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Electrical Insulation, IEEE Transactions on

Issue 6 • Date Dec. 1992

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Displaying Results 1 - 22 of 22
  • Comments, with reply, on "Fractal character of DC trees in polymethylmethacrylate" by M. Fujii et al

    Page(s): 1222 - 1223
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    In the above-named work by M. Fujii et al. (ibid., vol.26, no.6, pp.1159-1162, Dec. 1991), the differing fractal descriptions of DC trees generated in polymethylmethacrylate (PMMA) are attributed to the local electric fields generated by space charges. The commenter argues that, while a fractal dimension approach to DC tree characterization is certainly useful, a knowledge of the value of the exponent would be much more informative from the point of view of understanding the physical mechanism involved in the stochastic development of trees. This is also important because it is eta which controls the dimension of the tree structure. In their reply, Fujii et al. points out that, since the mechanisms of the streamers in solids are not understood well at present, it is difficult to discuss the difference of the patterns of the negative and the positive point electrodes. Therefore, the difference of the patterns for both polarities cannot be discussed using only the field dependence parameter eta .<> View full abstract»

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  • Effects of filler level in RTV silicone rubber coatings used in HV insulators

    Page(s): 1065 - 1072
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    The ability of room temperature vulcanizing (RTV) silicone rubber insulator coatings to suppress leakage current, thereby preventing power outages due to contamination, was studied. The RTV coatings were applied to porcelain suspension and line post insulators for a comparative study of the performance of coated and uncoated insulators. A study of the role of alumina trihydrate (ATH) which is used to impart tracking and erosion resistance to RTV coating is reported. A study of the diffusion of silicone fluid from the bulk to the surface of the RTV silicone rubber having various ATH filler levels using attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy is also reported. It has been found that the diffusion rate of the silicone fluid from the bulk to the surface of the coating after extensive dry band arcing decreases with increasing filler level View full abstract»

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  • Ultralow-frequency dielectric properties of EPR with filler

    Page(s): 1073 - 1082
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    The dielectric properties in the ultralow-frequency regions of ethylene propylene rubber with fillers are studied. These are important materials, especially for electrical cables and wires in nuclear power generating stations. The dielectric parameters α, τo, and ∈ro-∈r∞ in the Cole-Cole equation are obtained by fitting the theoretical curve with the experimental results on the discharge current. The present polarization is caused by the interfacial polarization between EPR and the fillers. Assuming that part of the filler clusters and takes the form of spheroids, the relaxation time is correlated with the shape of the spheroid. The observed distribution of relaxation times is interpreted as being due to the dispersion of various shapes of spheroids. As the concentration of filler increases in the low-concentration region, the characteristic relaxation time τo decreases. The decrement of τo can be understood with the variation of the cluster shape with the filler concentration View full abstract»

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  • An experimental technique to study space charge injection in polymeric materials under AC voltage

    Page(s): 1095 - 1100
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    An experimental technique for detecting space charge injection in polymeric material under divergent AC field is described. The automated charge detector can be used for studying charge injection as a function of voltage and time in the 0.001-0.01-pC range. Charge injection is a strong effect above a critical voltage Vc. For a moderate overvoltage, three different phases can be distinguished in the evolution of the dielectric properties: (1) a discharge-free regime, in which low-frequency space charge injection occurs; (2) a transition regime, with high-frequency discharge-like pulses superimposed on the charge injection signal; and (3) an electrical tree propagation regime View full abstract»

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  • The validation of the thermal step method

    Page(s): 1152 - 1158
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    The validation of the thermal step method, which makes it possible to determine the electric field and the space charge distributions in insulating materials, is described. Validation was done by testing the numerical treatment, and then by depositing real corona charges in a well-localized zone of the sample. Other validation techniques, such as thermal step inversion and the simulation of the measured current from calculated space charges, are also addressed View full abstract»

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  • Computational analysis of the surface permittivity and charging of dielectrics with the SEM-mirror technique

    Page(s): 1127 - 1135
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    Computational analysis based on the charge simulation method (CSM) is used to predict the electric potential distribution produced by charges implanted within the surface layer of a solid dielectric. The potential distribution thus obtained is used to match the experimental distribution using the scanning electron microscope (SEM) mirror technique. Using an optimization iteration process, a comparison is obtained for quartz, alumina, and yttria. The computational analysis assumes a surface layer with a permittivity higher than that of the bulk. The justification for this assumption is discussed. The electric potential distribution produced by a surface with increased surface defects, as determined by the SEM mirror technique, indicates a surface with a higher permittivity. It is also shown that the ability of such a surface to store charges is decreased. The results are consistent with physical models based on band structure (or trapping energy) considerations View full abstract»

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  • Radiation-induced electrical discharges in complex structures

    Page(s): 1166 - 1178
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    Polyethylene terephthalate (PET) foils were exposed to 25-keV electron beams of 3 nA/cm2, which has been known to charge the PET surface to -15 kV or more and to produce rapid pulse discharges sufficient to lower the magnitude of the surface potential significantly. The major experimental finding is that the pulse discharges often propagate to nearby mutually biased electrodes and can create a full vacuum breakdown between stainless steel electrodes, rarely if biased at as little as 100 V, but often if biased at 300 V. The phenomena appear to be associated with the pulsed creation of dense plasma from the discharging dielectric, and the pulsed volume of gas evolved is equivalent to roughly 0.1 to 0.4 mm3 at 1 atm for nominally 50-cm2 samples. The plasma could evolve to interconnect more than two electrodes. Additionally, PET pulse rates were strongly dependent upon the proximity of sharp edges, either metallic or dielectric View full abstract»

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  • Dielectric study at microwave frequencies of water-treed crosslinked polyethylene

    Page(s): 1083 - 1088
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    The dielectric properties of water-treed crosslinked polyethylene (XLPE) were studied by using a hollow microwave waveguide system between 135 MHz and 13.6 GHz. The dielectric loss of water-treed XLPE is proportional to water content. Water-treed XLPE has a low relaxation frequency, which is probably due to bound water. The binding water in trees was confirmed by the lower apparent water diffusion rate in treed XLPE than in undamaged controls measured with a radioactive tracer method View full abstract»

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  • Study of the contact electrification of polymers using contact and separation current

    Page(s): 1144 - 1151
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    An instrument was developed that measures the current resulting from surface charge rearrangement during individual contact and separation events involving an insulating sample and a metal probe. It is shown that the values of peak current measured can be used to determine the position of the insulator in a triboelectric series. A modified field mill instrument was used to verify the accuracy of the triboelectric series determined using the contact-separation technique on a set of polymer films. Results agree with published triboelectric series in all cases for which there was agreement among the published series. All measurements were made in a controlled environment on samples which had been stored in a nitrogen atmosphere. The instrument offers the advantages of short measurement time and the ability to make nondestructive measurements on samples of arbitrary thickness including films which are <10 nm thick. Measurements can be made on either conducting or insulating samples View full abstract»

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  • Evaluation of ethylene-propylene rubber as electrical insulating material for a superconducting cable

    Page(s): 1108 - 1117
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    Mechanical and electrical properties of ethylene propylene rubber (EPR) pieces were studied in comparison with polyethylene (PE) in the cryogenic temperature region to examine the possibility of its use as solid electrical insulating material in superconducting cables. The mechanical properties of EPR are preferable to those of PE mainly because of its low shrinkage. The electric strength and dielectric loss tangent of EPR are comparable with those of PE at liquid helium temperature. Based on the encouraging results, an extruded EPR-insulated cable was fabricated and its 15-m length was cooled down to liquid helium temperature successfully. Partial discharge experiments at that temperature showed good electrical characteristics. This is a breakthrough in terms of the electrical insulation design of cryogenic cables View full abstract»

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  • Deflection of streamer channels in high magnetic field

    Page(s): 1179 - 1185
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    Streamer propagation on a dielectric film under transverse magnetic field of 3 T produced by a superconducting magnet is investigated using the Lichtenberg figure technique. A streamer propagation mode is proposed, taking account of the electric field in the space charge region left behind the streamer and the field enhancement effect at the streamer tip. Through comparison of the experimental results of the streamer deflection characteristics with the theoretical prediction, the electric field in the space charge region is supposed to reach a value of 2 to 3 kV/cm. The internal field causes voltage drop across the space charge region, while the electric field is enhanced in the concentrated positive streamer head. On the other hand, the negative streamer spreads uniformly and the field concentration effect at the tip is weak View full abstract»

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  • A study of electrical endurance of MPPF capacitors and selection of end-point criteria

    Page(s): 1193 - 1201
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    The self-healing property of MPPF capacitors results in reduction in capacitance and increase in tanδ with aging. With a view to establishing the service reliability of MPPF capacitors, long-term aging tests were performed. For estimation of the life of nonmetalized film capacitors, local breakdown of PP film at the weak spot is generally considered as the end-point criterion. However, this cannot be selected for MPPF capacitors because the metalized film continues to operate even after several such breakdowns due to its self-healing property. Sufficient information on some important aspects regarding life estimation of MPPF capacitors, e.g., the property to be selected as aging index, choice of its end point, the statistical model to be adapted, etc., is not available. In the current study capacitance and tanδ were examined as indicative of aging properties. The life of MPPF capacitors was estimated, selecting suitable end-point criteria and employing an appropriate statistical model View full abstract»

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  • Statistical approach for determining breakdown voltage of gas-insulated cables

    Page(s): 1186 - 1192
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    Several goodness-of-fit test procedures were applied to DC-breakdown conditioning test results, obtained in SF6 at 0.4 MPa in concentric-coaxial systems, and to other published data to determine whether the three-parameter Weibull distribution is the only match. The three-parameter Weibull distribution was found to be the best suited among probabilistic exponential distributions representing the area-dependent statistical characteristics of compressed-gas insulated systems. Parameters of the three-parameter Weibull distribution were estimated by simultaneous solution of maximum-likelihood equations for all conditioning data. In contrast to the experimental results given in the relevant literature, the estimates of the parameters were found to be dependent upon the pressure and geometry of the gap. The value of the shape parameter β≃7 seems to hold only for the conditioning data at 0.6 MPa. The lowest asymptotic value of the threshold estimator-minimum breakdown field strength was observed to be ~141 kV/cm View full abstract»

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  • Dielectric properties of normal supercooled water obtained in alcohol/water mixtures

    Page(s): 1136 - 1143
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    The complex permittivity of (alcohol)x/(water)1-x mixtures was studied between 1 mHz and 10 MHz, from ambient down to the glass transition temperature, in the molar fraction range 0.1⩽x⩽1. Using methanol, ethanol, propanediol 1-2, and propanetriol 1-2-3 as the second component at sufficiently high concentration (x⩾0.1) eliminates the anomalous properties of pure water, and further explorations at x=0 lead to the normal part of static permittivity and relaxation time of supercooled water down to its glass transition temperature Tg=-135°C. These calculated values are nearly independent of the alcohol used. The static permittivity between +100 and -135°C can be adjusted by the relation ∈s=-22.2+29950/T, which agrees well with the random network model. The dielectric relaxation time can be fitted to the free-volume percolation model of M.H. Cohen and G.S. Grest (1984). As long as the solutions remain homogeneous, dielectric spectra exhibit a single absorption peak, which broadens near the glass transition temperature View full abstract»

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  • Effect of additives on treeing initiation as a function of oxygen concentration in polyethylene

    Page(s): 1089 - 1094
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    The effect of additives on treeing initiation was studied in low-density and cross-linked polyethylene (LDPE and XLPE) under various conditions. The additives suppress the initiation of the deteriorated region, which is a precursor of the tree initiation, and increase the tree starting voltage. This effect was found to be a function of oxygen concentration absorbed in PE; observable when the sample is equilibrated with atmospheres with a range of oxygen from 1 ppm to 10%. Also the effect of additives is not as clear at 77 K as it is at room temperature, and differs for impulse and AC voltage. These results can be explained by the following model. (1) Free radicals are generated in the high field region by carriers injected from the electrode. (2) The free radicals react with oxygen to produce chain scission of the PE molecule, especially if no additives are present. (3) Additives stabilize the free radicals against reaction with oxygen molecules View full abstract»

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  • Partial discharge behavior of epoxy resin-impregnated transformer coils

    Page(s): 1118 - 1126
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    Partial discharge (PD) investigations of industrially made epoxy-resin-impregnated transformer coils and coils produced under laboratory conditions in a temperature range between -30 and 180°C are described. The PD characteristics of the industrial coils can be reproduced with the simplified lab coils, and these results can be transferred to the industrial coils. The PD measurements show that the PD inception voltage decreases with rising temperature, which is explained with the superposition of two temperature-dependent phenomena. First, the temperature-dependent rising values of the relative permittivity of the epoxy resin strengthen the electrical field inside the gas cavity. Second, the ignition condition, i.e., the relative gas density in the cavity, changes due to diffusion. From the observed voltage dependence of the repetition rate and phase position of the PD pulses and with the help of a capacitive electrical circuit, it is shown that the measured PDs are discharges in gas-filled cavities View full abstract»

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  • Comparison of reliability of conventional and rapid aging methods for insulating materials

    Page(s): 1159 - 1165
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    It is shown that the conventional aging (CA) method of insulating materials is only an approximation, and simulates the actual service of insulation. Since the test is complex and uncertain, CA cannot be absolute. Although most rapid thermal endurance test methods of insulating materials are based on CA, once established, they are independent test methods. Further, the dispersion of CA data is large and repeating the tests is difficult due to its long time and high costs. Rapid methods, such as thermogravimetric point slope (TPS), have smaller scatter, higher success ratio, and good accuracy and are easy to repeat. Therefore, taken as a whole, the reliability of TPS may well be identical with, or even better than, that of CA for determining the thermal endurance properties of insulating materials View full abstract»

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  • Breakdown of a thin dielectric liquid layer

    Page(s): 1217 - 1221
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    Measurements of electrical breakdown on thin (10-100-μm) liquid dielectric (capacitor impregnant) layers for different interfacial situations are discussed. For this purpose a new uniform field electrode arrangement that represents interfacial conditions close to those of industrial practice was developed. It was investigated experimentally how the dielectric strength depends on such factors as electrode area, gap spacing, nature of the interface in contact with the liquid, and temperature. The breakdown phenomena are shown to be either related to high-field conduction (i.e., boiling or electrohydrodynamic cavitation leading to the generation of a vapor bubble in the prebreakdown regime) or related to the presence of particles or to local field enhancements View full abstract»

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  • Decay of charge deposited on the wall of gaseous void

    Page(s): 1202 - 1207
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    After partial discharge activity within a gaseous void, charges accumulate on the wall of the void. The decay of such charges due to surface currents at the void wall is studied analytically, and the factors affecting this decay are indicated. The results show that in terms of the basic time constant, the decay can take a considerable amount of time. The decay rate is significantly reduced by an increase in the permittivity of the bulk medium. The dominating influence of this permittivity is likewise reflected in the increased duration and thereby prolonged inhomogeneity of the electric field sustained in the void. However, the absolute value of this field is reduced with an increase in bulk permittivity. It is concluded that the present choice of a point charge to simulate the wall charge has the disadvantage that such a source is associated with a field singularity, and thus it is not possible to represent the maximum field at the void wall in a realistic manner View full abstract»

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  • Electrical and structural studies of plasma-polymerized fluorocarbon films

    Page(s): 1101 - 1107
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    Plasma-polymerized fluorocarbon films up to 8 μm in thickness were prepared by high-frequency glow discharge deposition, the purpose being to investigate the materials' charge storage (electret) properties. Film compositions were characterized by infrared and by X-ray photoelectron spectroscopies. Under `mild' plasma conditions, materials with high fluorine concentration (F/C<1.9) could be obtained, while films with lower F/C were found to be partially oxidized. Besides surface potential decay rate of corona charged specimens, other electrical measurements performed include those of complex permittivity and of thermally stimulated depolarization. The latter revealed relaxations (designated γ and β) at -80°C and +50°C, attributed to local motions of chain segments and to rotation of carbonyl dipoles, respectively. While fluorine-rich films have low dielectric loss and relatively good charge stability (characteristic charge decay time τ>1700 h), a systematic decrease in τ and increase in tanδ could be observed for specimens with decreasing F/C ratio View full abstract»

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  • Effects of frequency on the electric strength of polymers

    Page(s): 1213 - 1216
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    The electric strength of air, in millimeter gaps, decreases at frequencies exceeding 100 kHz, but the effect on the discharge inception voltage Vi in solid insulation is too small to explain the reduction in the short-time breakdown voltage Vst of solids at radio frequencies. When breakdown is caused by internal discharges or by surface discharges in air, V st at 50 Hz is usually between 5 Vi, and 20 Vi, so the reduction in Vst when the frequency is raised may be explained by the increased discharge repetition frequency causing more rapid degradation with a risk of cumulative heating at lower stresses. When tests are made in oil, Vst/Vi is smaller than in air, so in this case a tenfold reduction in Vst at higher frequencies may be caused by a reduction in the electric strength of the oil View full abstract»

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  • Efficient field calculation in three-core belted cable by charge simulation using complex charges

    Page(s): 1208 - 1212
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    Sheathed three-core belted power cable is simulated by a charge simulation method employing complex charges. Each conductor is simulated by Nc line charges of infinite length. An image charge for each fictitious charge is considered in such a way that the potential of the sheath is always kept at zero. Hence, no charge is necessary to simulate the sheath, which results in efficient modeling. Since complex charges are used for simulation, separate computer runs for different instants in time are not necessary 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