportional to the electrode spacing. Measurements using the parallel-plate methodology establish a mapping of the dielectric spectrum of EHV-Weidmann HIVAL pressboard impregnated with Shell Diala A transformer oil, as a function of temperature and water content. This mapping is then used to determine spatial moisture profiles in pressboard in other experiments which make use of a three-wavelength interdigitated sensor View full abstract»

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  • Rejection of narrow-band noise and repetitive pulses in on-site PD measurements

    Page(s): 433 - 446
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    This paper deals with the possibilities of noise suppression in partial discharge (PD) measurements under on-site conditions. After the presentation of previous work, a new concept of designing a digital FIR filter with a fixed order to suppress narrow-band disturbances is presented. The method is compared to a design using adaptive filters. A filter device, which was developed as an extension for a conventional PD detector, is presented. In the second part of the paper, a method for suppressing pulse shaped noise synchronous to the test voltage is discussed. The disturbing pulses are detected automatically in a PD signal by comparing several recorded cycles of the test voltage. There are two possibilities to suppress the pulses. The first one is an off-line method that allows attenuation degrees up to 30 dB, the second one works on line, and can process continuous PD signals. For the latter case a suppression of 14 dB for worst case pulses is possible with the developed hardware. In order to demonstrate the abilities of the filters, several measurements are shown View full abstract»

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  • Electrical conduction in fluoropolymer films

    Page(s): 475 - 482
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    Isothermal charging, discharging and transport currents in fluoropolymer films are measured in the temperature range of 50 to 200°C at electric fields to 40 MV/m. The currents are measured as a function of time elapsed after the application of steady state voltages in the range of 1 to 104 s. Charging currents increase with time to 103 s, depending upon the electric field and temperature of the polymer. For longer duration the currents decrease with time. Discharging currents decrease monotonically with time and the discharge current-time characteristics are dissimilar to charging current-time characteristics. Transport currents which are derived by subtracting the discharging currents from charging currents are shown to arise due to Schottky emission assisted by the electric field. At shorter time intervals (<103 s), it is suggested that the electric field at the cathode increases due to heterocharges and gives rise to currents increasing with time, while for longer times, gradual filling of the traps results in a decrease of the current. Low frequency dielectric loss factors are evaluated from the discharging current measurements View full abstract»

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  • The role of microscopic defects in multistress aging of micaceous insulation

    Page(s): 426 - 432
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    The aging mechanism of mica/epoxy insulations under multiple stresses is investigated with both breakdown tests and a scanning electron microscope (SEM). The result of simultaneous multistress testing shows that thermal and mechanical stresses have a synergistic effect on electric life. Similar results are also obtained for residual electrical breakdown characteristics after sequential application of thermal aging and mechanical stress. It is confirmed with SEM pictures that thermal stress causes isolated delaminations at the mica/epoxy interface. Using a newly developed SEM apparatus with servomechanisms, the development of delamination and cracks are observed during mechanical fatigue tests in realtime. A multistress aging model is proposed with relation to the generation and development of the microscopic defects View full abstract»

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  • Characterization of an insulated SCL non-relativistic electron-beam diode operating at 300 kV/cm

    Page(s): 368 - 375
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    High current density (>100 A/cm2) electron beam diodes operating beyond 250 kV/cm in a 1 cm gap configuration are described. The principal features of the electron source are the high current density and high field without the onset of vacuum arcs or arc collapse prior to gap closure. The electron beam diode was controlled by a variable pulse-width output Marx generator. Fields to 600 kV/cm were applied for 25 to 30 ns, and to 300 kV/cm during space-charge limited current conduction at pulse lengths of 100 to 120 ns. Evidence of the transition to space-charge limited unipolar flow and transition to bipolar space-charge limited flow was obtained. Traces of the typical behavior and the different transitions are shown. Beam uniformity was measured by using a set of Faraday cups. The Faraday cup setup was then used to demonstrate suppression of electron emission from surfaces coated with a dielectric film View full abstract»

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  • Effect of temperature on the partial discharge initiation voltage of capacitors

    Page(s): 499 - 502
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    The paper describes a novel test arrangement to characterize the temperature dependence of partial discharge (PD) inception voltage in capacitors. The test arrangement consist of a temperature chamber, HV supply and PD detection circuit. The corona free operation voltage is 0 to 3.8 kV, temperature range is -50 to 200°C. Different type of capacitor PD inception voltage were measured at different temperatures. It was found that most commercial capacitor PD was lower than the rated voltage and increased with the temperature. In contrast, the higher quality chip type ceramic capacitor PD was above the rated voltage and decreased with the temperature. The paper concludes that the prediction of capacitor performance requires the measurement of PD at different temperatures View full abstract»

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  • Area and volume effects on breakdown strength in liquid nitrogen

    Page(s): 376 - 384
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    We measured dc and ac breakdown voltages in liquid nitrogen (LN 2) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN2 did not increase monotonously but partially decreased as the sphere diameter increased at a constant gap length. Thus, the existence of the area and the volume effects on the breakdown voltage in LN2 was verified quantitatively; the breakdown strength decreased when increasing the {SEA}90 (90% stressed electrode area) and the {SLV}90 (90% stressed liquid volume). By changing the experimental conditions, it was verified that both area and volume effects, having a mutual correlation, simultaneously lead to the degradation of the breakdown strength in LN2. In order to examine the area and the volume effects for the larger {SEA}90 and {SLV}90, we also measured the breakdown voltage with a coaxial cylindrical electrode. It was concluded that the dc and ac breakdown strengths in LN2 decreased as the {SEA}90 and the {SLV}90 increased varying widely from 100 to 105 mm2 and from 10 -1 to 105 mm3, respectively View full abstract»

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  • Using the UHF technique to investigate PD produced by defects in solid insulation

    Page(s): 448 - 459
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    The partial discharges produced by internal defects in solid insulation have been investigated using the ultra high frequency (UHF) technique. The work has shown that the discharges produced by a virgin cavity generate a UHF signal that may be detected using this UHF technique. The discharges are observed as a series of UHF pulses that occur less frequently as time progresses. The reduction in the number of UHF pulses has been attributed to a progressive change from the streamer discharge mechanism, that is observed with a virgin cavity, to the Townsend mechanism, which occurs when the cavity has been exposed to prolonged discharge activity. It also has been shown that the discharges associated with tree formation generate a detectable UHF signal. The paper describes a method of distinguishing between the streamer discharges and the discharges that cause treeing View full abstract»

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  • Space charge in XLPE power cable under dc electrical stress and heat treatment

    Page(s): 467 - 474
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    It is well known that the existence and accumulation of space charge in insulation can be harmful to polymer power cable. Although there is much research done on space charge distribution in various samples, there are still some important problems left unsolved, such as the mechanism of formation and elimination of space charge in dielectrics. Because of its superiority, the pulsed electro-acoustic (PEA) measurement method is used widely in the measurement of space charge in various geometries of samples such as the multiple layer plate and cylindrical cables. The space charge in crosslinked polyethylene (XLPE) power cable under dc voltage and heat treatment is investigated by the PEA method in this paper. After heat treatment, the space charge, which previously formed in cable insulation under the applied voltage, disappears. If electrical stress is applied to the insulation again, space charge will appear once more. However, after the cable is heated and degassed in vacuum for a long time, no matter how long the dc voltage is applied to the cable, no more space charge will be formed in the cable. The mechanism of formation and elimination of space charge under the effect of electrical and thermal stress is discussed View full abstract»

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  • Weibull statistical analysis of area and volume effects on the breakdown strength in liquid nitrogen

    Page(s): 385 - 393
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    In order to examine the area and the volume effects on breakdown strength in liquid nitrogen (LN2), we measured dc and ac breakdown voltages in LN2 with sphere to plane and coaxial cylindrical electrode configurations. We also carried out statistical analysis of the experimental results using the Weibull distribution and discussed with the statistical stressed electrode area (SSEA) and/or liquid volume (SSLV). The dc and ac breakdown strength in LN2 decreased to 1/5 with increasing SSEA and SSLV over the wide range from 100to 105 mm2 and from 10-1 to 105 mm3, respectively. The Weibull shape parameter m for the sphere to plane and the coaxial cylindrical electrodes was estimated to be 6 to 8 and 11 to 13, respectively; the breakdown strength in LN2 saturated with increasing SSEA and SSLV. From these results, it was verified that the area and the volume effects definitely determined the breakdown strength in LN2. The results of the Weibull statistics were compared with the conventional concept of the stressed electrode area (SEA) and liquid volume (SLV). Consequently, SSEA or SSLV was found to be nearly equal to 80 to 85% of SEA or SLV View full abstract»

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  • Diagnostics of radiation exposure of dielectrics by fluorescence techniques

    Page(s): 503 - 505
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    We have examined the inherent fluorescence of epoxy resin and polyethylene samples exposed to radiation from a 60Co gamma source. We find that the fluorescence intensity increases with radiation dose. There is also a significant change, in polythene in particular, of the shape of the fluorescence spectrum. Accordingly, we suggest that a fluorescence intensity ratio approach will be useful, as it is in diagnostics of power transmission cable insulation aging, to assess the aging of dielectrics in radiation environments View full abstract»

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  • Charge distributions in PC/SAN/PCL polymer blends

    Page(s): 460 - 466
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    The formation of space charge in polycarbonate (PC)/poly (styrene-co-acrylonitrile) (SAN) blends containing poly (ε-caprolactone) (PCL) as a compatibilizer for improving the miscibility, has been studied by measuring spatial charge distributions. It is found that the miscibility of PC/SAN blends is improved as the PCL content increases. The amount of charge accumulated in the blends decreases when the PCL content increases to 15% by weight and then increases when the PCL content is >15% wt. The decrease of charge at the PCL content <15% wt was attributed to the combined effects of changes in conductivities and the molecular interactions at the domain interfaces. The increase of charge at the PCL content >15% wt was attributed to the trapping of charge at the amorphous and crystalline phases of PC and PCL View full abstract»

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  • Effect of short HV pulses on bacteria and fungi

    Page(s): 418 - 425
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    The survival of three kinds of microorganisms under strong-pulse electric field conditions was investigated with a possible application of the electric pulse method for sterilization of consumable liquids. The results of the investigations of survival ratio of Gram-negative (Escherichia coli, Yersinia enterocolitica) and Gram-positive (Staphylococcus aureus, Listeria monocytogenes) bacteria and yeastlike fungi (Candida albicans) are presented. The HV pulses with peak voltage U=0 to 100 kV and rise time tn= 0.5 to 1.2 μs were applied. The microorganisms were suspended in an NaCl solution with γ=6 to 13 mS/cm conductivity and pH=7.2. The experimental setup and the dependency of the microorganism survival ratio on the rise time, peak voltage and on the number of pulses applied, are presented. It has been found that the lethal effect on microorganisms caused by HV pulses depends on the pulse parameters as well as on the kind of microorganism being treated 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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