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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 6 • Date Dec. 2005

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  • IEEE Transactions on Dielectrics and Electrical Insulation

    Page(s): 0_1
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    Freely Available from IEEE
  • [Inside front cover]

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  • On the interpretation of some electrical aging and relaxation phenomena in solid dielectrics

    Page(s): 1089 - 1107
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    Electrical aging and relaxation in solid dielectrics are phenomena studied for decades but still largely poorly understood. The lack of understanding, or sometimes the misunderstanding, are partly due to inadequate or deficient theories and partly due to wrong interpretation of theories or concepts developed for other materials than dielectrics. In addition, a serious analysis of some of the empirical and approximate relationships often used to interpret polymer relaxations reveals that they are not only limited and restrictive but also that they are based on very dubious assumptions. Dry or wet electrical aging is much more complex and often includes many phenomena with synergetic effects such as thermal aging, electrochemical reactions, space charge effects, etc. In this lecture, we intend to show that some of the theories often cited in the literature are wrong or inappropriate. We show why they cannot describe the phenomena at play and we present some alternatives for future work in the domains of relaxation and electrical aging. The relations between aging, space charges and polarization of polyethylene (PE) under the influence of high electrical fields (above 20 kV/mm) are discussed in light of our electrical aging model. It is our contention that strong charge injection occurs only after nanocavity formation, i.e. above the critical field. The amorphous phase is then significantly deformed and weak van der Waals attraction bonds are broken, leading to another faster aging regime. The possible relation between the nanocavity formation at moderate fields and bonds breaking at higher fields proposed in our aging model and various polarization measurements is discussed. One objective of this paper is to encourage the development of more complex and complete theories specific to dielectrics. Some experimental work needed to achieve this goal is pointed out. View full abstract»

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  • Silicone rubber dielectrics modified by inorganic fillers for outdoor high voltage insulation applications

    Page(s): 1108 - 1115
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    The paper discusses the mechanisms by which inorganic fillers in silicone rubber dielectrics enhance the properties of thermal conductivity, relative permittivity, and electrical conductivity making them useful in outdoor high voltage insulation applications. The addition of alumina trihydrate or silica fillers to silicone elastomers, forming binary composites with enhanced thermal conductivity, is discussed in relation to filler type, particle size, shape, and concentration, and its use as a housing material for non-ceramic insulators to minimize material erosion at dry band arcing sites by lowering hot spot temperature. Also discussed is the enhanced relative permittivity of silicone dielectrics that is obtained through the addition of barium titanate powder which can be further increased with the addition of aluminium powder forming a tertiary composite, resulting in a significant grading of the surface electric field when applied as a housing material to high voltage bushings. Controlled electrical conductivity of silicone dielectrics is discussed through the use of antimony-doped tin oxide filler binary composites and when applied as a housing material to outdoor bushings, the pollution performance is greatly enhanced. View full abstract»

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  • Contribution of surface conductivity to the current forms of partial discharges in voids

    Page(s): 1116 - 1124
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    The current form of partial discharge (PD) in a void was divided into fast and slow components, with short and long fall times, respectively. Comparing these with the current shape of PD in a void between two metal surfaces, in which the slow component was not observed, it was suggested that the formation of fast component in a usual void was due to the instantaneous discharge between the discharge areas of the PD, i.e. the region to which the carriers in a PD process can instantaneously propagate, and the slow one might be due to the low conductivity in the regions outside the discharge areas. An equivalent circuit was proposed to describe the effect and numerical simulations were carried out. It was suggested that the fall part of the first PD after polarity reversal might reflect the surface conductivity change in aging process. View full abstract»

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  • Electric field computation in wet cable insulation using finite element approach

    Page(s): 1125 - 1133
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    Accurate assessment of cable insulation conditions can be achieved by implementing advanced diagnostic and simulation techniques that assist the measurement and monitoring of the properties related to aging and failure of the insulation system. It is well recognized that the electric field distribution is the dominant factor in the initiation of degradation process in the insulation system. In such a system, and due to the presence of pollutants such as water or moisture, local field enhancement occurs resulting in field stress values enough to cause local breakdown of the insulation. The finite element simulation technique is used to evaluate the electric field inside the power cable. A model that illustrates the water-dielectric interface within the cable insulation system is proposed. The difficulties associated with the building of such a model, which contains elliptically shaped water particles and unusually configured insulation areas, are illustrated. Finally, the link between the local field concentration in the vicinity of water particles and the possibility of insulation failure, which can be developed to a complete breakdown, is discussed. View full abstract»

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  • A fuzzy logic algorithm to detect electrical trees in polymeric insulation systems

    Page(s): 1134 - 1144
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    Inter-time (i.e., the time that separates a PD event from the next one) distribution analysis has been recently proposed as a tool to infer the presence of trees in solid insulation systems. In this paper, inter-time distribution analysis is improved to infer more accurately trees at an early growth stage. Measurements from both artificial specimens as well as practical objects are presented to support the effectiveness of this tool. Moreover, considerations on inter-time analysis robustness with respect to both noise interference and other partial discharge sources are reported. View full abstract»

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  • Thermal and ferroelectric properties of potassium nitrate: polyvinyl fluoride composite films

    Page(s): 1145 - 1150
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    The thermal and ferroelectric properties of potassium nitrate (KNO3): polyvinyl fluoride (PVF) composite films have been studied. The dielectric constant has been measured as a function of temperature and shows a sharp change during heating and cooling cycles. Differential scanning calorimetry (DSC) has been employed to characterize the phase transitions and corresponding enthalpies in the composite. A good correlation has been found between the transition peak temperatures of the DSC and the transition temperatures of the dielectric measurements in the composite films. The DSC experiments suggest that phase III is retained at room temperature in the composite films. The switching current investigations support the presence of ferroelectric phase in the composite film. The capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of the composite films exhibit essential butterfly features of a ferroelectric material. View full abstract»

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  • Dielectric characteristics of spin-coated dielectric films using on-wafer parallel-plate capacitors at microwave frequencies

    Page(s): 1151 - 1161
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    Dielectric properties of spin-coated dielectric insulators suitable for high-speed device fabrication are investigated. Complex dielectric permittivities and tangential losses of two polyimides, bisbenzocyclobutene (BCB), and a spin-on-glass (SOG) were extracted from the measured microwave reflection coefficient, S11, of parallel-plate capacitors over a frequency range of 50 MHz to 40 GHz. A model for the dielectric permittivity as a function of frequency is developed based on measured data with a minimum square error of less than 10-4 between measured and modeled microwave reflection coefficients. A circuit model for the pad capacitance is obtained based on geometrical and physical considerations. The relationship between the dielectric loss and its thickness is considered. Experimental results are fitted to Debye and Cole-Cole models. View full abstract»

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  • Effects of atmospheric pressure on DC resistance to tracking of polymer insulating materials

    Page(s): 1162 - 1171
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    Polymer insulating materials are required for use in high altitude regions. For the use of polymer insulating materials in such regions, it must be confirmed whether the resistance to tracking at high altitudes is different from that at altitudes near the sea level. In this paper, an artificial atmospheric chamber was set up to investigate the resistance to tracking on a polymer insulating surface at DC voltage application under a reduced pressure. The tests were carried out in accordance with IEC 60112. The test results showed that the dependences of the resistance to tracking for polymer insulating materials upon the decrease in ambient pressure could be classified into three types. One type is for paper-based phenolic laminate and polycarbonate, where the resistance increases with the decreasing ambient pressure; the second is for polybutylene terephthalate where the resistance decreases with the decreasing ambient pressure; the last type is for epoxy resin where the resistance is almost independent of the decrease in the pressure. A Gaussian wavelet analysis was applied to discharge currents to discuss correlations among the resistance to tracking, the discharge energy level and the ambient pressure. It was found that the data of resistance to tracking with some of the materials obtained at altitudes near sea level are inadequate for use in a high-altitude environment. Problems may occur with respect to the safety and reliability of polymer insulating materials when the DC resistance to tracking decreases under low-pressure conditions. View full abstract»

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  • Influences of probe geometry and experimental errors on spatial resolution of surface charge measurement with electrostatic probe

    Page(s): 1172 - 1181
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    When an electrostatic probe is used for surface charge measurement on an insulating plate with constant thickness or a long insulating pipe, the configuration of the measuring system is regarded as shift-invariant and the relation between the surface charge density and the probe output can be treated in the spatial frequency domain through Fourier analysis. In the inverse calculation from the probe output to the charge distribution, a Wiener-filter technique is effectively used to suppress the excessive amplification of the noise. Influences of the probe geometry and experimental errors on the spatial resolution of the total measuring system, including the restoration process with the Wiener filter, are quantitatively discussed through the analysis of transfer characteristics in the spatial frequency domain. View full abstract»

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  • Signal denoising techniques for partial discharge measurements

    Page(s): 1182 - 1191
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    One of the major challenges of on-site partial discharge (PD) measurements is the recovery of PD signals from a noisy environment. The different sources of noise include thermal or resistor noise added by the measuring circuit, and high-frequency sinusoidal signals that electromagnetically couple from radio broadcasts and/or carrier wave communications. Sophisticated methods are required to detect PD signals correctly. Fortunately, advances in analog-to-digital conversion (ADC) technology, and recent developments in digital signal processing (DSP) enable easy extraction of PD signals. This paper deals with the denoising of PD signals caused by corona discharges. Several techniques are investigated and employed on simulated as well as real PD data. View full abstract»

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  • Breakdown electric field calculations of hot SF6 for high voltage circuit breaker applications

    Page(s): 1192 - 1200
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    The critical reduced electric field strengths of hot SF6 corresponding to the dielectric recovery phase of a high voltage circuit breaker are calculated for a large temperature range (300-3000 K). Calculations are based on a multi-term Boltzmann equation solution using, in comparison to the literature works, improved cross section sets for the interactions of electrons with various SF6 dissociated products. The obtained critical electric fields show a reasonable agreement with the available data. These results are then used in hydrodynamics simulations which correctly predicts the circuit breaker behaviors observed in the case of a successful breaking test as well as in a failed one. View full abstract»

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  • A novel technique to evaluate the erosion resistance of silicone rubber composites for high voltage outdoor insulation using infrared laser erosion

    Page(s): 1201 - 1208
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    The standard methods available to test tracking and erosion resistance of filled silicone rubber do not allow to delineate the fundamentals of the thermal degradation, although the heat from the dry band arcing is the main degradation factor. In this work a thermal imaging camera is used to investigate the performance of filled silicone rubber in an inclined plane test, and a scheme is established to relate the temperature and the electrical discharge energy. Further, an infrared laser technique, based on constant energy approach, is developed to study the material performance under laser heating. The technique consists of applying a certain amount of infrared laser energy during a set time period, in order to produce erosion in the silicone rubber samples. Through correlation studies the applicability of the proposed laser technique to rank silicone rubber samples with various fillers has been validated. The laser tests show identical ranking of samples when compared to samples ranked using inclined plane tests, yet offering significant advantages of being faster, simpler, and reproducible. View full abstract»

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  • Heat treatment of cross-linked polyethylene and its effect on morphology and space charge evolution

    Page(s): 1209 - 1221
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    It is believed that space charge build-up in cross-linked polyethylene (XLPE) insulation is the main cause of premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is the process of subjecting the XLPE to a moderately high temperature, thereby removing volatile residual by-products and additives that are commonly employed in cable manufacturing to improve insulation performance. While there have been reports comparing space charge activities (as-received) un-degassed and degassed samples, little attention has been placed on degassing conditions effect on space charge. In this paper, investigations of morphology and space charge distribution of planar XLPE samples was carried out as parameters such as degassing temperature, degassing duration and sample thickness were adjusted. Changes in the morphological structure of the material were probed with the differential scanning calorimeter and the transmission electron microscopy, whereas space charge distribution of the samples subjected to DC electric stress was monitored for 24 h using the laser induced pressure pulsed technique. It was revealed that degassing temperature, duration and the thickness of the degassed sample has a direct effect on the morphological structure and space charge distributions of the samples. It is believed that the presence and amount of by-products takes precedence over the morphology of the material as far as space charge evolution is concerned. View full abstract»

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  • An improved optically based PD detection system for continuous on-line monitoring of HV cables

    Page(s): 1222 - 1234
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    An optically-based remote sensing technique using a laser source, polarization scrambler, standard single mode fibers, fiber polarizer, electro-optic (EO) modulator and optical receiver, for continuous on-line monitoring of partial discharges (PD) in high voltage cable joints is proposed. Both the fiber polarizer and EO modulator, which are placed near cable joints, are passive without any power requirements. An EO modulator with no electrical bias, which operates close to the half-intensity point, was used and proved to have much better light transmission performance and higher measurement sensitivity than the bias modulator approach investigated earlier. Various multiplexing techniques including optical switches and a fused fiber coupler have been investigated in order to implement multi-channel monitoring so that one laser source can be used to provide light to several fibers. The revised PD optical remote sensing technique has been applied to practical PD measurements on both 132 kV cable joint at the University of Southampton in the UK and 400 kV cable joints at Sudkabel GmbH in Germany. Obtained results have verified the principle and demonstrated the effectiveness of the developed technique. View full abstract»

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  • Dielectric properties, of metallized paper-film capacitors

    Page(s): 1235 - 1240
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    A capacitor formed of fluid-impregnated paper metalized on one side and a polymer film results in a relaxation peak caused by the much more rapid polarization of the dielectric fluid impregnated paper relative to the polymer film. This results in the capacitance being a function of frequency, with a substantial increase in capacitance at very low frequencies. A theory for such a capacitor is developed and compared with measurements on actual capacitors with relatively good agreement. The implications of the theory for discharge current and discharge efficiency are discussed. View full abstract»

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  • Transport phenomena of poly(3-alkylthiophene) at the solid-liquid phase transition

    Page(s): 1241 - 1248
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    We have tried to confirm the sign inversion of the photocarrier in regiorandom poly(3-octadecylthiophene), PAT18, by means of the time-of-flight (TOF) method. It was found that the hole mobility decreases with increasing temperature. At the temperature near the solid-liquid phase transition, the hole mobility decreases drastically, and eventually the melting temperature, it was impossible to evaluate the hole mobility from noisy transient photocurrent. On the other hand, transient photocurrents based on electron transport are observed above the melting point. The negative carrier mobility was evaluated in the range from 10-6 to 10-5 cm2/Vs, which is comparably the hole mobility at the solid state. Moreover, the electrical conductivity changes drastically at the instant of the solid-liquid phase transition, and transport in the liquid state was not influenced remarkably by applied hydrostatic pressure, which suggests electronic transport and not ionic transport. These facts suggest that the same mechanism, for example interchain hopping limits carrier transport, and the negative carrier is electron. This unique phenomenon is interpreted as modulation of electronic energy state caused by conformational change of the main chain. View full abstract»

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  • Modeling metal oxide surge arrester for the modern polarization based diagnostics

    Page(s): 1249 - 1258
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    Recently a number of new non-destructive diagnostic techniques have been investigated for the reliable condition assessment of the ageing of metal oxide surge arrester (MOSA). Among them polarization/depolarization current and return voltage measurement techniques showed very promising results. This paper presents an insulation polarization model for investigating polarization-based diagnostics for metal oxide surge arrester. The simulations of the proposed model are validated and verified by comparing the data from the polarization/depolarization current measurements. The simulation of return voltage for MOSA is also performed to validate the insulation polarization model. Finally, the usefulness of this model for MOSA diagnostics has been highlighted in this paper. View full abstract»

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  • Electrical tracking performance of LLDPE-natural rubber blends by employing combination of leakage current level and rate of carbon track propagation

    Page(s): 1259 - 1265
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    Electrical tracking develops from surface discharge activity associated with the flow of leakage current on insulator surface under wet and contaminated conditions. Arcs created from this surface discharge phenomenon burn the polymer insulator material and create carbonized tracks in the long run. This paper reports on electrical tracking performance through the observation of leakage current and carbon track development of blends of linear low-density polyethylene with natural rubber (LLDPE/NR) either filled with or without alumina trihydrate. An electrical tracking and erosion test using the inclined-plane tracking method is conducted to study the overall performance of surface tracking properties. The combined effect of leakage current level and carbon track propagation rate is used as a new technique for describing the level of electrical tracking performance based on the proposed normalized degradation index. Morphological analysis is also carried out to investigate the surface microstructure before and after the tracking test. The experimental results show that different material compositions affect the leakage current and carbon track development. In addition, the calculated normalized degradation index has shown some correlation with the degree of surface deterioration. View full abstract»

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  • Barrier effect of treeing in composite insulating materials with heat-adhesive interfaces of different polymers

    Page(s): 1266 - 1271
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    A polymer/polymer composite system is widely used in high-voltage equipment and power cables. The interface of this composite system plays an important role in the electrical degradation, such as electrical treeing or partial discharge, of the composite system. Therefore, it is important to study the effect of an interface on the electrical treeing of polymer/polymer-composite insulating materials. In this paper, we describe the tree growth and complete breakdown characteristics of specimens with a polymer barrier film layer molded in ethylene vinyl acetate copolymer (EVA), in which the interface of EVA and the barrier film is perpendicular to the electric field under an AC voltage. It is found that with a barrier film, the tree growth is retarded on the barrier film, which then punctures the film, or develops along the edge of the film. It is also found that the pressure of decomposing gas in the tree channels plays an important role in tree propagation. 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