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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 1 • Date Feb. 2007

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  • Table of contents - Feb. 2007, Vol 14 No 1

    Page(s): c1
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  • IEEE Dielectrics and Electrical Insulation Society - Staff

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  • Editorial - After Five Years

    Page(s): 1 - 2
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  • A novel wavelet transform technique for on-line partial discharge measurements. 1. WT de-noising algorithm

    Page(s): 3 - 14
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    Medium and high voltage power cables are widely used in the electrical industry with substantial growth over the last 20-30 years ago, particular in the use of XLPE insulated systems. Ageing of the cable insulation is becoming an increasing problem that requires development of reliable methods for on-line condition assessment. For insulation condition assessment of MV and HV cables, partial discharge (PD) monitoring is one of the most effective techniques. However on-site and on-line PD measurements are affected by electromagnetic interference (EMI) that makes sensitive PD detection very difficult, if not impossible. This paper describes implementation of wavelet transform techniques to reject noise from on-line partial discharge measurements on cables. A new wavelet threshold determination method is proposed with the technique. With implementation of this novel de-noising method, PD measurement sensitivity has been greatly improved. In addition, a full AC cycle data recovery can be achieved instead of focusing only on recovering individual PD pulses. Other wavelet threshold de-noising methods are discussed and examined under a noisy environment to compare their performance with the new method proposed here. The method described here has been found to be superior to the other wavelet-based methods View full abstract»

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  • A novel wavelet transform technique for on-line partial discharge measurements. 2. On-site noise rejection application

    Page(s): 15 - 22
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    For pt.I see ibid., p.3-14, (2007). Insulation assessment of HV cables requires continuous partial discharge (PD) monitoring to identify the nature of insulation defects and to determine any degradation trends. However to recover PD signals with sufficient sensitivity to determine such insulation degradation in substations with high levels of electromagnetic interference is a major challenge. This paper is the second of two papers addressing this challenge for on-line PD measurements in a noisy environment. The first paper described a wavelet transform-based method of interference rejection. This paper applies that method to the problem of on-site testing, using both laboratory tests and on-site tests. The laboratory tests were used to stimulate the noisy on-site testing environment, with use of transient pulse-like noise, discrete spectral interference (DSI) and white noise. These noise types have been successfully rejected by the method proposed in the first paper. In addition, on-site tests have been undertaken and have been able to detect PD signals in an old 11 kV substation multi-cable installation View full abstract»

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  • Inferring ceramic insulator pollution by an innovative approach resorting to PD detection

    Page(s): 23 - 29
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    This paper presents a technique for outdoor ceramic insulator pollution assessment based on partial discharge detection and analysis. Laboratory tests and data coming from a monitoring activity carried out on some insulators of transmission lines are reported. They support the effectiveness of partial discharge detection and analysis as a tool to carry out condition-based maintenance of insulators of overhead lines. In particular, it can be seen that partial discharge analysis highlights the mechanisms bringing insulators to flashover (dry band arc activity) and is, therefore, well suited to predict increasing pollution and imminent flashover. As a consequence, algorithms are proposed to provide a quantitative evaluation of pollution severity View full abstract»

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  • Study on mathematical model for VHF partial discharge of typical insulated defects in GIS

    Page(s): 30 - 38
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    Very high frequency (VHF) measurement is a valid technique for partial discharge (PD) detection, and PD mathematical models are very important to study PD denoising and waveshape pattern recognition etc. However, these mathematical models based on the traditional PD measurement have been established according to RC or LCR resistance detection so that they are not suitable for VHF measurements. PD signals are detected based on four typical physical models of defects in gas insulated substation (GIS) using VHF method. The VHF PD mathematical models are established, and the principles and methods of establishment are summarized. Furthermore, the parameters of the mathematical models are given and the validity of PD mathematical model is proved by fitting error, power spectrum and time-frequency analysis. Results show that the VHF PD mathematical models can facilitate theoretical simulation of different insulated defects by using the values of given parameters. In the end, applications of the VHF PD mathematical models and complex wavelet transform for extracting the relevant signals from a white noise background are illustrated View full abstract»

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  • Partial discharge inception and propagation characteristics of magnet wire for inverter-fed motor under surge voltage application

    Page(s): 39 - 45
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    In inverter-fed motor coils, surge voltages with the rise time of several tens or hundreds of nano-second may cause partial discharge (PD) and degradation of electrical insulation performance of the inverter-fed motor coils. This paper discusses PD inception characteristics as well as PD propagation characteristics after PD inception for magnet wire of inverter-fed motor under surge voltage application. Experimental results firstly revealed that PD inception voltage (PDIV) decreased with the increase in the length of enamel-coated wire, which was evaluated in terms of the stressed wire contact length under surge voltage application, i.e. size effect. We proposed a regression line for the size effect on PDIV for the electrical insulation design of inverter-fed motor coils. Secondly, PD propagation characteristics were also investigated under the higher voltage application, and their mechanisms were discussed in terms of generation probability of initial electrons, space charge behavior in the wedge-shaped air gap, charging on the enamel surface and so on View full abstract»

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  • Phenomena and mechanism of electric charges on spacers in gas insulated switchgears

    Page(s): 46 - 52
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    The phenomenon of accumulated charges on solid insulator surfaces is one of the critical parameters to consider at the insulation design stage, for AC electric power equipment as well as for DC equipment, so it is important to investigate the characteristics and predominant factors underlying various charging mechanisms. Several researches related to this theme have been reported, but independently, and cross-sectional comparison and evaluation from a unified viewpoint are meaningful. In this paper, the resistance of solid insulator is first discussed, showing that the resistances found by diverse measurements are in a fairly good agreement under similar conditions of the temperature and electric field. Next, three kinds of electric charging mechanisms, i.e. volume conduction, surface conduction and electric field emission are characterized in terms of the time constant, applied voltage and charge distribution. Then, eight cases of recent measurements on the charging time are investigated and their charging mechanisms are classified. Electric field emissions are likely to occur with model spacers made in routine GIS manufacturing process at the electric field level used. Further, three examples in cases with metallic particles are introduced, of simulating charge from edge on the tank, spacer surface charging phenomena, and influence of charge on spacer surface flashover. It is expected that this paper wilt be helpful for understanding charging phenomena e.g. on insulation spacers in gas insulated switchgears View full abstract»

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  • Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

    Page(s): 53 - 64
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    The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps View full abstract»

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  • Temperature effect on space charge dynamics in XLPE insulation

    Page(s): 65 - 76
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    This paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kVdc/mm at four temperatures 25, 50, 70 and 90 degC for 24 h. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The DC threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown View full abstract»

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  • Phase-resolved pulsed electro-acoustic technique to detect space charge in solid dielectrics subjected to AC voltage

    Page(s): 77 - 82
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    The PEA technique has been modified to study the space charge development in solid dielectrics subjected to AC electric field. Narrow (5 ns) electrical pulses are applied at various phase angles of the AC waveform. Special software, developed to precisely synchronize the pulse generator with the high voltage supply, applied the narrow pulses at 0deg phase angle and then in steps of 10deg till 360deg. By processing the PEA data at various phase angles of the AC waveform, without resorting to complex mathematical analysis, the electric field at which charges are injected into the polymeric insulation was determined. The phase resolved PEA technique can also provide the dynamics of space charge development under AC fields View full abstract»

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  • Fundamental insulation characteristic of high-pressure CO2 gas under actual equipment conditions

    Page(s): 83 - 90
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    SF6 gas has been widely used in electrical power equipment such as circuit breakers and transformers due to its superior insulation and interruption characteristics. However since 1997, SF6 gas has been designated a greenhouse gas subject to emission restrictions at COP3 (The 3rd session of the Conference Of the Parties to the United Nations Framework Convention on Climate Change) so a new insulating gas is needed as a substitute for SF6 gas. This research considers the use of high-pressure CO2 gas as an insulator while stressing the environment aspects. Fundamental insulation data for the insulating gas acquired supposing gas insulated switchgears (GIS) consists of; (1) insulation breakdown characteristics under clean conditions and, (2) insulation breakdown characteristics with metallic particle contamination. The parameters in this case were assumed from an actual apparatus viewpoint, to be a high gas pressure up to 2.0 MPa, an electrode size capable of determining the surface area effect, the electrode surface roughness, and metallic particle length, etc. at the base electrode of the 72 kV GIS. As a result, experiments using these parameters revealed insulation characteristics for high-pressure CO2 gas and that negative lightning impulse decided the insulation design, as well as the present SF6 GIS. The need for taking measures to suppress PD under AC voltage and also the need for restricting metallic foreign particles around the central conductor and insulating spacer were recognized View full abstract»

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  • AC particle-triggered corona discharge in low pressure SF/sub 6/ gas

    Page(s): 91 - 100
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    This paper deals with AC particle-triggered corona discharge as a follow-up to our previous research with DC voltage to clarify the particle-triggered corona discharge process in SF6 gas. Corona current pulses, charges associated with a corona current pulse, and corona light pulses were observed with an aluminum ellipsoidal particle suspended in a parallel plane electrode system under a SF6 gas pressure range of 30 kPalesPles50 kPa by changing the particle position. Corona mode, phase (Phi)-charge (q) characteristics as well as corona discharge processes were discussed and the following results were obtained. AC corona mode depended on the instantaneous applied voltage, voltage gradient as well as the particle position and then, the Phi-q characteristics were also affected by those parameters. Charges flow into the floating particle due to coronas on the both tips of particle and excite field fluctuations around the opposite side of particle in addition to the applied AC field. The field fluctuation in SF6 gas by the corona charges was about 4% of the applied field and much lower than that in air gap which was about 70%. That is, the corona development was suppressed effectively by high electron affinity of SF6 gas even in the case of floating particle. The less effective interference between coronas on the both side of particle in SF6 gas results in an obscure local minimum in the breakdown voltage characteristics as the particle is in the vicinity of electrode as contrasted with a drastic fall in the breakdown voltage by the particle in air. View full abstract»

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  • Insulation properties and degradation mechanism of insulating spacers in gas insulated switchgear (GIS) for repeated/long voltage application

    Page(s): 101 - 110
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    Gas insulated switchgear (GIS) has been increasingly being used in recent years and high reliability and rational insulation design are required. This research provided V-N characteristics (the dielectric breakdown voltage versus number of repetitions characteristics) regarding the internal insulation and creeping insulation of the epoxy spacer, which are the main insulation elements of GIS, in order to help set LIWV (lightning impulse withstand voltage) and SIWV (switching impulse withstand voltage). The insulation strength decreased the most when the lighting impulse voltage was applied to internal insulation of the spacer, however, taking the absolute values into consideration, it turned out that the impact of frequent surges is slight. In the experiment in which alternating current voltage is applied for a long period of time, it was found that there is no decline in the insulation properties even after the voltage is applied for the equivalent of 30 years when the electric field intensity is 12 kVrms/mm or less although the combination with the multiple lightning impulse application may bring about damages to the spacer insulation. The degradation mechanism caused by generation of micro-pits was also understood through simultaneous microscopic observation of the surface and of the interface between the electrode and epoxy View full abstract»

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  • Progression of positive corona on cylindrical insulating surfaces. I. Influence of dielectric material

    Page(s): 111 - 118
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    Experiments are described on the propagation of corona discharges over the surfaces of cylindrical PTFE and porcelain insulators under positive impulse voltages. A rod-plane electrode arrangement has been used to which a single value of peak voltage, with the same impulse profile, has been applied throughout. Quantities measured were inception times and voltages, corona current and light output at selected axial regions of the corona. Still photographs of the corona appearance have been taken. Comparison has been made between the surface corona and that in air alone. Interpretation is based on knowledge of the electric field profiles, as modified by the insulators and the possible effects of the surface on the discharge. These are discussed in terms of the presence or absence of charge attachment, the effects of the surface on electron production and the effect of the permittivity of the materials View full abstract»

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  • Numerical modeling of near corona wire electrohydrodynamic flow in a wire-plate electrostatic precipitator

    Page(s): 119 - 124
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    Numerical modelling of the flow velocity fields for the near corona wire electrohydrodynamic (EHD) flow was conducted. Solutions of the steady, two-dimensional momentum equations have been computed for a wire-plate type electrostatic precipitator (ESP). The equations were solved in the conservative finite-difference form on a fine uniform rectilinear grid of sufficient resolution to accurately capture the momentum boundary layers. The numerical procedure for the differential equations was used by SIMPLEST algorithm. The CFD code coupled with Poisson's electric field, ion transport equations and the momentum equation with electric body force, was used for the numerical simulation with the Chen-Kim k-epsiv turbulent model. The numerical results show that EHD secondary flow was clearly visible in the downstream regions of the corona wire despite the low Reynolds number for the electrode (Re cw=12.4). Secondary flow vortices caused by the EHD increases with increasing discharge current or EHD number, hence pressure drop of ESP increases View full abstract»

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  • A comparison of the pollution performance of long rod and disc type HVDC insulators

    Page(s): 125 - 129
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    Long rod insulators made of porcelain and disc type insulators made of porcelain or glass are the main types of HV insulators used since more than 60 years in overhead transmission and distribution lines and also will be the dominating products in the future. Historically based, there exists much published work on the electrical pollution performance of disc (cap and pin) type HV insulators. Only little knowledge exists about the good service pollution performance of the long rod type insulators. This paper compares the test results obtained from artificial pollution tests according to IEC 61245 with both types of HV insulators under same test conditions. The tests were performed with DC voltage at the FGH (Forschungsgemeinschaft fur Elektrische Anlagen), an independent German high voltage test laboratory. It is well known that the sensitivity of insulator strings to pollution effects is much higher at DC than at AC (50 Hz power frequency) voltage. The test results show an equivalent pollution performance of long rod and disc (cap and pin) type insulators under the same pollution conditions and the same electrical stresses View full abstract»

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  • Aging of silicone rubber composite insulators on 400 kV transmission lines

    Page(s): 130 - 136
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    A number of silicone rubber composite insulators have been examined following 15 years of service on a coastal 400 kV transmission line in the UK. Extensive measurements of their hydrophobicity, as determined by contact angle, are given along with a description of their appearance. The hydrophobicity change varied from the low voltage end to the high voltage end with the lowest contact angles being found in the middle of the string. The sheds also aged differently around their circumference and this was reflected in discoloration differences on different sides of the insulator, in addition to hydrophobicity changes. The greatest change to properties was witnessed on the core of the insulators on which contact angles of less than 70 degrees were recorded View full abstract»

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  • Properties of interfaces in silicone rubber

    Page(s): 137 - 145
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    A salt fog test and an inclined plane test have been combined in order to trace changes in electrical and mechanical properties as well as in chemical stability of high temperature cured silicone rubber (HTV) samples containing interfaces. The interfaces were created in two different ways. The first technique, named adding/curing, was based on pressing together an uncured HTV silicone plate towards a piece of already cured one, followed by curing at atmospheric pressure and high temperature. The second type of interface was made by joining two pieces of cured HTV rubber by silicone glue (RTV-1). Thereafter both types of samples were divided in two parts, one of which was additionally exposed to UV-radiation before testing. The combined salt fog inclined plane test did not reveal significant differences in the development of leakage currents, in the change of surface roughness, or in the degree of surface oxidation in comparison to reference samples of cured HTV rubber without interfaces. At the same time the leakage currents became lower on the UV-exposed samples. It is suggested that the reduction in leakage currents resulted from a combination of increased surface roughness and increased hydrophobicity. Reduction of mechanical properties of UV-exposed samples was also observed. The tensile testing showed that samples containing the adding/curing interfaces maintained their mechanical strength after ageing while the strength of the glued samples was significantly reduced, indicating that the adding/curing system has better long-term mechanical stability compared to the glued system View full abstract»

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  • Evaluation of breakdown characteristics of oil-immersed transformers under non-standard lightning impulse waveforms - definition of non-standard lightning impulse waveforms and insulation characteristics for waveforms including pulses

    Page(s): 146 - 155
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    To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of oil-immersed transformers and thus cut the equipment cost while maintaining the high reliability in its insulation performance, it is necessary to grasp in an organized way the insulation characteristics under non-standard lightning impulse voltage waveforms that represent actual surge waveforms encountered in the field and compare them with the characteristics under the standard lightning impulse waveform quantitatively. As described in this paper, the first step in a series of study for the purpose above was taken by analyzing lightning surge waveforms and restriking surge waveforms such as disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations and identifying four typical non-standard lightning impulse waveforms with basic frequencies of 0.24 to 1.0 MHz. Then, two of these non-standard lightning impulse waveforms, the single-pulse waveform which is the most basic type and the waveform with a pulse in the crest and a subsequent flat section, were used to measure the breakdown voltage and the partial discharge inception voltage while changing the parameters, on three models that represent the insulation elements of windings of oil-immersed transformers. Then, the resultant average breakdown voltages were evaluated in terms of the overvoltage durations, leading to a result of formulating them in a unified way. In the tested range, the dielectric breakdown values under non-standard lightning impulse waveforms were higher, marking 52% at the maximum, than those under standard lightning impulse waveforms in all the cases, suggesting a possibility of lowering the insulation specifications of an oil-immersed transformer View full abstract»

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  • Dielectric response of mineral oil impregnated cellulose and the impact of aging

    Page(s): 156 - 169
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    Dielectric response is a non-invasive diagnostic method with a potential to give information about the status of the solid insulation in a power transformer. To investigate what conditions the method can and cannot reveal, frequency domain spectroscopy has been applied to oil-impregnated paper and pressboard with and without moisture and/or acids added, and to paper at various stages of aging. Water and carboxylic acids are important aging indicators. It is found that moisture is clearly detectable. Also low molecular weight carboxylic acids will affect the dielectric response in a way similar to moisture, but high concentrations are needed for a contribution comparable to that of moisture. It is difficult to tell moisture and acids apart from the responses obtained. High molecular weight carboxylic acids make little or no contribution to the dielectric response of the solid insulation in a paper/pressboard - oil system. It was also observed that keeping paper or pressboard at elevated temperatures had a conditioning effect on the response View full abstract»

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  • Correlation between trapping parameters and surface insulation strength of solid dielectric under pulse voltage in vacuum

    Page(s): 170 - 178
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    As well known, the surface conditions of solid materials strongly affect the flashover phenomena under high electric field. The artificial doping ingredients could change the energy distribution and the density of electron and hole traps. The trapping parameters in the surface layer have some influences on the surface insulation strength. In order to adjust different trapping parameters, the composite materials of epoxy resin intermingled with aluminum hydroxide are prepared. The trapping parameters of the samples with different filler concentrations are investigated through the method of measuring the surface potential based on the isothermal current theory. The surface insulation performance in vacuum of each sample is tested according to the flashover voltage under pulsed excitation. Based on the experiment, the correlation between the trapping parameters and surface insulation strength is discussed and some novel models are presented View full abstract»

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  • On-line monitoring of pressure in vacuum interrupters

    Page(s): 179 - 184
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    Methods for on-line monitoring of the pressure in vacuum interrupters were investigated. Some experiments were carried out to investigate the characteristics of an inverted magnetron vacuum gauge in pulsed operation mode. According to the experimental results, a kind of vacuum chamber with intellectualized vacuum sensor was developed to implement the on-line monitoring of pressure in vacuum interrupters. The vacuum sensor can measure pressures ranging from 3times10-4 Pa to 0.5 Pa View full abstract»

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  • Effects of temperature and impurities on the DC conductivity of snow

    Page(s): 185 - 193
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    Several major parameters affecting the electrical behavior of natural snow, namely volume conductivity and density, liquid water content and conductivity of water melted from snow, are crucial for the characterization of the electrical performance of snow-covered HV insulators. However, little study has been devoted to this subject, despite of its importance. These parameters are found to vary significantly with snow composition and purity as well as with other parameters, such as temperature, airborne pollutants, electric field strength and polarity. From laboratory experiments carried out on a large number of snow samples, it was found that DC conductivity of snow shows a peak at about -2degC. This apparently curious behavior near the melting temperature is attributed to important changes to the microstructure of snow. A correlation between DC conductivity and snow temperature was established 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