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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 2 • Date April 1999

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Displaying Results 1 - 18 of 18
  • Comment on "1-dimensional model for nonlinear stress control in cable terminations" [with reply]

    Publication Year: 1999 , Page(s): 267 - 270
    Cited by:  Papers (1)
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    For the original paper see ibid., vol. 4, no. 6, p.785-91 (1997). In the aforementioned paper by Rhyner and Bou-Diab, a problem of considerable engineering interest concerning the evaluation of the voltage profile in a cable termination equipped with stress grading materials has been discussed. In the paper, the potential distribution inside the termination is evaluated both for linear and nonlinear materials by adopting the well-known RC transmission line model. Although both the problem and the model adopted are not new, the authors introduce and discuss two novel aspects, of theoretical and practical interest for the electrical engineer involved in the design of cable terminations. In this Comment on the paper, Tucci aims to show that there are some weak points in the general theoretical model adopted and some of the approximations used in the analysis of particular cases. In reply, the authors answer the criticism and comments raised by Tucci. View full abstract»

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  • Calculation of the uniform breakdown field strength of SF/sub 6/ gas [Erratum]

    Publication Year: 1999 , Page(s): 271
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    First Page of the Article
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  • Toward a quantitative analysis of the mirror method for characterizing insulation

    Publication Year: 1999 , Page(s): 202 - 210
    Cited by:  Papers (13)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (636 KB)  

    The electrostatic mirror method developed by Le Gressus can be used to characterize the charge trapping properties of insulators. It is based on the measurements of the dark spot observed on a scanning electron microscope (SEM) screen working in the secondary electron mode when a beam of low energy electrons is used to probe the area of the sample where a charge has been trapped previously at high energy. The dark spot is the image of the end section of the emitter tube. The radius of the dark spot for a probing beam of energy eV is nearly proportional to the curvature radius of the equipotential surface V. For a trapped charge qt with spherical symmetry (e.g. a point charge), the radius of curvature R(V) varies as 1/V, so that the plot of 1/R(V) is a straight line of slope λ/qt, with λ=2πε0r+1). However, if the charge is anisotropic, the 1/R(V) curve deviates from the straight line for probing electrons approaching the charge at high enough V. This work presents analytical derivations of the 1/R(V) function for a charged segment perpendicular to the sample surface and for a charged disc parallel to that surface, and also a good approximation for a charged cylinder. It shows that an oblate trapped charge leads to curves deviating downward from the above straight line and displaying a well-characterized maximum which usually is observed. Conversely, a prolate charge leads to an upward deviation. The work also considers the influence of the electrostatic image of the trapped charge in the substrate. Whereas this is negligible for thick samples, it becomes relevant for thin ones. The theoretical predictions are confirmed by the experimental data obtained with 100 μm thick samples, and the effect of the image charge has allowed detection of the film lamination influence on the anisotropy of the trapped charge, It is likely that this work might contribute significantly to the understanding of the trapping and detrapping processes in aging and breakdown mechanisms and, more practically, to the standardization of industrial insulators View full abstract»

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  • Calculation of thermally stimulated depolarization current from isothermal dielectric response data

    Publication Year: 1999 , Page(s): 137 - 144
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (588 KB)  

    Thermally stimulated depolarization is modeled from dielectric response functions deduced from isothermal measurements. The key assumption made is that the same state of polarization can be achieved both via the real non-isothermal thermally stimulated depolarization (TSD) process and via an equivalent isothermal process. The change of polarization with temperature and time is traced through a continuous adjustment of the starting and stopping times and polarizing field of the fictitious isothermal process. The theory is applicable to general cases of non-Debye relaxation with variable activation energy. A comparison of the modeled and measured TSD thermograms is presented for films View full abstract»

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  • A general phenomenological life model for insulating materials under combined stresses

    Publication Year: 1999 , Page(s): 250 - 258
    Cited by:  Papers (7)
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    In this paper, the general phenomenological life model proposed by the author for insulating materials with both a threshold or non-threshold behaviour, is applied to experimental data pertinent to two insulating materials for power cables. The first material, EPR, was found to be a non-threshold material for stresses close to or larger than the service stress, but extrapolation at low temperature seems to show a tendency toward a threshold. Application of the general model does appear very appropriate for this insulation and a good fit to the data has been found. The case of the second material, an old generation of XLPE, is different and the general model does not fit the data. However, an excellent fit is reached by introducing some improvements to the model, where possible and correct. Further complications seem necessary to conform this model to various practical cases, and make its application more difficult. For practical applications, such as selection of materials and/or design of apparatus, the use of properties and indices obtainable by means of long tests, such as those suggested and carried out by the author's research group, is simpler and more rapid. However, the usefulness of the model also consists in studying the various processes occurring during combined-stress aging of materials, all of which the model can take into account by simple mathematical expressions. This constitutes a significant advance in approaching the complex physical mechanisms involved in aging View full abstract»

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  • Electrical properties of silane crosslinked polyethylene in comparison with DCP crosslinked polyethylene

    Publication Year: 1999 , Page(s): 164 - 168
    Cited by:  Papers (6)
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    The electrical properties such as water tree length, electrical conduction, ac breakdown strength and space charge of silane crosslinked polyethylene (SXLPE) were investigated, with a purpose to compare this material with ordinary dicumyl peroxide (DCP) crosslinked polyethylene (XLPE). Experimental results show that SXLPE has a smaller water tree length, much lower conduction current density, a little bit higher ac breakdown strength than XLPE, and on SXLPE only a very small homocharge is seen. The better behaviour of SXLPE under electric stress is attributed to the absence of residual curing byproducts present in XLPE and the change of chemical structure by grafting and curing reactions View full abstract»

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  • Insulation coordination from the probabilistic point of view

    Publication Year: 1999 , Page(s): 259 - 266
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (684 KB)  

    The problem of the design of the insulation characteristics of an electric power system was solved, up to very recent days, and is still solved in a large majority of cases, in a deterministic way, i.e. in choosing the highest expected dielectric stress from one side and the lowest expected dielectric strength of the equipment from the other side, and by providing a suitable gap between the two, in order to cover errors, unknown factors, and aging of the equipment. In this paper, the principles of an optimal insulation design is recalled and the cost of failure risk is introduced into the economical optimum design of transmission systems. A mathematical expression is given for two simple cases of optimum design to which the insulation coordination of a transmission system can be reduced in general. The importance of knowing the breakdown probability of an insulating component, as a function of the crest value of a given overvoltage, in these optimum designs is also pointed out and the methods to calculate such probability are exposed. A description of the experimental method is also given, which can be applied to any component samples, and of the calculation methods which enable one to derive the breakdown probability of a complex component from knowledge of the breakdown probability of its elements. Also examined is how the withstand test selections modify the breakdown probability distribution of a component. Finally, it is shown how on the basis of the above mentioned methods, one can have only estimations of breakdown probability distributions and how the uncertainties of such determinations can be taken into account in intelligent designing of insulation View full abstract»

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  • Ion dynamics near charged electrodes with excluded volume effect

    Publication Year: 1999 , Page(s): 226 - 235
    Cited by:  Papers (4)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (640 KB)  

    The goal of this paper is to analyze ion relaxation around a charged electrode taking into account finite ion volume and frequency. Time-dependent and electrostatic equations for ion behaviour near an electrode are developed and include the effects of nonequilibrium and steric hindrance. Time-dependent wave equations for the potentials are developed in the Lorentz gauge. The charge density is expressed as a sum of the quasi-steady state solution plus a nonequilibrium term. For slowly changing potentials the ions are in quasi-equilibrium and we show that in this limit the potential satisfies the sinh-Gordon equation. New solutions to this nonlinear time-dependent equation are developed. In the low voltage limit the equation reduces to the Klein-Gordon equation, time-dependent analog of the Debye-Huckel equation. We also present equations for capacitance and surface charge View full abstract»

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  • Thermal and electrical analysis of alumina and beryllia coax high-power windows under irradiation

    Publication Year: 1999 , Page(s): 169 - 174
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (640 KB)  

    Characteristics of dielectric insulators in vacuum windows of coaxial 10 to 100 MHz transmission lines in high-power steady-state use under irradiation are simulated with respect to electric, nuclear, mechanical, and thermal properties. Neutron fluence ~5×1018 n/cm2 at the window is obtained to be sufficiently small to allow beryllia, but not alumina, to be used as dielectric. In beryllia (10-3 displacements per atom (dpa) due to irradiation) or in un-irradiated alumina (97.5% purity), the temperature is found to rise by not more than 125°C with maximum stress <140 MPa for 50 kV peak voltage at 60 MHz, provided niobium, titanium or materials with similar thermal expansion coefficients are used in water cooled conductors. The tangential electric field is kept well below the surface discharge limit 2 MV/m by using potential rings together with a sufficiently large inclination angle of the conical ceramic with respect to the radial coaxial direction, but high normal fields exceeding the vacuum breakdown limit are obtained near the potential rings. Abandoning the potential rings and deforming the equipotential lines by shaping the ceramic-conductor joint can reduce both tangential and normal field components below the breakdown limit, which appears to be in agreement with recent voltage test experiments View full abstract»

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  • Computer simulation on formation of space charge packets in XLPE films

    Publication Year: 1999 , Page(s): 152 - 158
    Cited by:  Papers (45)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (528 KB)  

    The packet-like space-charge behaviour in an oxidized crosslinked polyethylene (XLPE) film doped with antioxidant is discussed through computer simulation. The model is based on the following assumptions, (1) both electrons and holes are injected from the electrodes and they migrate to the opposite electrodes by hopping transport, (2) holes are also supplied from acceptor sites depending on the field, (3) the holes are excited to the hopping level only via an intermediate level. In this model, a space-charge packet is not caused by the excess of mobile electrons but by the deficiency in mobile holes. The ionized acceptors left behind by hole migration form an apparent negative space charge packet and it propagates towards the anode as the high field region moves View full abstract»

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  • A new algorithm for mixed Weibull analysis of partial discharge amplitude distributions

    Publication Year: 1999 , Page(s): 242 - 249
    Cited by:  Papers (11)
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    In this paper, an heuristic algorithm is presented for the evaluation of the parameters of additive-Weibull distributions having more than two elementary functions, which is a necessary condition when testing complex insulating structures with different partial discharge (PD) sources. The algorithm has, in principle, no limit in the number of component Weibull functions. Furthermore, it should be emphasized that it is not required to have a very close initial guess of the parameters for solution convergence, unlike other algorithms used at present. In order to check the validity of the proposed algorithm in finding the elementary components of a mixed Weibull function, many PD experimental tests have been performed on some lumped capacitance specimens in order to compare the experimental cumulative probability of pulse amplitude distributions (PAD) with the ones obtained from the Weibull analysis. In this aim, the results of the Cramer Von Mises (CVM) test performed on the experimental results are reported. Some considerations are also given in the case of testing commercial HV components having complex PAD View full abstract»

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  • Furfural analysis for assessing degradation of thermally upgraded papers in transformer insulation

    Publication Year: 1999 , Page(s): 159 - 163
    Cited by:  Papers (10)
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    Furfural analysis by high performance liquid chromatography (HPLC) has gained increasing favor as a means of estimating degradation of insulating papers in power transformers. We have studied this technique in some detail as applied to thermally upgraded kraft papers. Significant differences in the results for papers of several manufacturers were found, which could influence diagnosis after extended operation. As a result, we believe that the current practice of simply identifying a transformer as manufactured with common or thermally upgraded paper will not be enough to interpret results at a later date. A detailed identification of the paper, and information on its accelerated aging behaviour should be kept on record View full abstract»

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  • Electric field distortion caused by asymmetric pollution on insulator surfaces

    Publication Year: 1999 , Page(s): 175 - 180
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (452 KB)  

    The pollution on the surface of an insulator is asymmetric, although insulators, in general, are axisymmetric. The distortion of fields on the surface of an asymmetrically polluted axisymmetric insulator is computed in this paper. The surface charge simulation method (SCSM) has been used with cylindrical, conical and toroidal surface elements. The volume resistivity of the insulation also has been taken into consideration, using complex permittivity for the insulating material View full abstract»

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  • Modeling the polarization spectrum in composite oil/paper insulation systems

    Publication Year: 1999 , Page(s): 145 - 151
    Cited by:  Papers (28)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (576 KB)  

    The determination of the polarization spectrum in the low frequency region of the dissipation factor, commonly has been used in practice to diagnose insulation conditions. The recovery voltage method (RVM) is one of the methods used to estimate the polarization spectrum. Nevertheless, current analysis of the RVM tests do not use all the information available from the test. In this paper, a mathematical model of the polarization spectrum is proposed. It is shown that, from the derived model, some of the hidden RVM peaks can be identified. Since the difficulty to derive a model from RVM data increases with the number of the peaks to be identified, a simpler model is always preferable. A scheme for determining the parameters of an equivalent circuit is also given, The composite oil/paper insulation is considered which is extensively used in cables and transformers. Cellulose paper, such as the Manila-Kraft type, which is used as insulation, exerts a major influence on the life of the equipment because of its hygroscopic nature. The importance of detecting and eliminating moisture from paper is crucial in oil-impregnated HV insulation. Analysis of the polarization spectrum has been used by several utilities to determine moisture in the paper. Recently, some test results indicated that the polarization spectrum may not clearly show important peaks corresponding to local moisture ingress and thus can mislead experts in the field, The developed model has been validated using RVM data of real power transformers View full abstract»

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  • Investigations on charge storage and transport in plasma-deposited inorganic electrets

    Publication Year: 1999 , Page(s): 236 - 241
    Cited by:  Papers (5)
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    The storage and transport of charges in plasma deposited inorganic electrets with low mechanical stress were investigated under different conditions. Silicon dioxide, nitride and oxynitride were chemically deposited on Si substrates by means of a radio frequency (13.56 MHz) plasma system. The samples were negatively charged by the constant voltage corona method. The surface potential was observed during aging at room temperature, at elevated temperatures of 200, 250 and 300°C and at a relative humidity of 95% at 60°C. The thermal activation process was examined by thermally stimulated discharge (TSC) measurements. Silicon oxynitride possessed the highest charge stability during the different experiments, correlating with the current maximum located at ~400°C. The charge transport in silicon dioxide, with a current maximum located at ~330°C, was described by a theoretical model which considered the drift and the partial retrapping of free charges in the bulk. Despite treatment with hexamethyldisilazane, it was not possible to stabilize the surface potential of silicon nitride samples even at room temperature View full abstract»

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  • Influence of single and multiple environmental stresses on tracking and erosion of RTV silicone rubber

    Publication Year: 1999 , Page(s): 211 - 225
    Cited by:  Papers (17)
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    Single and multiple effects of UV, corona, thermal, water absorption and acid rain stresses on the tracking and erosion characteristics of room temperature vulcanized silicone rubber (RTV) are investigated. It is shown that corona stress and water absorption stress decrease the tracking and erosion resistance of RTV, while thermal stress and UV stress improve it. Synthetic acid rain, 500× stronger than actual rain in Japan, cannot induce the change in the tracking and erosion characteristics. RTV subjected to simultaneous multiple stresses also is evaluated. Significant variations in the tracking and erosion resistance appear. Chemical and morphological analysis for assessing the aging level is carried out by scanning electron microscope (SEM)-energy dispersive X-ray (EDX), attenuated total reflection (ATR)-Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC). In this study, DSC plays an important role to detect boiling and combustion temperatures of byproducts affecting tracking and erosion. Noteworthy results are obtained in that the decrease in the resistance to tracking and erosion is related to the formation of byproducts that burn at low temperature (170 to 275°C) View full abstract»

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  • A novel physical model for partial discharge in narrow channels

    Publication Year: 1999 , Page(s): 181 - 190
    Cited by:  Papers (28)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (796 KB)  

    In order to reveal the mechanism of partial discharge (PD) in electrical trees, an artificial single channel is studied as model for tree channels. In a long-narrow channel, the behaviour of PD is similar to that in an actual tree. When the length of the channel is decreased, the φ-q-n pattern of PD in the channel changes from wing to turtle or rabbit like, and the range of light emission in the channel increases when the applied voltage is increased. Considering the influence of a PD on the total field distribution and thus on a later PD, a physical model is put forward to describe the PD behaviour. It is assumed that PD is induced by the high field at the needle tip, then propagates along the channel, and stops propagating when the field in the channel becomes lower than a critical value due to field divergence. Mathematical analysis on an ideal needle-plane electrode system clearly explains the physical meaning of the model. And the computer simulation by using the superposition method agrees with the experimental results very well. This model is also valid to describe the behaviour of PD in electrical trees View full abstract»

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  • PD resistance of thermally aged polyethylene and carbonyl-containing model polymers

    Publication Year: 1999 , Page(s): 191 - 201
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (928 KB)  

    Films of polyethylene (PE) with different degrees of crystallinity, including a crosslinked cable grade and a crosslinked grade containing 2-vinylnaphthalene, were aged in air at 373, 388 and 403 K for periods to 19000 h and subsequently exposed to external partial discharge (PD) in dry air at 298 K. Stress-strain measurements in a tensile testing machine, differential scanning calorimetry to assess the antioxidant concentration, crystallinity and crystal thickness, infrared spectroscopy to determine the concentrations of oxidation products, and size exclusion chromatography (sec) were used to characterize the samples before and after the oven aging. The thermal oxidation led to a combination of chain scission, as revealed by sec on the soluble fraction of aged samples, and crosslinking which was evident from the gel formation. Oxidation-induced chain scission increased the crystallinity of the crystal lamella stacks stable at the aging temperature, and lead to a pronounced brittleness of these materials. The fraction that was molten at the aging temperature and crystallized during the subsequent cooling phase dropped substantially in crystallinity after oxidation. Mere oxidation was not sufficient to cause a decrease in PD life, unless the carbonyl content was above a certain threshold value above which the PD life decreased rapidly with increasing concentration of carbonyl groups. It is suggested that the carbonyl groups enhance radical formation and thereby accelerate surface erosion, causing early PD breakdown. The PD life was not influenced by the degree of crystallinity, the average crystal thickness, the concentration of antioxidant or by the fracture toughness. Vinylnaphthalene acted in crosslinked PE as a stabilizer and prolonged the incubation period for oxidation 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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Reuben Hackam