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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 2 • Date April 2006

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  • IEEE Transactions on Dielectrics and Electrical Insulation Magazine - April 2006 - Vol. 13 - No. 2

    Page(s): 0_1
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    Freely Available from IEEE
  • IEEE Dielectrics and Electrical Insulation Society

    Page(s): 0_2
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    Freely Available from IEEE
  • Numerical methods in the simulation of charge transport in solid dielectrics

    Page(s): 239 - 246
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (230 KB) |  | HTML iconHTML  

    This paper introduces some numerical methods pertaining to the simulation of charge transport in solid dielectrics that can be of interest to the electrical engineering community. These methods are generally scattered over numerical analysis and computing literature making difficult for the non-specialist to select the appropriate computing schemes. In the first part we consider an example of a set of physical parameters needed to describe charge transport and we introduce mathematical equations that must be solved. A series of numerical schemes are compared for one-dimensional convection problems in a second part. Numerical results show that a third order upwind scheme named QUICKEST, combined with a flux limiter (ULTIMATE) is competent for the resolution of the transport equation in solid dielectrics, avoiding numerical oscillations and numerical diffusion. The third part deals with numerical resolution of the Poisson's equation comparing two different methods. View full abstract»

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  • An RCL sensor for measuring dielectrically lossy materials in the MHz frequency range. Part I. Comparison of hydrogel model simulation with actual hydrogel impedance measurements

    Page(s): 247 - 256
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    There is a requirement for the development of non-invasive continuous blood glucose monitoring devices to meet the clinical demands of the rapidly increasing number of people currently developing diabetes mellitus. Impedance spectroscopy is a technology that meets the requirements of such devices. An NI CGMD is being developed as a device that couples a sensor to the skin to form an RCL sensor. The reliability of such an RCL sensor model has been investigated by comparing electrodynamical simulations to in-vitro measurements of dielectrically "lossy" materials. The sensor has been modeled and simulated in FEMLAB (finite element modeling laboratory). In-vitro measurements are performed on hydrogels, representing the lossy material, by the aid of a Rohde & Schwarz VNA (vector network analyzer). From the quantitative agreement of the results we conclude, that the proposed qualitative model is appropriate for the characterization of the RCL sensor and suggests that more detailed models can be used to elucidate the behavior of human skin tissue. View full abstract»

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  • The behavior of N2+SF6 gas mixtures subjected to orthogonal electric and magnetic fields

    Page(s): 257 - 263
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    The limiting equivalent electric fields in N2+SF6 binary gas mixtures due to Townsend discharges are evaluated directly from a Monte-Carlo simulation when the mixture is subjected to orthogonal electric and magnetic fields. Along with the limiting equivalent electric fields, transverse and perpendicular drift velocities, electron mean energies and collision frequencies are also determined within the scope of the Monte-Carlo simulation. The equivalent reduced electric field (EREF) concept for the corresponding limiting electric fields is also investigated for the calculated mean energy levels and collision frequencies. The EREF values are found to be in good agreement with the previously published limiting electric field data. View full abstract»

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  • Effect of insulator diameter on AC flashover voltage of an ice-covered insulator string

    Page(s): 264 - 271
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    The effect of the diameter of an insulator covered with ice on its flashover voltage was investigated. The insulator diameter was simulated and varied by controlling the width of a layer of ice artificially accreted on a short string of 5 IEEE standard units. The 50% withstand voltage (V50) was experimentally determined using the method described in IEC 60507. The results show that the V50 decreases as the width of the ice layer increases. Moreover, a mathematical model for predicting the critical flashover voltage of ice-covered insulators is proposed, and is validated against the experimental results. The model is then applied to ice-covered industrial insulators with different diameters yielding good concordance between the results from the model and the experimental ones. View full abstract»

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  • Space charge measurements on multi-dielectrics by means of the pulsed electroacoustic method

    Page(s): 272 - 281
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    The pulsed electroacoustic (PEA) method is now widely accepted as one of the most simple and effective techniques for the measurement of the dynamic space charge distribution in solid dielectrics. Recently, the PEA method has been applied also to laminar test objects composed of two or more layers of different dielectrics (multi-dielectrics). However, when a multi-dielectric is tested by means of the PEA method, the different acoustic and electric properties of the materials affect the detected space charge signal. In this paper, the principle of the PEA technique is reviewed in case the test object is a multi-dielectric. The generation, transmission and reflection of electrically-induced acoustic waves are described. Based on the proposed approach, results of PEA measurements performed on various kinds of multi-dielectrics are presented and discussed. View full abstract»

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  • Dielectric breakdown of epoxy-based composites: relative influence of physical and chemical aging

    Page(s): 282 - 292
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    The effect of aging on the dielectric strength of epoxy-inorganic particle composites used for insulators in the high voltage industry is reported. A differential scanning calorimetry analysis of an insulator aged twenty years in actual service conditions indicated both a chemical degradation and a structural recovery of the polymer network. This composite exhibited however a breakdown field comparable to that of a fresh sample with the same formulation. An accelerated physical aging was thus performed which lead to a large increase in the high voltage performance of the newly processes composite over time. This improvement was attributed to a densification of the thermoset resin, which impeded tree growth. It was also observed that the choice of the electrode geometry greatly alters the measurements under high electric field. In a quasi-homogeneous field configuration, the breakdown was mainly governed by the major flaws at the sample scale, namely the reinforcing particles. On the contrary, under a divergent field (with a point-plane electrode arrangement), the field was essentially localized at the point electrode tip, and the major flaws might not be reachable by the damage tree. It hence appeared that the measurements performed in a quasi-homogeneous field are not very sensitive to the variations within the polymeric matrix as are the measurements under a divergent field. View full abstract»

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  • Spectroscopic measurement and analysis of water and oil in transformer insulating paper

    Page(s): 293 - 308
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    A portable fiber-optic spectroscopic system (TRANSPEC) has been developed for nondestructive measurement of aged transformer insulating paper. Following successful measurement of degree of polymerization (DP) of a range of transformer-aged paper samples, the system has now been shown to separate the oil and paper information for measurement of DP in oil-wetted paper samples. In addition, the system has been shown to be capable of the prediction of both oil and water content of paper to a high accuracy, and is also capable of identifying and quantifying different water species. Spectroscopic measurements have been used together with gravimetric water adsorption measurements to investigate the kinetics of uptake of water vapour into paper from air in the case of nominally dry and oil-impregnated samples. Relationships between water adsorption parameters and properties of the paper insulation have also been investigated using measurements under controlled conditions. View full abstract»

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  • Non-destructive measurement of the degradation of transformer insulating paper

    Page(s): 309 - 318
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1200 KB) |  | HTML iconHTML  

    Knowledge of the condition of power transformer winding insulation paper is fundamental to making optimum asset replacement decisions in the power industry. The ability to assess the aged condition of Kraft paper quickly and non-destructively using portable instrumentation would significantly increase the opportunities for gaining this knowledge. Insulation paper degrades over time in-service and its degree of polymerization (DP) reduces, eventually affecting its mechanical strength. At low DP levels the insulation may start to disintegrate and the risk of electrical breakdown increases. Currently-used methods of estimating DP are either approximate or destructive. The use of spectroscopy together with multivariate statistical analysis (MVSA) provides a powerful non-destructive evaluation of the condition of paper. From initial feasibility studies, we have developed a simple, portable system (TRANSPEC) using fiber-optics and broad-band spectroscopy that can measure the degree of polymerization of various aged transformer papers to a precision of approximately 30 DP units with a spatial resolution of 14 mm. The system can also measure the chemical composition and condition of the insulating mineral oil. MVSA regression models were constructed from library spectral data, and these models are used to predict the DP of other papers with parameters that fall within the range spanned by the set of calibration samples. Separating oil and moisture information from wetted paper is possible and will be reported in a separate publication. With a single TRANSPEC system, non-destructive in-situ analysis of the DP of insulating paper is possible, providing a rapid cost-effective method for transformer insulation condition assessment and monitoring, which correlates well with current destructive methods. View full abstract»

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  • Effects of nano- and micro-filler mixture on electrical insulation properties of epoxy based composites

    Page(s): 319 - 326
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    This paper focuses on the electrical insulation properties of a newly prepared composite material by nano- and micro-filler mixture. Nano- and micro-filler mixture composites were made by dispersing nano-scale layered silicate fillers and micro-scale silica fillers in epoxy resin. To investigate the effects of nano- and micro-filler mixture, the thermal expansion coefficient and insulation breakdown properties by a needle-plate electrode method were measured for the filler mixture composite and the conventional filled epoxy. The filler mixture composite had almost the same thermal expansion coefficient as the conventional filled epoxy. In a continuous voltage rising test, the filler mixture composite had 7% higher insulation breakdown strength than the conventional filled epoxy. Moreover, under constant AC voltage (10 kV at 1 kHz), the filler mixture composite had an insulation breakdown time of more than 20,000 minutes whereas the conventional filled epoxy had a breakdown time of 830 minutes. Electron microscope observation showed that the area surrounded by dispersed micro-scale silica fillers were also filled with the nano-scale layered silicate fillers. Furthermore, the estimate of spacing between the fillers and the filler/epoxy interface area showed a more densely-packed structure of the filler mixture composite than the conventional filled epoxy. The morphological feature of the filler mixture composite seems to improve its insulation breakdown strength and time. View full abstract»

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  • Common insulating properties in insulating materials

    Page(s): 327 - 335
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    Insulating materials exist in a variety of different forms, including gas, liquid, solid, and vacuum, and different forms of insulating materials have significantly different dielectric strengths and insulating properties. Power transmission/substation facilities for UHV AC and 500 kV DC power transmission and 500 kV underground power transmission facilities have been designed and constructed by taking advantage of the insulating properties of various insulating materials and structures so that they conform to the demanded insulation specifications. Conventionally, insulating material technology has been developed by making a close study of apparatuses and insulating materials individually. In this paper, as a new attempt, the insulating properties of various insulating materials/apparatuses are cross-sectionally compared and examined from an entirely new viewpoint through the assessment of commonalities and differences between them. A listing of lightning impulse and AC dielectric strength data is prepared and systematically positioned from the standpoint of material and structure, and compared and evaluated in a unified way from the viewpoint of typical indices of insulating properties including the "n" value of the V-t characteristic, Weibull parameters, and impulse ratio. Further, an insulation assessment of apparatuses is systematically made with consideration given to their sizes and structures as well as the insulating materials themselves. As a result, it was found, for example, that the design field strengths of power capacitors were high and those of generators were not, and there was a noticeable difference in design field strength between SF/sub 6/ gas insulated apparatuses and oil-immersed transformers. It is expected that the analysis conducted from an entirely new angle may provide a new approach to the development of apparatus insulation technology and to the rationalization of insulation design. View full abstract»

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  • Analysis of the electric field and force in an arrangement of a conducting sphere and a plane electrode with a dielectric barrier

    Page(s): 336 - 344
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    This paper presents an analysis of the electric field and dielectrophoretic force in an arrangement consisting of an uncharged conducting sphere and a plane electrode with a dielectric barrier. The electric field is calculated by using the method of multipole images using an iterative algorithm proposed for calculating the images of the dielectric barrier of finite thickness. The calculation results show electric field intensification due to the presence of the dielectric barrier having higher permittivity, /spl epsiv//sub S/, than that of the surrounding medium, /spl epsiv//sub E/; however, if the barrier is separated from the conducting sphere by at least the sphere radius, its influence is negligible. Inside the dielectric barrier, the electric field on the axis of symmetry becomes more uniform and the average field significantly increases with decreasing its thickness. For a case where dielectric barrier is sufficiently thin, the electric field at the contact point and the force on the conducting sphere vary approximately as power functions of /spl epsiv//sub S///spl epsiv//sub E/. View full abstract»

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  • Effect of mechanical vibration on the behavior of partial discharges in generator windings

    Page(s): 345 - 352
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    As mechanical vibration usually exists in HV motors and generators, its influence on the partial discharge (PD) needs to be studied so as to improve the reliability of on-line PD monitoring method. In this report, the mechanical vibration frequency was synchronized to the power frequency and the effects of vibration phase and frequency were investigated. Because the length of the discharge gap changed with the vibration, obvious variation in the slot-PD patterns was observed at different vibration lead phase and frequency. Moreover, when the vibration amplitude was large, it was also found that the PD behavior at low voltage just above the PD inception voltage was different from that at higher voltage. This was explained in terms of the effects of the field variation in the gap due to the vibration and the charges left on the insulating surface due to PD. View full abstract»

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  • Calibration methods of carbon nanotube gas sensor for partial discharge detection in SF/sub 6/

    Page(s): 353 - 361
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (569 KB) |  | HTML iconHTML  

    The authors proposed a new type of gas sensor for high sensitive detection of decomposition products generated by partial discharge (PD) in SF/sub 6/ gas. The sensor employed carbon nanotubes (CNTs) as gas sensing transducer and was fabricated by electrokinetic manipulation of CNTs using dielectrophoresis. Due to complicated gas decomposition process of SF/sub 6/ gas, calibration of the CNT gas sensor is an essential and challenging subject in order to realize reliable and stable detection of PD under practical conditions. In this paper, two methods for the CNT gas sensor calibration were proposed and tested. The first method was based on initial conductance dependence of the CNT gas sensor response. The CNT gas sensor response to PD increased almost linearly with the amount of CNT trapped onto the sensor electrode, which could be quantified by the initial conductance of the sensor. However, the calibration accuracy of this method was about 50 % and was not high enough for practical use. The second method employed NO/sub 2/ (nitrogen dioxide) as calibration gas because of the similarity in the CNT sensor response to PD decomposition products and NO2. It was found that the accuracy of the NO/sub 2/ calibration was about 10 % and far better than the first method. Finally, effectiveness of the NO/sub 2/ calibration was demonstrated by a PD monitoring test in which two CNT sensors were simultaneously exposed to SF/sub 6/ decomposition products with identical concentration. View full abstract»

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  • Application of functionally graded material for solid insulator in gaseous insulation system

    Page(s): 362 - 372
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    Functionally graded materials (FGM) have spatial distribution of a material property in order to achieve efficient stress control. An application of the FGM to a solid insulator (spacer) for a gaseous insulation system, like gas insulated switchgear, is expected to improve electric field (E-field) distribution around the spacer. In this paper, we describe the applicability of the FGM spacer to gas insulated power equipment. In the FGM spacer, we gave spatial distribution of dielectric permittivity to control the E-field distribution inside and outside the spacer. This paper includes following key results for the applications of the FGM. Firstly, E-field simulation results when applying the FGM by a finite element method are presented, in which we show the effective reduction of the maximum field strength by applying the FGM. Next, a fabrication technique of the FGM spacer sample with not only step-by-step but also continuous changes of permittivity is presented by use of centrifugal force. Finally, dielectric breakdown tests using FGM samples which are accurately controlled the spatial distribution of permittivity are carried out under lightning impulse voltage applications. The test result indicates the increase of breakdown voltage (BDV). From these results, we verified the applicability and the fabrication technique of FGM spacer for improvement of the dielectric strength in the gaseous insulation system. View full abstract»

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  • The effect of barriers on electrical tree propagation in composite insulation materials

    Page(s): 373 - 382
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    The propagation of electrical trees is of a particular interest for the power engineering industry as it is one of the major causes for breakdown in high voltage equipments. Composites, materials with barriers and a surrounding polymer matrix are used to extend time to breakdown of the insulations. Since the material, structure or processing of the barriers are various and can determine time to breakdown significantly, the major influences of barriers on the propagation of electrical trees is described in this paper in experiments and numerical simulations. Although it was the motivation, to determine tree growth in the composite structure of mica-epoxy winding insulations, the results can also be used for other composite materials. The trees grew in needle-plane samples and their growth characteristic was optically analysed. The results show that tree propagation can be slowed down when introducing a barrier between the needle and the plane electrode and may cause significantly increased time to breakdown values. The increase depends on the barrier materials used, their thickness, the dielectric strength of the interface to the surrounding epoxy resin and the width of the barrier. When choosing multiple barriers, their arrangement influences time to breakdown significantly. Overlapped barriers show much higher time to breakdown values than impinged ones. From the results, it can be concluded that the selection of the barrier materials, their processing and their arrangement play a major role towards time to breakdown of composite insulation materials. View full abstract»

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  • Dielectric response studies on insulating system of high voltage rotating machines

    Page(s): 383 - 393
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (337 KB) |  | HTML iconHTML  

    A significant method to characterize the electrical properties of materials and to evaluate the condition of the insulating systems is the dielectric response study which can be done in the time or frequency domain. The dielectric response measuring results can be affected by different parameters like measuring voltage, temperature, water content and aging condition of insulating system. In this work numerous measurements are presented, thus to understand, how the dielectric response measurement results at the insulating system of high voltage rotating machines can be affected by these parameters in the time and frequency domain. Because of the importance of the dielectric response application as a non-destructive diagnostic method the possibilities and limitations of this method to reflect the deterioration processes due to thermal and electrical stress were also investigated. In addition the possibility to transfer the measuring data from the time to frequency domain was investigated and the calculated and measured data were compared. For the test object actual stator bars were used that were manufactured with epoxy VPI technology. View full abstract»

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  • The stoichiometry of thermal oxidation

    Page(s): 394 - 399
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    At a stoichiometrical relation of silicon and oxygen during the growth of a rapid thermal silicon-oxide layer (RTO) the electrical parameters of the corresponding metal-insulator-semiconductor capacitor show significant behavior. Combining the Henry-Dalton law and the Boyle-Mariotte law with the Deal-Grove model allows an estimation of the fundamental (T, t, C) process, parameters, temperature, time and concentration of the reactive gas for a stoichiometrical processing of ultra thin SiO/sub 2/ films. Electrical measurements show the (T, t, C)-dependency of I-V, time dependent dielectric breakdown (TDDB) and C-V measurements especially of direct tunneling currents j/sub DT/, relative difference of tunneling currents /spl Delta//sub RDT/, charge-to-breakdown Q/sub BD/ and interface state density D/sub it/. View full abstract»

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  • Systematization of insulation design technology for various electric power apparatus

    Page(s): 400 - 407
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (237 KB) |  | HTML iconHTML  

    This paper deals with cross-apparatus comparisons of an insulation design of electric power apparatus. Focusing on a gas insulated switchgear (GIS), an oil-filled transformer, a cable, and a capacitor, we summarize the different and common features of their insulation design on the basis of not only referring literatures but also the authors' experiences. The insulation design process has three evaluation steps. Each step optimizes its evaluation subject by considering the nature of an insulation medium or an insulation component, and the optimization result is different. However, it is common that the insulation design process has three steps without exceptions. The technology supporting the three steps is also common. View full abstract»

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  • Electric potential near a tip of a local discharge on a conductive film

    Page(s): 408 - 414
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    To develop a propagation model of a local discharge on a wet polluted insulator, the authors have examined characteristics of the local discharge on an electrolytic solution simulating the wet polluted insulator surface. The local discharge was propagated on the solution under the impulse voltage. A shape of a contacting surface between the local discharge and the solution has been already obtained by the authors. By using the contacting surface, an electric potential near a tip of the local discharge on a conductive film of 1 mm in thickness was calculated. Furthermore, the potential was measured with a probe. The calculated values and the measured values of the electric potential agreed well regardless of the propagation length of the local discharge, the width of the conductive film, the resistivity of the conductive film, the peak value of the applied voltage and the applied voltage waveform. View full abstract»

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  • The degradation assessment of epoxy/mica insulation under multi-stresses aging

    Page(s): 415 - 422
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    The actual stator bars of 18 kV/300 MW generator were used for accelerated multi-stress aging tests to investigate the degradation of epoxy mica insulation. The stator insulation of large generator was a composition system of epoxy resin, mica and glass fiber. Ultra-wide band PD detecting technique was introduced to measure the PD pulse of the specimens during aging. The data obtained were analyzed using joint time-frequency analysis methods and finally Gabor spectrum was selected to analyze the characteristics of single impulse wave in different aging period. The results of analysis indicate that following increasing aging time, the high-frequency component of single impulse wave continuously increased, while the energy of discharge alternated from low-frequency long time lag to high-frequency short time lag and high-frequency long time lag. Scanning electronic microscope was used to observe the pattern of interface during the aging process, which show that changes in interface is the main reason of aging. The variation of components in insulation was also analyzed by the technique of infrared spectrum and X-ray energy spectrum and the results indicate that epoxy resin creates hydrolytic reaction and the mica undergoes structural change. View full abstract»

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  • Quantification of degradation in nonceramic housing materials by laser ablation

    Page(s): 423 - 429
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    A laser irradiation technique has been explored to quantify visible as well as invisible degradation of housing materials used for nonceramic insulators. Nonceramic insulators that were removed from service and new samples of the housing that were subjected to accelerated testing in the laboratory have been evaluated. The method uses a constant power laser source for delivering the same energy to the test specimen. The proposed method is fairly quick and could be looked upon as an additional method for characterizing aging of nonceramic insulators. It has been shown that this method provides a better characterization of aging than is obtained from Fourier Transform Infrared Spectroscopy. The housing materials evaluated include silicone rubber, ethylene propylene diene monomer rubber and cycloaliphatic epoxy resin. View full abstract»

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  • Stress grading materials for cable terminations under fast-rise time pulses

    Page(s): 430 - 435
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    The paper discusses the material properties and designs of stress grading coatings for cable terminations that can help to reduce the adverse effects of stepped voltages present in voltage source converters. Simulations using transient and quasistatic finite element method, as well as experimental measurements of stress grading materials are presented in this study. The use of two stress grading layers in cable terminations is suggested as an alternative design to reduce the hot spot temperature in conductive stress grading coatings. View full abstract»

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  • Voltage-time and voltage-number characteristics of insulation elements with oil-filled transformers in EHV and UHV classes

    Page(s): 436 - 444
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (504 KB) |  | HTML iconHTML  

    In order to attain reduction in insulation test voltages, voltage-time (V-t) characteristics under AC voltage and voltage-number (V-N) characteristics under lightning impulse and switching impulse voltages were experimentally investigated on the turn-to-turn insulation model, section-to-section insulation model and barrier-oil-duct insulation model of core- and shell-type transformers in EHV and UHV Classes. First, V-t characteristics for AC voltage were obtained in a short time range of a few tens of milliseconds to a long time range of three to four months. The n-values (inclination) of V-t characteristics are distributed around 40 for the short time and several hundreds for the long time characteristics. These values are of essence in determining the AC withstand voltage test. Next, V-N characteristics for both lightning and switching impulse voltages were acquired for up to 1,000 times of application. The n-values (inclination) of V-N characteristics are in most cases distributed around 70 for both impulse waveforms and about 40 in only turn-to-turn insulation models This result will be useful for evaluating the effect of frequent surges on apparatus insulation. Finally, insulation coordination studies were performed based on these experimental results. 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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Editor-in-Chief
Reuben Hackam