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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 3 • Date June 2000

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Displaying Results 1 - 22 of 22
  • A fast hybrid system for PD measurement

    Page(s): 440 - 445
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    A fast partial discharge (PD) measurement system is described, which employs analog as well as digital signal processing to achieve high sensitivity and discharge pulse resolution with low spectral distortion. Data on apparent charge, time of pulse occurrence and instantaneous applied voltage are stored in memory for subsequent information retrieval and treatment. The PD measurement system is designed to provide an integrated response to rapid rise time pulses of 1 to 2 ns, with suitable filtering added to reject extraneous noise. View full abstract»

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  • Impacts of thermal aging and water absorption on the surface electrical and chemical properties of cycloaliphatic epoxy resin

    Page(s): 424 - 431
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    New techniques for measuring surface degradation are applied to the study of surface tracking resistance of materials intended for outdoor use. Cycloaliphatic epoxy resin (CAE) with 200 pph silica powder was aged under accelerated conditions for 100 h (100 to 250°C) and then its tracking resistance was assessed. The tracking resistance of CAE that was exposed to water absorption (0 to 3.3%) also is evaluated. Changes on the surface layer (1 to 10 μm) with thermal treatment are revealed by employing scanning electron microscopy (SEM)-energy dispersive X-ray (EDX) analysis, attenuated total reflection (ATR)-Fourier transform infrared (FTIR), reflective visible spectroscopy and thermal gravimetry (TG)-differential thermal analysis (DTA) mass spectrometry (MS). The tracking resistance of CAE aged above 170°C is particularly reduced. Surface resistivity declines with rising temperature of thermal aging. The lowering of surface resistivity enlarges the leakage current passing at the aged surface layer and increases the surface temperature of CAE. This would accelerate thermal aging. It also is shown that absorbed water in CAE decreases tracking resistance. The effect of absorbed water during the initial stage of a tracking test is described. Under saline pollution and moisture, thermal aging affects not only surface tracking but also electrochemical erosion derived from sodium hydroxide which forms through electrolysis of sodium chloride. Electrochemical erosion enhances water uptake for CAE, which can lead to tracking resulting from absorbed water View full abstract»

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  • Low-molecular weight silicone fluid in RTV silicone rubber coatings

    Page(s): 461 - 462
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    The author comments on a recent paper by H. Deng and R. Hackam (see ibid., vol.6, no.1, pp.84-94, 1999), which studied several factors affecting the level of low molecular weight (LMW) silicone fluid which is present in room temperature vulcanized (RTV) silicone rubber (SIR) coatings. A reply by the original authors is included View full abstract»

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  • A generalized dielectric polarization evolution equation

    Page(s): 374 - 386
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    In this paper a non-equilibrium statistical-mechanical theory of dielectric relaxation is developed. This approach differs from previous work in that a generalized nonlocal evolution equation for the polarization is constructed. General equations of motion are presented for the polarization, internal energy, and entropy which include effects of memory. These equations can be expressed in terms of reduced-correlation functions, and are valid for non-equilibrium and arbitrary field strengths. Expressions for an effective local field also are developed. The Fourier transform of the evolution equation yields a general compact expression for the Fourier transform of the memory function and a specific form for the susceptibility. The kernel, Fourier transform of the memory function are developed, and relaxation-time functions for special cases. In the limit of a single relaxation time, a Debye response is obtained. In the subsequent special cases exponential and Gaussian forms for the memory functions are assumed. The final special case relates a power-law circuit transfer function to the theory of Dissado and Hill. In this case the memory kernel and relaxation times are derived from the Dissado-Hill response function View full abstract»

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  • The stability of ferroelectric polarization of PVDF upon irradiation

    Page(s): 353 - 359
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    Polymeric actuator and sensor materials suffer from a low thermal and temporal stability. We report on an attempt to slow down the underlying structural relaxation in polyvinylidenefluoride (PVDF) films by irradiation with electrons of 1.5 MeV. This treatment yielded a dose-dependent crosslinking of the macromolecular chains. It caused a considerable rise of the ferroelectric remanent and saturation polarization, and the material became a harder ferroelectric. These changes were almost completely and irreversibly lost upon subsequent annealing. Electron spin resonance (ESR) measurements revealed that a large number of radicals and trapped electrons remained after irradiation (~1 per crystallite), and that the electric conductivity increased from 0.11 pS/m in the un-irradiated but poled sample to 1.1 pS/m at 450 kGy. The increase in conductivity with dose is caused completely by a corresponding increase of the product μn of the charge carrier mobility μ and the number density n. Based on a discussion of depolarization effects, we conclude that the change in ferroelectric behavior upon irradiation is caused mainly by the activity of those charges which are implanted or created by irradiation, and that the majority of them are annihilated by thermal treatment. However, the reduction of polarization with time, or at elevated temperature, is considerably slowed down due to the existence of crosslinks View full abstract»

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  • Evaluation of dc insulation performance based on space-charge measurements and accelerated life tests

    Page(s): 322 - 328
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    Space charge observation is becoming the most widely used technique to evaluate polymeric materials for dc insulation applications, particularly HV cables. However, quantities are still lacking that can help to summarize and interpret the huge amount of data resulting from space charge measurements, and that also are associated with the electrical performance of the insulation. The purpose of this paper is to propose test methods and parameters, based on the pulsed electroacoustic technique, which can be used for the evaluation of the electrical performance of polymeric insulation under dc fields. These parameters are related to the space charge magnitude and mobility as well as to the internal electric field amplification, the rate of charge accumulation, and the threshold field above which charge is stored in the insulation. In order to investigate the meaning of the parameters proposed for the assessment of insulation performance, the correlation between these parameters associated with space charge and insulation life is discussed. Examples are reported with reference to five different candidate materials for dc cable insulation View full abstract»

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  • On the effect of anisotropy in nonlinear composite materials for stress grading applications-a numerical study

    Page(s): 387 - 393
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    The production technology of carbon-black loaded polyolefins adopted for stress grading purposes in ac cable terminations may impart, in addition to the nonlinear electrical properties, an anisotropic behavior to the material. In this paper a numerical study is presented in order to study the influence of both anisotropy and nonlinearity on the electrical field dynamics inside cable terminations adopting stress grading carbon-black loaded composites. The nonlinear anisotropic conductivity is properly taken into account in the model, allowing a sound understanding of the influence of such property on the stress grading performances of the considered accessory. The numerical results for sinusoidal power frequency and standard impulse voltages demonstrate the role of the different conduction mechanisms on stress grading efficiency and give useful indications for an effective material characterization View full abstract»

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  • The electrical degradation threshold of polyethylene investigated by space charge and conduction current measurements

    Page(s): 309 - 315
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    Measurement of dc charging current and space-charge observations, performed on specimens made by low-density polyethylene (PE) and crosslinked PE, are considered for dc electrical threshold investigation. The threshold is inferred from charging-current measurements through steady-state voltage-current characteristics, as well as from the total trapped charge, obtained by space-charge measurements carried out at 0 V and under short-circuit after poling for a pre-selected time. It is shown that both techniques provide similar information regarding the electrical threshold values, which vary for the two tested materials. Being the threshold associated with mechanisms of charge storage, its evaluation can help in insulation design and material characterization when electric field, especially dc, is a significant stress in service. Other significant information for design purposes comes from the rate of charge accumulation as function of the applied field, which is estimated through the charge-field characteristic derived from space-charge measurements View full abstract»

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  • Study and characterization of PD in impregnated polypropylene capacitors

    Page(s): 446 - 457
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    Measurements of partial discharge (PD) in models of impregnated polypropylene capacitors with and without specific defects have been conducted. Two different regimes of discharges (in the liquid and in gas bubbles) are illustrated. The effect of temperature on the decrease of the insulation property is underlined. Four specific defects have been artificially introduced in the test cell (poor impregnation, bad contact, fold in the polypropylene (PP) film, cavity). The relevance of parameters such as skewness and kurtosis to characterize PD distributions is discussed. New parameters based upon the mean position of the PD in the voltage phase are proposed for a better recognition of the defects inducing PD View full abstract»

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  • The influence of defects on the short-term breakdown characteristics and long-term dc performance of LDPE insulation

    Page(s): 401 - 407
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    Extruded polyethylene is used as the bulk insulation for ac HV cables because of its high electric resistivity and breakdown strength. Although the material at present has limited use in dc power cables, it is used extensively in submarine optical communication cable systems. This paper reports on the study of the short-term characteristics and long-term performance of low-density polyethylene (LDPE) insulation under dc electric stress. The results are presented in which controlled defects as found in practical systems (voids, metallic and non-metallic particles) were introduced into well-characterized polymer material so that their influence on electrical strength and breakdown mechanisms could be determined. Samples were compression-molded under laboratory conditions and subjected to ramp-to-failure and electrical aging tests at various stresses. Weibull statistics are used to analyze the results. Examination of electrically aged samples indicates an oxidation degradation around the defects due to stress enhancement. From the failed samples a value of n in the inverse power model was estimated. The residual life of aged samples was also examined View full abstract»

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  • Electric field reduction due to charge accumulation in a dielectric-covered electrode system

    Page(s): 340 - 345
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    Outlines for increased insulation performance of an air gap through the use of dielectric coatings are given. Theoretically, it is shown that the homogeneous electric field in a plane-parallel electrode system can be reduced if the electrodes are covered with a thick dielectric coating. Free charges will be affected by the electric field between the electrodes and are deposited at the dielectric surfaces. As a consequence, a counteracting electric field component results, which accordingly causes a reduction of the total electric field in the air gap and an enhancement of the field in the dielectric layers, i.e. the electric field is forced into the dielectric coatings by the charges. This effect has important implications in HV engineering. Introductory experiments supporting the idea have been carried out with promising results. It was confirmed that the withstand voltage of a plane-parallel electrode geometry with an open air gap, for dc as well as unipolar impulse voltage, could be increased considerably if the electrodes were covered with thick polymeric layers. Charge formation at the electrode surfaces as well as in the air gap is believed to be responsible for this effect. In today's insulation systems, this is known to work only for time-independent electric fields, i.e. generally dc voltages. Further experimental work is required and will be performed in order to scrutinize thoroughly and clarify the phenomenon, its capabilities and limitations View full abstract»

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  • Multilayer high gradient insulator technology

    Page(s): 334 - 339
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    We are investigating a novel insulator concept that involves the use of alternating layers of conductors and insulators with periods on the order of <1 mm. These structures perform many times better (~1.5 to >4× higher breakdown electric field strength) than conventional insulators in long pulse, short pulse, and alternating polarity applications. A previously defined scaling law of d0:5 , where d is the insulator length, appears to apply to these new structures when the scaling parameter d is replaced by the layer period d1. This observation implies that each layer within the structure behaves independently in the breakdown process. We present our ongoing studies investigating the degradation of the breakdown electric field strength resulting from surface roughness, the effect of gas pressure, and the performance of the insulator structure under bipolar stress. Further, we present our initial work on scaling and modeling studies View full abstract»

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  • Electrical contact between a local discharge on an electrolytic solution and the solution surface [wet polluted insulators]

    Page(s): 360 - 365
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    An electrolytic solution surface was used to simulate the surface of wet polluted insulators. A local discharge propagates on the solution surface when a voltage is applied to the surface. The electrical contact between the local discharge on the electrolytic solution and the solution surface was examined. The distribution of the current from the local discharge to the solution is estimated from the potential distribution measured with probes placed in the solution. The resistivity of the solution is varied from ~80 to 370 Ω cm. Impulse voltages of 2/80 μs, 10/800 μs and 100/2500 μs are used as the applied voltage. The peak value of the applied voltage was 1.05 to 1.4× the 50% flashover voltage V50 under each set of experimental conditions. As a result, it is found that most of the discharge current flows from the tip part of the local discharge, and the current from the local discharge column is considerably lower. Furthermore, the distribution of the current from the local discharge to the solution is found to be barely influenced by the resistivity of the solution, the peak value of the applied voltage, or the applied voltage waveform View full abstract»

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  • Temperature distribution along an outdoor insulator subjected to different pollution levels

    Page(s): 416 - 423
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    The appearance of dry bands on polluted insulator surfaces may cause the ignition of electrical arcs, which in turn give rise to tracking and erosion, or may elongate until they bridge the electrodes causing flashover. Efficient models able to predict the occurrence of such a severe problem are of paramount importance for the engineer involved in the design of power line insulation. The present paper concerns the evaluation of the temperature distribution along the surface of an outdoor insulator subjected to pollution of different severity in order to predict dry band formation. The proposed numerical procedure, based on the joint solution of Maxwell equations in their quasi-static formulation and thermal equations, has been applied to study the case of a suspension cap-and-pin glass insulator, covered with pollution layers of different conductivity and thickness. The dependence of the conductivity of the layer on the temperature has been taken into account properly. The influence of uniform and nonuniform pollution distribution on the temperature profile is discussed for an applied sinusoidal power frequency voltage View full abstract»

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  • Simulation of streamer discharges as finitely conducting channels [comments and reply]

    Page(s): 458 - 460
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    The recent paper by Gao et al. [see ibid., vol.6, p35-42, 1999] is most interesting and develops a useful model. Three comments will be made: on the model itself and the `within-streamer' field used; on the interpretation; and on the streamer diameter View full abstract»

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  • Effects of O2 rapid thermal annealing on the microstructural properties and reliability of RF-sputtered Ta2O5 films

    Page(s): 316 - 321
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    The microstructural properties and reliability of sputtered Ta2O5 films treated by various temperatures of rapid thermal annealing (RTA) in O2 atmosphere have been systematically investigated. Analytical results revealed that whenever the RTA temperature was >650°C, the noncrystallinity of as-grown Ta2O5 film would be effectively improved from an amorphous phase to the β-Ta2O5 phase. Leakage current measurement indicated that leakage current decreases with increasing annealing temperature in a low RTA temperature range (⩽650°C) and, contrarily, increases with increasing annealing temperature in a high RTA temperature range (650 to 950°C). The former result was asserted in that reducing pinholes and oxygen vacancies played key factors. However, the latter result arose due to significant Si diffusion into the Ta2O5 film, causing a leaky transition layer distributed along the grain boundary to form the leakage path. Finally, the time-dependent dielectric-breakdown experiments revealed that 950°C O2 RTA treated Ta2 O5 film possessed the superior crystallinity, creating less interfacial hole trap states at the junction of Ta2O5/Si and exhibiting the best long-term reliability View full abstract»

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  • Thermal step and TSDC measurement in PVC

    Page(s): 329 - 333
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    The space charge behavior of some polyvinylchloride (PVC) samples with different molecular microstructures, obtained through chemical modification of PVC has been analyzed by means of the thermal step (TS) and the thermally stimulated discharge current (TSDC) techniques. The magnitude of the space charge distribution has been shown to be lower as the degree of chemical modification increases. Combining the TS and the TSDC methods allowed us to study minority carriers, which could not be observed by a single TS measurement View full abstract»

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  • Space charge characterization in aged LDPE amalgamated insulation regions from underwater telecommunication systems

    Page(s): 346 - 352
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    The requirement of insertion of optical amplifier units and joints, into long-distance submarine telecommunication systems, results in regions of the polymeric insulation which have to be reinstated, using injection molding techniques. These techniques introduce internal amalgamated interfaces, which subsequently can be the sites of possible electrical insulation failure in the system. This paper reports on space charge measurements, (using the pulsed electroacoustic (PEA) technique), from microtomed thin film samples taken from the interface region of typical system joints which have undergone accelerated aging tests under a dc voltage at room temperature. The space charge distributions obtained for the samples are discussed with reference to the underlying mechanisms of both the bulk low-density polyethylene (LDPE) and the surface (LDPE/electrode interface). The effect of varying the electrode material (i.e. Au and Al) is discussed with reference to previously published data. Finally the effects of long-term aging on space charge accumulation within LDPE are considered briefly View full abstract»

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  • Analysis of electrical activity associated with inclined-plane tracking and erosion of insulating materials

    Page(s): 394 - 400
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    Surface electrical activity and physical changes were measured and compared for several types of outdoor insulation materials, using a standard test method. The leakage current activities during the inclined-plane test (ASTM D2303) were measured by using four representative materials: a silicone with high (>70% by weight) loading of alumina trihydrate (ATH), a silicone with no ATH, a poly ethylene vinyl acetate (EVA), and a glazed porcelain. Quantitatively, the severity of the inclined-plane test was defined by detailed leakage current measurements. Qualitatively, it is observed that the test was severe enough to damage the glaze on porcelain. The study enabled us to compare polymers with porcelain, silicones containing no ATH vs. high loading of ATH, and silicones with EVA. The analysis showed that electrical activity, particularly the average leakage current and the distribution of peak current, depended on the surface wettability of the materials by the contaminant solution. The compound formulation is more important than the generic polymer types. It was demonstrated that silicone with no ATH, and EVA with only a medium level of ATH, exhibited excellent tracking and erosion resistance, comparable to the silicone highly filled with ATH. The presence of ATH is not absolutely necessary to achieve the superior tracking and erosion resistance of a silicone elastomer View full abstract»

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  • Statistical models for failure modes of polymeric materials for HV outdoor insulation

    Page(s): 408 - 415
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    In this paper, a methodology to build the required models for the performance comparison of materials that usually pass the standard inclined plane tracking and erosion tests, is presented. Samples made of silicone rubber and cycloaliphatic epoxy resin were evaluated to present the practical aspects of the proposed methodology. Factorial experiments, using the modified version of the inclined plane test method, were conducted to obtain the response variable. Generalized linear models provided the base for the proposed statistical models since the selected response variable was not normally distributed View full abstract»

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  • Increase in capacitance and tan δ between conductors on printed circuit board at low frequency due to ionic migration

    Page(s): 366 - 373
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    An endurance test for a printed circuit board to ionic migration was carried out for 2000 h in a chamber controlled at 85°C and 85%RH. The capacitance C and tan δ between the conductors on the board were measured to find their correlation with ionic migration in the board. The measurement frequency of C and tan δ ranged from 1 to 1000 Hz. The configuration of the test circuit was designed according to IPC-SM-840. The material of the insulating board was fiberglass-reinforced epoxy resin. The distance between the Cu conductors was 0.165 mm, and 70 V dc was applied continuously. In the low frequency region (<30 Hz), C and tan δ gradually increase after ~800 h voltage application. During the gradual increase, steeply transient increases in C and tan δ are detected at about the same time when a deposit was seen to have formed on the board between the conductors, although no significant decrease in insulating resistance between the conductors is observed. It was confirmed that the deposit could be detected early from the steep increases in C and tan δ in the low frequency region. In the high frequency region (>30 Hz), on the other hand, gradual slight increases are observed from ~1100 h after voltage application, but no steep increases. The dependence of C and tan δ on frequency at the measurement is due to the low mobility of Cu ions in the water film on the insulating board View full abstract»

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  • The ion-specific behavior of watertree growth in LDPE

    Page(s): 432 - 439
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    The growth of water trees in low-density polyethylene (PE) has been studied. Trees were grown in solutions containing a fixed anion, Cl -, but different cations with valences of +1 (Li+), +2 (Mn2+) and +3 (Fe3+). An ion-specific effect on tree growth and tree appearance has been found. Generally, these results have been shown to be correlated with either the ion-water interactions or the interionic attractions. However, for the specific case of adding small amounts of 3+ cations into univalent ion solutions, such a correlation was not found: it is likely that the hydrogen ions, introduced by the multivalent ions through their electrostatic interaction with water molecules, play an important role. The abnormal transport behavior of such hydrogen ions in the solution and probably in the polymer, may expand the overall region affected by treeing but reduce the visibility of the individual trees themselves. A similar process may account for the particularly translucent appearance of trees formed with pure FeCl3 solutions. This argument is further demonstrated through tree growth experiments using KCl solution and its mixtures with small amounts of HCl. It was found that this additive accelerated tree growth. It is argued that small amounts of KOH might be expected to show an effect similar to HCl because of the similar transport behavior of OH- ions and H+ ions respectively and such enhanced tree growth was demonstrated, using small quantities of KOH added to KC1 solutions 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