Showing papers in "IEEE Transactions on Dielectrics and Electrical Insulation in 2004"
TL;DR: In this article, the future of mesoscopic properties of nanocomposite polymers is discussed, and several interesting results to indicate the foreseeable future have been revealed, some of which are described on materials and processing, together with basic concepts and future direction.
Abstract: Polymer nanocomposites are defined as polymers in which small amounts of nanometer size fillers are homogeneously dispersed by only several weight percentages. Addition of just a few weight percent of the nanofillers has profound impact on the physical, chemical, mechanical and electrical properties of polymers. Such change is often favorable for engineering purpose. This nanocomposite technology has emerged from the field of engineering plastics, and potentially expanded its application to structural materials, coatings, and packaging to medical/biomedical products, and electronic and photonic devices. Recently these 'hi-tech' materials with excellent properties have begun to attract research people in the field of dielectrics and electrical insulation. Since new properties are brought about from the interactions of nanofillers with polymer matrices, mesoscopic properties are expected to come out, which would be interesting to both scientists and engineers. Improved characteristics are. expected as dielectrics and electrical insulation. Several interesting results to indicate the foreseeable future have been revealed, some of which are described on materials and processing in the paper together with basic concepts and future direction.
889 citations
TL;DR: In this article, it is argued that the behavior of dielectric particles as they shrink in size through micrometric to nanometric scales will be increasingly dominated by the properties of their interfaces with the environment.
Abstract: It is argued that the behavior of dielectric particles as they shrink in size through micrometric to nanometric scales will be increasingly dominated by the properties of their interfaces with the environment. The various interatomic and intermolecular forces that determine the structure of these interfaces are reviewed with special emphasis on their electrical nature. A number of situations in which passive and dynamic dielectric properties are traceable to nanometric interfacial properties are considered. It is also demonstrated that such interfaces are nanometric electromechanical (NEM) systems which acting collectively also explain piezoelectricity in macroscopic systems. Interfaces are naturally nanometric entities and must have a major role in the future development of nanotechnology. Their ubiquitous employment in living systems is noted and comparison suggests synergistic opportunities.
692 citations
TL;DR: In this article, the authors discuss the current status, development needs and future potential to build or engineer nanostructured materials for dielectric applications in the electrical power industry.
Abstract: While specialty applications of nanotechnology in the photonics and electronics areas have seen a tremendous growth in the past several years, the use of nanodielectrics in the electrical industry (high power density and high voltage) has not shown the same level of activity. In addition to a review of nanodielectrics, we discuss in this paper, our perspective on the current status, development needs and future potential to build or engineer nanostructured materials for dielectric applications in the electrical power industry. Short and long-term future research and development needs are considered from the point of view of industrial applications.
489 citations
TL;DR: In this article, the polarization and depolarization current (PDC) analysis is used for determining the conductivity and moisture content of insulation materials in a transformer, which is a non-destructive dielectric testing method.
Abstract: Moisture and ageing strongly influence the dielectric properties of oil/paper insulation system of power transformer. Moisture measurement in oil sample generally gives inconclusive information since oil/paper moisture equilibrium is temperature dependent and takes a long time to be in equilibrium. Direct moisture measurement of paper sample is not practicable for in-service transformers. The measurement and evaluation of the "dielectric response" and conductivity is one possible way of diagnosing a transformer insulation condition. In a recent research project, polarization and depolarization current measurement has been used for assessing the condition of oil/paper insulation. The polarization and depolarization current (PDC) analysis is a nondestructive dielectric testing method for determining the conductivity and moisture content of insulation materials in a transformer. On the basis of this analysis it is possible to take further actions like oil-refurbishment, drying or replacement of the winding of the transformer. This paper presents a description of the PDC technique with the physical and mathematical background and some results of PDC measurements on several transformers. Analyses and interpretation of the field test data are also presented in this paper.
298 citations
TL;DR: In this article, the relation between partial discharge quantities (e.g., inception voltage, repetition frequency, amplitude) and electrical properties, associated with charge accumulation, which can be directly evaluated through space charge measurements.
Abstract: It has been observed that voltage waveforms generated by power electronic converters may affect significantly the reliability of electric motor insulation. Since partial discharges are considered to be the main cause of the reliability loss, new enamel insulations for magnet wires are being developed in order to withstand better stress amplification. The electrical characterization of these insulating materials is often carried out through aging tests which may provide estimation of life under different stress levels and conditions. However, deeper investigation of aging phenomena due to supply voltage waveforms is needed, especially regarding the relation between aging factors and stress conditions. This paper deals with this topic, showing experimental evidences of relation between partial discharge quantities (e.g., inception voltage, repetition frequency, amplitude) and electrical properties, associated with charge accumulation, which can be directly evaluated through space charge measurements. Characterization of insulating materials and comparison of materials candidate for application in power electronic waveform environment can be carried out resorting to the methodology proposed here. This approach can provide, therefore, a useful feedback to wire manufacturers regarding, e.g., the choice of additive nature and enamel components for magnet wires in power-electronic controlled motors.
247 citations
TL;DR: In this article, partial discharge degradation of polyamide both without nanoscale fillers (nanofillers) and with 2,4 and 5 wt% additions of nanophillers was investigated.
Abstract: Partial discharge (PD) degradation of polyamide both without nanoscale fillers (nanofillers) and with 2,4 and 5 wt% additions of nanofillers was investigated. Such materials were subjected to PDs using the IEC (b) electrodes for evaluation. Comparisons were made as to the surface roughness observed by scanning electron microscopy and atomic force microscopy. It was found that the change in the surface roughness is far smaller in specimens with nanofillers than those without nanofillers, and that the 2 wt% addition is sufficient for improvement of the surface roughness. Furthermore, it was elucidated that the difference of surface roughness of the degraded area due to PDs among the specimens originates from the difference in their crystalline structures. These results indicate that polyamide nanocomposite is more resistance to PDs than polyamide without nanofillers.
229 citations
TL;DR: In this paper, the authors show that electrical properties of nanocomposite insulating materials for DC applications, specifically space charge, conductivity and breakdown voltage, can improve significantly with respect to the basis, unfilled materials.
Abstract: Nanostructured materials are attracting increased interest and applications. Exciting perspectives may be offered by electrical insulation. Polymeric nanofilled materials may find new and/or upgraded applications in the electrical and electronic industry, replacing conventional insulation to provide improved performances in electrical apparatus, as regards, e.g., reliability, environmental compatibility and power rating. This paper shows that electrical properties of nanocomposite insulating materials for DC applications, specifically space charge, conductivity and breakdown voltage, can improve significantly with respect to the basis, unfilled materials. Reference is made to two polymeric materials, i.e. poly(ethylene-covinylacetate) (EVA) and polypropylene (PP), that are widely used as electrical insulation, e.g. for cables and capacitors. The nanofiller consists of an organophilic layered silicate, specifically an extra-pure synthetic fluorohectorite modified by means of interlayer exchange of sodium cations for protonated octadecylamine NH/sub 3//sup +/ (ODA), in a weight concentration of maximum 6%. In both materials the space charge accumulation rate as a function of applied electric field is significantly reduced, while the electrical conductivity is raised. The breakdown voltage can increase for the nanofilled materials.
222 citations
TL;DR: A personal view of the nanodielectric field is given in this paper, with a tentative definition of the concept of nanodieslectrics being offered at the end of the paper.
Abstract: The neologism nanotechnology exists today and is attracting a considerable amount of interest and activity. It is complex in nature with multiple ramifications. It has a vast scope and in fact includes older as well as newer concepts. At this point in time, a greater focus on the field status and impact on new materials, especially dielectrics and their insulating properties, would certainly be desirable. A personal view is thus being presented here, with a tentative definition of the concept of nanodielectrics being offered at the end.
174 citations
TL;DR: In this paper, the influence of the interface between electrode and polymer or polymer and polymer on the space charge dynamics has been studied, and it has been found that the types of interface between polymer and electrode play a significant role in determining the charge distribution in the insulation.
Abstract: This paper reports on an investigation into the space charge formation and decay at different material interfaces. In particular, the influence of the interface between electrode and polymer or polymer and polymer on the space charge dynamics has been studied. Planar samples were subjected to high DC electric stresses for extended periods of time and space charge measurements taken using the pulsed electroacoustic (PEA) technique. It has been found that the types of interface between electrode and polymer play a significant role in determining the charge distribution in the insulation and that the interface between polymer and polymer acts as a potential barrier to electrons while allowing positive charge carriers through easily.
150 citations
TL;DR: In this article, a correlation study showed a strong relationship between the erosion resistance and the thermal conductivities of the tested samples, highlighting the importance of an outdoor insulating material to have high thermal conductivity in order to withstand dry band arcing.
Abstract: Silicone rubber samples having various concentrations and mean particle sizes of either alumina tri-hydrate or silica filters, prepared by room temperature and heat cured under pressure (hot pressed), are tested for erosion resistance in the ASTM D2303 inclined plane tracking and erosion test. Their corresponding thermal conductivities are determined using a transient temperature technique in which an infrared laser is employed as the heat source and a thermal imaging camera as a temperature detection device. Scanning electron microscope observations show greater filler bonding to the silicone matrix in the hot pressed samples than in the room temperature vulcanized samples leading higher thermal conductivity and increased resistance to erosion, for both ATH and silica filled samples. The correlation study shows a strong relationship between the erosion resistance and the thermal conductivity of the tested samples, highlighting the importance of an outdoor insulating material to have high thermal conductivity in order to withstand dry band arcing. The results can be used to provide guidance on filler selection for silicone rubber compounding for outdoor insulation applications.
130 citations
TL;DR: In this paper, the authors present an understanding of the (quasi)-piezo-and -pyroelectricity in cellular polymers and show that the piezoelectric-like response of these materials is intrinsic, with positive d/sub 33/ and negative D/sub 31/ and d/Sub 32/ coefficients.
Abstract: Cellular space-charge electrets have recently emerged as a new class of materials for electromechanical devices, offering chances for a wide range of applications and challenges for materials optimization. However, many fundamental and applied aspects of the physics of these novel materials are not yet explored. Here we summarize our present understanding of the (quasi)-piezo- and -pyroelectricity in such materials. In contrast to the dipole-density piezoelectricity in ferroelectric polymers, the piezoelectric-like response of cellular polymers is intrinsic, with positive d/sub 33/ and negative d/sub 31/ and d/sub 32/ piezoelectric-like coefficients. Similarities with ferroelectric materials are outlined, especially switching of "polarization" and (quasi)-piezoelectricity. First steps towards patterned charging of cellular polymers are reported, an immediate consequence of the possibility for "polarization"-switchingin cellular materials. The results on cellular space-charge electrets suggest that well-known electret devices like microphones may be seen in a new light. Examples include dielectric and electromechanical hysteresis loops obtained with a commercially available electret microphone. In view of the results, cellular polymers may henceforth be called "ferroelectrets" and their material behavior "ferroelectretic". From an applied point of view, the performance of a Fresnel zone plate for focussing ultrasound is shown. Such a device may pave the way for a simple tool in nondestructive materials inspection, and demonstrates the large potential of cellular polymers for applications.
TL;DR: In this article, defects modification to P(VDF-TrFE) via either high energy electron irradiation treatment or copolymerizing VDF-trFE with a small amount of chlorinated monomer to form a random terpolymer was demonstrated.
Abstract: Making use of defects modification to P(VDF-TrFE) via either high energy electron irradiation treatment or copolymerizing VDF-TrFE with a small amount of chlorinated monomer to form a random terpolymer, we demonstrate that high electromechanical responses can be realized in P(VDF-TrFE) based polymers. It will be shown that in the stretched and irradiated 68/32 mol% copolymer, a transverse strain of 4.5% and a transverse electromechanical coupling factor k/sub 31/ of 0.65 can be induced under a field of 85 MV/m. In addition, the irradiated copolymer also exhibits a high elastic energy density, /spl sim/ 1 J/cm/sup 3/. For PVDF based terpolymers such as P(VDF-TrFE-CFE) terpolymer (CFE: chlorofluoroethylene), an electrostrictive strain of more than 7% can be obtained. To elucidate the microstructure changes due to the defects modification in P(VDF-TrFE) based polymers, synchrotron X-ray measurement was carried out on the irradiated copolymers and the results show that, the irradiation converts the polar-phase into a nonpolar phase. In addition, X-ray date show that the polar-phase can be induced, at the expense of the nonpolar phase, by external fields, confirming that the field induced conformation change is responsible for the observed high electromechanical responses. Although the modified PVDF based polymer exhibits the highest room temperature dielectric constant (60 versus below 10), it is still far below those in the inorganic materials. Experimental results show that by using delocalized electrons in conjugated bonds an all-organic composite with a dielectric constant more than 400 can be achieved. As a result, a strain of near 2% with an elastic energy density higher than 0.1 J/cm/sup 3/ can be induced under a low applied field of 13 V//spl mu/m. The strain is proportional to the applied field and the composite has an elastic modulus near 1 GPa.
TL;DR: In this paper, a representation of the decay and potential return curves that separates different components of dielectric response, by using tdv/dt vs log t plots, is proposed.
Abstract: Our work shows, through measurements on polypropylene films, the validity of surface potential measurements as a technique for dielectric properties analysis. We propose a representation of the decay and potential return curves that separates different components of dielectric response, by using tdv/dt vs log t plots. Using this transformation, and varying some experimental conditions we have identified different contributions to the potential decay. From our experimental observations we suggest a qualitative interpretation of the initial seconds of potential decay for negative corona charged samples that uses a model controlled by surface trap emission and charge exchanges between corona discharge and the insulator surface.
TL;DR: Aging and degradation of 345 kV EPDM transmission line insulators removed from service is presented in this article, where the surface structural changes were studied in detail using advanced surface analysis techniques, such as attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscope (XPS).
Abstract: Aging and degradation of 345 kV ethylene propylene diene monomer (EPDM) transmission line insulators removed from service is presented. These suspension type insulators were installed in a New Hampshire coastal area in 1995 and were removed in 2000 after unexplained outages in that structure. The purpose of this paper is to better understand the aging and degradation mechanisms of EPDM insulators in service. The insulators showed severe chalking and discoloration and partial loss of hydrophobicity on the side facing the sun. The surface structural changes were studied in detail using advanced surface analysis techniques, such as attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). For the first time, the significant differences in surface properties between the chalked/discolored (white) and the other surfaces (dark) were studied quantitatively. The Fourier transform infrared (FTIR) absorption spectra showed a significant decomposition of the CH groups of the white surface, elucidating the effect of photo-oxidation on the EPDM polymer. The SEM micrographs showed the cracking of the surfaces. The XPS spectra showed the formation of various polar carboxyl groups and the presence of high surface energy compounds, such as silica, and silicates. This study provided valuable basic information on the changes in the surface properties of EPDM insulators during service in a coastal environment.
TL;DR: In this paper, a simplified time averaged one-dimensional physical model based on Poisson's equation for electric field and transport equation for electrons was developed to predict plasma parameters in a pure nitrogen environment.
Abstract: Numerical modelling of ferroelectric packed bed nonthermal plasma reactor has been conducted to predict plasma parameters in a pure nitrogen environment. Simplified time averaged one-dimensional physical model based on Poisson's equation for electric field and transport equation for electrons was developed. The mean electron energy was obtained by a swarm relationship from calculated electric field profiles and plasma neutral conditions. For chemical model, N/sup +/, N/sub 2//sup +/, N/sub 3//sup +/, N/sub 4//sup +/, N/sup */, N/sub 2//sup */ and electron were considered where N/sup */ and N/sub 2//sup */ are the total excited atoms and molecules, respectively. The results show that all the plasma parameters increase with increasing applied AC voltage and pellet dielectric constant. The numerical results also show that the dominant ion is N/sub 4//sup +/ and the metastable molecule density is much higher than radical and the electron densities at atmospheric gas pressure.
TL;DR: In this article, the results of measurements and analyses of the acoustic emission (AE) pulses generated in setups making the modeling of basic PD forms that can occur in oil insulations possible were presented.
Abstract: The subject matter of this paper refers to the improvement of the acoustic emission (AE) method when used for detection, measurement and location of partial discharges (PDs) in oil insulation systems of power appliances. The detailed subject matter refers to the issues connected with the application of modern methods of digital processing of signals obtained during technical high-power measurements. The paper presents the results of measurements and analyses of the AE pulses generated in setups making the modeling of basic PD forms that can occur in oil insulations possible. The research concentrated mainly on the following types of PDs: point-plane, multipoint-plane, multipoint-plane with a layer of pressboard, surface, generated in gas bubbles and on the indeterminate-potential particles moving in oil. In each case, the acoustic pulses generated by PDs in both positive and negative half times of the voltage supplying the spark gaps under study were analyzed. The AE pulses measured were subject to the wavelet analysis. For each of the PD forms the continuous wavelet transform (CWT) was determined and the corresponding time-frequency distributions were drawn. Moreover, for the AE pulses generated by PDs of the surface and multipoint-plane types, the discrete wavelet transform (DWT) was calculated, the runs of the particular details for six frequency levels and the approximation run were presented. In reference to the DWT analysis for the particular details, the probability density functions (PDF) and the autocorrelation functions (ACF) were determined at the particular levels of decomposition. Also columnar diagrams showing the energy value that is transferred at the particular levels of decomposition were presented. Moreover, the energy density spectrum runs determined through a FFT were presented for comparison. The Morlet wavelet was used to determine the CWT, and the symlet wavelet was used to calculate the DWT. Next, the distributions obtained for the particular PD forms underwent a comparative analysis taking into account the voltage polarization. It was observed that different frequency structures occur for the particular PD forms. Also, a different time of coherent structure duration for the particular PD forms was observed.
TL;DR: In this paper, the problems of HVAC/DC gas insulated systems (GIS) dielectric strength coordination and support spacers design criteria are discussed in order to protect the spacers against flashover and consequently exclude them from consideration when coordinating an insulation of GIS.
Abstract: The problems of HVAC/DC gas insulated systems (GIS) dielectric strength coordination and support spacers design criteria are discussed in this paper. The most important challenge is to protect the spacers against flashover and consequently exclude them from consideration when coordinating an insulation of GIS. Both experimental and numerical studies demonstrate that electric field formation mechanisms and insulation behavior at SF/sub 6//epoxy interface are substantially different for AC and DC voltage excitations. Nevertheless, in actual power systems, HVDC apparatus are frequently subjected to impulse and time-varying voltages, implying capacitive field grading, and vice versa, HVAC apparatus can actually be subjected to harmful pseudo-DC voltages occurring in certain operating modes. Therefore, the above-mentioned problem is considered in order to find generalized criteria which will provide a reliable spacer performance in both HVAC/DC gas insulated systems. To illustrate the specificity of SF/sub 6//epoxy insulation behavior at HVDC stress, static V-T characteristics for post-type spacer models were studied experimentally. Dielectric strength coordination requirements and constraints arising for AC/DC GIS conditions were discussed. Some design and technological alternatives directed to improve the DC spacer reliability were analyzed. Based on these results, generalized design criteria for AC/DC conical and post type spacers were formulated. For conventional range of GIS operating stress and insulation parameters, it has been shown that proposed design rules can be realizable with no additional enlargement of apparatus dimensions. Such an approach provides unification and interchangeability of support spacers in HVAC/DC SF/sub 6/ apparatus, which could be technologically worthwhile and economically beneficial for manufacturers and power utilities.
TL;DR: In this paper, a wavelet transform was used to detect the zero-cross point of the base current at 50 Hz without the need for voltage information, and the source information regarding the distortion level of the current half-wave separated by the zerocross point and a time lag to onset in current attributable to arcs were used in order to obtain the above three components.
Abstract: Leakage current produced on contaminated ceramic and polymeric insulating surfaces was analyzed using a wavelet transform technique providing time-frequency domain information. Salt-fog tests with and without the deposition of non-soluble contamination were conducted in a laboratory, during which the leakage current was separated into sinusoidal, local arc, or their transition components per halfwave of power frequency. The wavelet transform allowed the detection of the zero-cross point of the base current at 50 Hz without the need for voltage information. The source information regarding the distortion level (I/sub 150 Hz//I/sub 50 Hz/) of the current half-wave separated by the zero-cross point and a time lag to onset in current attributable to arcs are used in order to obtain the above three components. The time-integrated values (cumulative charges) of these components were found to correlate with hydrophobicity and contamination level. It was shown that time variations of the cumulative charges and of their component ratios were useful for estimating the conditions of ceramic and polymeric insulating surfaces.
TL;DR: In this paper, the quasistatic and dynamic d/sub 33/ coefficients of polymer electrets consisting of cellular polypropylene (PP) or of porous polytetrafluoroethylene (PTFE) with thicknesses of 50 to 100 /spl mu/m are studied.
Abstract: Piezoelectric properties of polymer electrets consisting of cellular polypropylene (PP) or of porous polytetrafluoroethylene (PTFE) with thicknesses of 50 to 100 /spl mu/m are studied. In addition, bilayer or multilayer structures composed of one of these polymers in solid or voided form plus an additional air layer are investigated. In particular, the quasistatic and the dynamic piezoelectric d/sub 33/ coefficients are determined with electro-mechanical, optical, and acoustic methods. The quasistatic coefficients are of the order of 100 to 350 pC/N for the cellular PP and much lower for the porous PTFE. With increasing frequency up to about 50 kHz the coefficient of cellular PP decreases where it starts to rise toward the resonance at approximately 300 kHz. The bilayer or multilayer structures show considerably higher quasistatic coefficients which reach up to 20,000 pC/N for certain combinations. As for the single-layer systems, a decrease toward higher frequencies is observed. The effect is linear as a function of load pressure at relatively low pressures, ranging up to 100 Pa for some air-gap systems and up to several kPa for the cellular PP.
TL;DR: In this paper, a technique is developed for extraction of the wave propagation properties of power cables from S-parameter measurements, extracting the complex propagation constant and the characteristic impedance, as well as the LCRG telegrapher's equation parameters.
Abstract: A technique is developed for extraction of the wave propagation properties of power cables from S-parameter measurements. The method extracts the complex propagation constant and the characteristic impedance, as well as the LCRG telegrapher's equation parameters. The extraction process is developed after clarifying the effect of the connection between the measurement port and the power cable. It is concluded that treating the connection solely as a characteristic impedance change could lead to considerable errors in the parameter extraction. Furthermore, the method corrects for electrical lengths, which are not accounted for by the standard network analyzer calibration. The extraction is demonstrated for a medium voltage cross linked polyethylene (XLPE) cable over the frequency range 300 kHz to 300 MHz. The results are compared to a time domain short pulse propagation method for cable characterization. Both measurement methods are evaluated against a cable model.
TL;DR: Semicrystalline terpolymers comprising vinylidene fluoride (VDF), trifluoroethylene (TrFE), and 1,1-chlorofluoro methylene (CFE) were prepared via a suspension polymerization process.
Abstract: Semicrystalline terpolymers comprising vinylidene fluoride (VDF), trifluoroethylene (TrFE), and 1,1-chlorofluoroethylene (CFE), terpolymers were prepared via a suspension polymerization process. For two specific compositions, relevant relaxor properties as well as high dielectric permittivity have been observed. More importantly, very large electrostrictive strains ( /spl sim/ 7%) were achieved in these terpolymers.
TL;DR: In this article, an electrostatic probe is set close to the spacer, and it moves along its surface maintaining a small gap, and the accumulated charge is inversely calculated from the measured data by utilizing the relationship that is obtained through the numerical electric field computation.
Abstract: A system for measuring charge distribution on an insulating spacer is developed. An electrostatic probe is set close to the spacer, and it moves along its surface maintaining a small gap. The accumulated charge is inversely calculated from the measured data by utilizing the relationship that is obtained through the numerical electric field computation. Using this system, the surface charge distribution on a truncated cone spacer of 80 mm diameter and 15 mm height is measured. The number of measured points is 3402, and the spatial resolution is 3.9 mm. After the application of DC 10 kV for 19 hours, the spacer surface is charged with a spotted pattern. The charge density reaches 60 pC/mm/sup 2/ at its maximum. In addition, the residual charge distribution of partial discharge from metallic particles on the spacer is observed.
TL;DR: In this paper, the authors compared the performance of two different theoretical models for charge transport in electron-beam irradiated insulators with various assumptions concerning generation, drift, trapping, and recombination of the charge carriers.
Abstract: Charge transport in electron-beam irradiated insulators can be studied with various theoretical models based on different assumptions concerning generation, drift, trapping, and recombination of the charge carriers. In the past, two models have been of particular interest. A first macroscopic approach is based on the concept of a radiation-induced conductivity (RIC) generated by the injected electrons. As opposed to this, the second microscopic scheme utilizes a detailed description of carrier generation and recombination in the insulator. While the macroscopic model requires the information of the RIC, the microscopic approach, resulting in a more complicated set of equations, calls for generation and recombination rates in addition to information about mobility and trapping of the more mobile carriers. Comparisons of numerical results from the two models for open-circuit conditions indicate that charge distributions and locations of charge peak and charge centroid are in fair to good agreement, depending on assumptions made for the main parameters. Further comparisons of the simulation results with experimental data for charge distributions and locations of charge peaks in fluoroethylenepropylene and polyimide show that the macroscopic model, with independently determined values for the RIC, yields good agreement with the experimental data. For the microscopic model, good agreement with experiment can also be achieved with properly chosen values of the partially unknown parameters.
TL;DR: In this paper, the surface charge distribution under impulse voltage is measured using a static capacitance probe, and a probe with very small charge leakage is designed to investigate the influence of charge accumulation on the flashover characteristics, and it is concluded that micro discharges in the gas near the insulator surface such as the corona caused by free and fixed metal particles is usually a prerequisite condition.
Abstract: The surface charge distribution under impulse voltage is measured using a static capacitance probe. A probe with very small charge leakage is designed. The condition of surface charge accumulation under impulse voltage is analyzed, and it is concluded that micro discharges in the gas near the insulator surface such as the corona caused by free and fixed metal particles is usually a prerequisite condition. The dynamic equation describing the relationship between surface charge density and the applied voltage is established, and the process of surface charge accumulation under impulse voltage is analyzed. Theoretical analysis and experimental results show that the decrease of wave front time of the impulse voltage can result in an increase of surface charge accumulation. A GIS spacer is used to investigate the influence of charge accumulation on the flashover characteristics. It is shown that the 50% impulse flashover voltage can be reduced by 23.4%, and the lower limit of the V-t characteristics can be lowered drastically if the polarity of the surface charge is opposite to that of the applied voltage.
TL;DR: In this paper, the authors investigated the electromagnetic response at microwave frequencies, using frequency domain network analysis, of cold-pressed powder compacts made of Ni, /spl gamma/-Fe/sub 2/O/sub 3/, Co, and ZnO nanosized powders.
Abstract: Advances in nanotechnology have led to a variety of new materials with strong potential applications to microwave and millimeter-wave components, e.g. dispersions of nanoscale particles, nanoparticle-filled polymers, self-assembled nanolattices of magnetic particles. More specifically, the properties of nanocomposites can be tailored for operation as insulators, ferro- and ferrimagnetic materials, highly conductive materials as well, for specific applications. In this study, we have investigated the electromagnetic response at microwave frequencies, using frequency domain network analysis, of cold-pressed powder compacts made of Ni, /spl gamma/-Fe/sub 2/O/sub 3/, Co, and ZnO nanosized powders. Effective complex permittivities of composites over the frequency range (100 MHz-10 GHz) as a function of composition were studied. Within the frequency range of measurements the real and imaginary parts of the effective permittivity of nanocomposites exhibit spectra which can be analytically well represented by power laws. The associated power law exponents, which are similar for the real and imaginary parts of the permittivity, are in the range 0.05-0.20 in agreement with data in the published literature. The dependences of the effective permittivity vs. composition are compared to those obtained from the effective medium theory of Bruggeman, which is found not to be adequate for all nanocomposites studied.
TL;DR: In this article, a high frequency measurement technique to characterize the semi-conducting screens in medium voltage cross-linked polyethylene (XLPE) cables has been developed, and the influence of the experimental set-up, sample preparation methods, pressure and temperature are investigated.
Abstract: Knowledge on the dependence of wave propagation characteristics on material properties and cable design is important in establishing diagnostic methods for cable insulation. In this study, a high frequency measurement technique to characterize the semi-conducting screens in medium voltage cross-linked polyethylene (XLPE) cables has been developed. The frequency ranges from 30 kHz to 500 MHz. The influence of the experimental set-up, sample preparation methods, pressure and temperature are investigated. A dielectric function is developed for the semiconducting screens and this is incorporated into a high frequency model for the cable. The propagation characteristics obtained from the high frequency cable model are compared with those obtained from measurements made on the same cables.
TL;DR: In this paper, a new technique is proposed that gives the polarization distribution in the form of a special 8th-degree polynomial, which can be used to analyze both simulated and experimental data.
Abstract: The laser intensity modulation method (LIMM) is widely used for the determination of the spatial distribution of polarization in ceramics, single crystals and polymers. A brief history of the origin and development of LIMM during the past 20 years is presented. The experimental technique and the derivation of the LIMM equation are described. The data analysis requires the numerical solution of a Fredholm integral equation of the 1st kind. This is an illposed problem that has multiple and very different solutions. In this paper, a new technique is proposed that gives the polarization distribution in the form of a special 8th-degree polynomial. Both simulated and experimental data are analyzed with the new technique.
TL;DR: The conditions under which corona first occurs for any set of electrodes are an important design consideration since corona can limit the performance of any given configuration of electrical conductors.
Abstract: The conditions under which corona first occurs for any set of electrodes are an important design consideration since corona can limit the performance of any given configuration of electrical conductors. Here, the 60 Hz ac corona onset voltage for a stranded wire in air is calculated. The criterion involves the characteristics, i.e. composition and pressure, of the gas in which the conductor is immersed and the electric field in the vicinity of the conductor surface. It is found that the calculated corona onset voltage agrees reasonably well with experimental data and that its behavior as a function of overall conductor radius and number of strands is as expected.
TL;DR: In this paper, a model to rank the tested samples using the inclined plane test is developed, which is validated by applying it to both the recorded harmonic power components and temperature, resulting in the correct ranking of damage on the samples tested in the inclined planes test.
Abstract: Silicone rubber samples, filled with 30% alumina trihydrate, subjected to the ASTM inclined plane test, show good correlation between the measured harmonic power components of dry band arcing and the surface temperature recorded with a thermovision camera. The simultaneous study of the low frequency fast Fourier transform and variations in temperature shows that when the temperature rises above 100/spl deg/C there is a well defined increase in the low frequency harmonic components of the measured power; however, this trend is not verified by the fundamental component. The eroded mass of the samples is measured independently by a laser approach and a model to rank the tested samples using the inclined plane test is developed. The model is validated by applying it to both the recorded harmonic power components and temperature, resulting in the correct ranking of damage on the samples tested in the inclined plane test.
TL;DR: In this paper, a model based on field criterion has been developed to represent the flashover phenomenon, which occurs due to surface pollution on high voltage insulators, under ac voltage.
Abstract: In this work, a model based on field criterion has been developed to represent the flashover phenomenon, which occurs due to surface pollution on high voltage insulators, under ac voltage. The values of potential and electric field on an insulator surface have been determined using the finite element method (FEM). The open model of the insulator has been used for calculating the resistance in series with the arc, in addition to the values of the leakage current and the arc gradient. As a new approach, this dynamic model uses Lagrange multipliers for the solution of the pollution flashover problem. Both the impedance and the electric field criterion have been used for the propagation of arc on the surface. A computer program called NFDM (new flashover dynamic model) has been developed to achieve this. The results obtained from the program have been compared with theoretical and experimental results of other researchers.