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Electrical Insulation, IEEE Transactions on

Issue 2 • Date Apr 1988

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Displaying Results 1 - 16 of 16
  • Partial discharges measured with an automated system in epoxy resin and polyethylene

    Publication Year: 1988 , Page(s): 215 - 225
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (644 KB)  

    The partial-discharge (PD) characteristics of a filled epoxy casting resin were compared in long-term tests with those of polyethylene (PE). For this purpose, a newly developed automatic measuring system was used to detect and process PD parameters such as the apparent amount of charge, the PD repetition rate, the apparent PD energy and the polarity of the impulses, together with the position of the latter with respect to the phase of the applied 50-Hz AC voltage. The measurements were made on needle-plate electrode model specimens. The results recorded over long stress periods at constant voltage show that the PD impulses in epoxy resins arise discontinuously and with constantly changing amplitude. So called impulse pauses lasting several hours were recorded in these materials. PE was found to differ fundamentally from epoxy resins in their PD characteristics. In the PE material, the PD impulses occurred fairly regularly and were of almost constant amplitude. As a result of the very high impulse frequency in PE over the whole period of measurement, breakdown development was relatively rapid. Interpretation of the results is based on the concept of the origin of PD channels in the extremely inhomogeneous AC field, and, in particular, use is made of the recorded phase histograms and of the changes observed in the apparent PD energy with time View full abstract»

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  • In-situ insulator surface charge measurements in dielectric bridged vacuum gaps using an electrostatic probe

    Publication Year: 1988 , Page(s): 261 - 273
    Cited by:  Papers (36)  |  Patents (15)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (896 KB)  

    Surface charge measurements on alumina and polymer insulators were carried out after stressing them with DC voltages in a high vacuum. The order of magnitude of surface charge density was found to be the same for materials with supposedly widely varying secondary-emission yields. Surface coatings on alumina insulators reduced charge accumulation because of increased surface conductivity and/or reduced secondary-emission yield, which led to significant improvement in voltage hold-off for alumina ceramics. Removing the cathode triple junction from the main body of the cylindrical insulator, reducing the X-ray activity in the gap, or relieving the stress at the critical junction did not significantly alter the surface charge characteristics of cylindrical insulators. Wet hydrogen firing of plain alumina reduced the voltage hold-off by 25% without altering the surface charge density. It is postulated that the charging of insulators in bridged vacuum gaps with DC stresses is due to internal secondary emission produced by ionization of the lattice in the surface layer of the insulating material, by primary electrons injected at the cathode triple junction. This mechanism of charge production differs from the current models where charging is believed to occur due to electrons hopping along the surface/vacuum interface View full abstract»

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  • Water treeing using the water needle method: the influence of the magnitude of the electric field at the needle tip

    Publication Year: 1988 , Page(s): 275 - 278
    Cited by:  Papers (13)
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    The initial growth kinetics of water trees in polyethylene is measured for different values of the radius of curvature of the needle tip and for different applied voltages. The initial propagation rate is shown to be determined by the magnitude of the electric field at the needle tip, except at its highest values. The local electric field at the solution/polymer interface or in its vicinity plays an essential role in the growth mechanisms of water trees; it determines the initial propagation rate up to values of about 100 to 300 V/μm. Above these field values the water tree growth is found to be controlled by slower processes, e.g. the migration of the liquid through the polymer. These results are used in discussing some experimental conditions which are required to get reproducible results in water needle tests View full abstract»

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  • Improvement of non-linear characteristics of multicomponent ZnO-based ceramics containing Nb2O5

    Publication Year: 1988 , Page(s): 279 - 286
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    The combined effect of minor variations in the Nb2O5 content and initial particle size on the current-voltage characteristics and energy handling capability of ZnO-based composites is reported. Samples containing 0.2 wt.% of Nb2O5 along with other additive oxides, prepared as a finely milled mixture, were found to possess the best overall properties, characterized by high nonlinearity index (≃60), low leakage current (0.6-1.2 μA/cm2) long life (>150 y), among the samples studied. Addition of Nb2O5 above 0.2 wt.%, maintaining other parameters constant, was found to degrade the electrical characteristics due to the excessive formation of Zn3Nb2O8 spinel phase View full abstract»

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  • Technique for extracting dielectric permittivity from data obscured by electrode polarization

    Publication Year: 1988 , Page(s): 197 - 200
    Cited by:  Papers (1)
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    A technique was developed by which the dipole orientation component can be extracted from the total measured permittivity in the presence of electrode polarization. The static dielectric permittivity and relaxation times for fluids containing mobile ions can thus be determined unambiguously. The approach is based on the use of nonlinear regression to fit an established model for the frequency dependence of permittivity to the measured data View full abstract»

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  • The effect of an oil-paper interface parallel to an electric field on the breakdown voltage at elevated temperatures

    Publication Year: 1988 , Page(s): 249 - 259
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (744 KB)  

    Results are reported for the measurement of the electrical breakdown location in the vicinity of an oil-paper interface over the temperature range from room temperature to 150°C. The data indicated that the electrical breakdown occurred at the interface from 15% to 43% of the time, depending on the details of the particular set of measurements. A theoretical analysis shows that this experimental result is consistent with the electric-field enhancement, the area over which the enhancement occurs, and the spread in the breakdown voltages for nominally identical tests View full abstract»

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  • X-ray emission and prebreakdown currents in plain and dielectric bridged vacuum gaps under DC excitation

    Publication Year: 1988 , Page(s): 231 - 242
    Cited by:  Papers (6)
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    X-ray activity in both plain and solid insulator bridged-vacuum gaps was investigated under DC stresses. The gap conditions under which X-ray emission takes place, and the correlation between X-ray activity and predischarge current were studied. Steady X-ray activity was observed for a plain gap with electrodes subjected to repeated breakdowns, and for gaps bridged with insulators having low secondary emission yields. The hysteresis in the V-I characteristics of both plain and bridged vacuum gaps is attributed to a regenerative feedback process between X-ray and photoemission processes. The nonresistive component of predischarged current for bridge vacuum gaps obeys the Fowler-Nordheim theory only in the high-field region (>60 kV/cm), and only in the absence of X-ray activity. Chromium oxide coatings on Wesgo AL-300 alumina ceramic gave a substantial improvement (>100%) in the voltage hold-off, but at the expense of increased prebreakdown current and X-ray activity. It has been shown that chromium oxide coatings do not improve the voltage hold-off unless a dense, low porosity, surface is produced by the coating View full abstract»

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  • Photocurrents and thermally stimulated currents in epoxy resin: effects of mechanical stress

    Publication Year: 1988 , Page(s): 201 - 208
    Cited by:  Papers (3)
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    Photocurrent and thermally stimulated current (TSC) measurements were carried out on epoxy resin films with and without mechanical stress. Considerable photoconduction was observed, which the authors interpret as indicating the importance of electronic conduction in epoxy resin. Both photocurrent and TSC samples under mechanical stress showed dependence on the polarity of the applied field, which was not seen in samples without mechanical stress. Under mechanical stress, the photocurrent under a positive bias (i.e. positive voltage being applied to the illuminated electrode) was larger than that under negative bias and the TSC from a photoelectric formed under positive bias was also larger than for a negative bias. This led to the conclusion that the defects caused by the mechanical stress act as hopping centers for holes at low temperatures enhancing hole transport. The defects also act as trapping centers and lead to a TSC peak P2≃247 K where the defects are diminished by stress relaxation. At high temperatures where the defect levels are ineffective, the hole conduction is almost equal to the electron conduction samples free of mechanical stress View full abstract»

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  • Dielectric study at microwave frequencies of halos in crosslinked polyethylene cable

    Publication Year: 1988 , Page(s): 319 - 323
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (284 KB)  

    The dielectric properties of halos in crosslinked polyethylene cable were studied using the method of time-domain reflectometry (TDR) between 1 MHz and 8 GHz. It was found that, although the halo is predominantly composed of free water, a low relaxation frequency is also present that is probably due to bound water or cumyl alcohol impurities. The water contents deduced from the TDR method and a Mitsubishi moisture meter are comparable. Water contents thus deduced were compared with measurements performed with a Mitsubishi moisture meter and the results found to be comparable View full abstract»

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  • Measurement of electric charges at the interface between two dielectric layers using an electro-acoustic transducer technique

    Publication Year: 1988 , Page(s): 287 - 295
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (552 KB)  

    An acoustic-electric field meter was developed that can measure electric field strength within two dielectric insulating layers. The meter was used for the measurement of accumulated charges at the interface between double insulating layers. For an accurate measurement of an electrical field in insulating materials, two experimental conditions were required: (1) an optimum electrode thickness, and (2) the optimum conversion constant from acoustic transducer pressure due to the electrical signal. A compensation equation to calculate the amount of accumulated charges for various thicknesses of insulating materials is presented. It was found that the amount of accumulated charge and the accumulation rate strongly depended on the condition of the interface between two dielectrics whether a metal electrode was present or not View full abstract»

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  • The breakdown process of a liquid trigatron

    Publication Year: 1988 , Page(s): 227 - 230
    Cited by:  Papers (2)
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    A description of the mechanism of a breakdown of the liquid trigatron is proposed on the basis of the experimental data obtained in the past and additional measurements presented in the paper. Liquid trigatrons are considered both with the trigger at the cathode and at the anode. The mechanism proposed is as follows: in a liquid trigatron, some of the liquid in the gap will be vaporized and a local region of low density will develop there due to heat produced in the plasma of the triggering spark. There will also be increased probability for electron ionization in the plasma region. If the time lag is long enough, this vaporization of part of the liquid in the gap will transform the nature of a process of breakdown and gas phase will be involved. The breakdown condition is formed basically by electric-field distortion and intensification due to the positive space charge in the gap. In consequence, for the cathode triggered trigatron, if the field in the gap is high enough, negative streamers will develop. When the main gap voltage V is higher, the distance for which plasma region together with growing space charge must move in the gap before the main discharge will start, will be shorter, as is shown by the measurements. For the anode triggered trigatron, the growing space charge reinforces the electric field ahead of the plasma to allow development of a positive streamer. In the range of high values of V the avalanche produced by the photoelectrons is large and the positive streamer can be initiated immediately. In effect the corresponding time lag will be very short, as the measurements show View full abstract»

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  • Volume low-frequency dispersion in a semi-insulating system

    Publication Year: 1988 , Page(s): 209 - 213
    Cited by:  Papers (2)
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    The phenomenon of low-frequency dispersion (LFD) is observed in semi-insulating solids in which slowly mobile electronic or ionic charge carriers dominate the dielectric response at low frequencies giving rise to very high effective capacitance values. This behavior may be associated with electrochemical activity. An experiment is described that was designed to answer the question of whether LFD may be associated with volume processes or whether it is essentially confined to interfacial activity. The investigators used the medium of humid cellulose in the form of sheets of filter paper soaked in a salt solution to provide ionic charge carriers. A stack of such filter papers sandwiched between metal electrodes, which may be regarded as representative of many others, gives volume LFD response distributed throughout the system, with only relatively negligible contributions from the end-electrode interfaces. While this does not rule out the existence of other interfacial LFD phenomena, it proves the existence of distributed volume LFD processes in ionically conducting systems View full abstract»

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  • Optical field determination in water

    Publication Year: 1988 , Page(s): 189 - 195
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (640 KB)  

    Electrostrictive pressure measurements were obtained by the use of a holographic interferometer. Use of this technique permitted a determination of the local electric field in water when subjected to a high-voltage impulse. The interferometer was illuminated by a pulse laser, which enabled detailed images of the rapidly changing field to be captured in a single 20-ns exposure. Using the same test cell and laser, field measurements were also obtained by the Kerr electrooptical technique. A comparison of the electrostriction and Kerr-derived field maps yielded good correlation, and revealed a superior sensitivity for the electrostriction method. Analysis is presented that links fundamental polarization modeling with the electrostrictive field measurements View full abstract»

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  • Experiments on tracking enhancement by copper

    Publication Year: 1988 , Page(s): 243 - 248
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (536 KB)  

    In an attempt to explain the difference in results obtained with the use of copper electrodes as compared to platinum electrodes, two hypotheses were investigated. The first advanced the supposition that copper salts accumulate on the surface and increase the electrical conductivity. Oscillograms of the electric current failed to support this hypothesis. The second hypothesis tested is that the copper changed the wetting of the surface. However, when the surface wetting was changed using platinum electrodes and inert powders no significant reduction occurred in the variation of the Comparative Tracking Index (CTI) test by the International Electrotechnical Commission (IEC) publication 112. The investigation has shown that the substantial difference in results which occurs when comparative tracking tests use copper rather than platinum electrodes cannot be explained by increased conductivity or by improved wetting ability of the liquid View full abstract»

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  • Potential and electric field distributions at a high voltage insulator shed

    Publication Year: 1988 , Page(s): 307 - 318
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (684 KB)  

    The charge simulation method is applied to a two-dielectric rotationally symmetrical geometry, consisting of a single section from an approximately periodic sequence of sections of a practical post-type insulator. The dependence on the physical dimensions is determined of the potential and the electric field at various locations inside and outside of the solid dielectric and at the interface. The dependence of these quantities on the radius of the curved portions as well as on the relative dielectric permittivity of the insulator material is found in the range 2.1 to 12000 for symmetrical and asymmetrical applied voltages. A study of the errors in the potential and the field components is carried out. The extension of these results to a practical insulator case is discussed and recommendations regarding some aspects of the design are provided View full abstract»

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  • Some aspects of the statistical modeling of partial discharge inception conditions

    Publication Year: 1988 , Page(s): 297 - 306
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (632 KB)  

    Statistical modeling procedures are presented for representing various conditions in different types of samples and equipment at, or near, the onset of partial discharge (PD). Using multiple-regression techniques it was possible to establish trends in PD behavior in terms of the repetition rate. The latter parameter was chosen as being sensitive to changes near inception, and it is measurable by conventional instrumentation (for example a multichannel pulse-height analyzer). The statistical repetition rate models also can be related to the probability of inception. The probabilistic definition derived complies with the simpler criteria often used in partial-discharge testing. Application of the methods is described by statistical models determined for five examples that include resin-insulated and oil-impregnated systems. Computer analysis showed that a high proportion of the variability of discharge behavior near inception could be estimated from the models: in particular, the effect of voltage in combination with increase in time, insulation thickness, and temperature changes. In addition, the models indicated that characteristic PD patterns are identifiable for resin voids and oil wedges. It is concluded that adoption of statistical techniques could result in the specification of PD inception in terms of probability, thus reducing the difficulties in interpreting the results from commercial tests 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