IEEE Transactions on Dielectrics and Electrical Insulation

Published by Institute of Electrical and Electronics Engineers
Online ISSN: 1070-9878
Publications
Article
This report deals with space charge behavior in PE (polyethylene) under dc fields. Direct observation of time-dependent space charge profiles in 3-mm thick XLPE (crosslinked low-density polyethylene) cable insulation under dc electric fields was performed using the pulsed electroacoustic method. Stable hetero charges were formed when the field was as low as 0.2 MV/cm, and intermittent generation of packet shaped space charges and their propagation through the insulation were observed when the field was as high as 0.7 MV/cm. These phenomena were reproduced in sheet specimens of XLPE and LDPE (low-density polyethylene). It was found that hetero charges resulted from heat treatment of the XLPE specimen containing antioxidant and acetophenone, which is one of the crosslinking by-products, suggesting dissociation of the antioxidant through solvation at high temperature by acetophenone. The packet charges were easily detected when acetophenone was diffused into the LDPE specimen. However, uniformity of acetophenone distribution prevented the packet charge generation. It is suggested on the basis of several experimental results that local ionization of impurities in the insulation through solvation by acetophenone takes place assisted by high field and leads to the packet charge generation. A numerical simulation was carried out based on the above model
 
Article
An investigation of the HV vacuum breakdown between polished, powder coated, and e-beam treated 304L and 316L stainless steel electrodes is described. Tests were performed with 160 ns, 1-cos(ωt), and 260 ns flat-top voltage pulses of up to 500 kV. The high voltage hold-off for the 160 ns pulse was ∼130 kV/mm for 2 mm gaps for 80-mm diameter polished stainless steel electrodes, and 15% lower for 120-mm polished and e-beam treated electrodes. The longer 260 ns pulse gave 15% lower hold-off for 80-mm electrodes. These electrodes showed voltage hold-off that scaled as the square root of the gap between 0.5 and 7 mm. This total voltage effect has been interpreted in the past as due to accelerated particles. We analyze our data in terms of this mechanism and show that only nanoparticles of molecular size could be responsible. We also discuss how ions or background gas could affect the breakdown thresholds but existing models do not predict square root dependence. We test how extremely fine powers affect hold-off and show that contaminated surfaces have relatively constant reduced breakdown E-fields that intersect the clean-electrode voltage-dependent breakdown at critical gaps defined by the type and quantity of contamination. The hold-off was ∼55 and 65 kV/mm with copper powder on the cathode and anode for 2 to 6.5 mm gaps, respectively, and ∼95 and 75 kV/mm for talc powder on the cathode and anode for gaps <3.5 and 6.5 mm. Optical diagnostics show no difference in the light emission from clean and contaminated electrode breakdown arcs.
 
Frequency dependence of the (a) real component of dielectric constant and (b) imaginary component of dielectric constant of CLBO glasses at various temperatures. The temperature dependence of dielectric constant for the as-quenched glasses was also investigated in the 310-450 K temperature range over the frequency range 100 kHz-10 MHz (Figure 4). It is noted from the figure that, at each frequency, the increment in dielectric constant with temperature (310 K450 K) is approximately linear. The temperature dependence of the dielectric constant at different frequencies was rationalized using Havinga's model [12].
Variation of dielectric constant with temperature at various frequencies and solid line is a linear fit.
Article
The frequency and temperature dependence of the dielectric constant and the electrical conductivity of the transparent glasses in the composition 0.5Cs<sub>2</sub>O-0.5Li<sub>2</sub>O-3B<sub>2</sub>O<sub>3</sub> (CLBO) were investigated in the 100 Hz - 10 MHz frequency range. The dielectric constant for the as-quenched glass increased with increasing temperature, exhibiting anomalies in the vicinity of the glass transition and crystallization temperatures. The temperature coefficient of dielectric constant was estimated (35 ± 2 ppm.K<sup>-1</sup>) using Havinga's formula. The dielectric loss at 313 K is 0.005 ± 0.0005 at all the frequencies understudy. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 1.73 ± 0.05 eV, close to that of the activation energy obtained for DC conductivity (1.6 ± 0.06 eV). The frequency dependent electrical conductivity was analyzed using Jonscher's power law. The combination of these dielectric characteristics suggests that these are good candidates for electrical energy storage device applications.
 
Article
This work contains results on the internal structure and dc conduction of 1,4-cispolybutadiene. X-ray and thermally stimulated depolarization (TSD) studies have been a source of information on phase transitions in the investigated material. The conduction measurements have been performed on thin films of thickness varying from 1 to 10 μm. Gold and aluminum have been applied as electrodes. The temperature range changed from 13 to 325 K. The investigated material was 1,4-cis polybutadiene of the Philips Petroleum Co, It contained 96% cis and 4% of the trans form of polybutadiene. It was observed that the internal structure of the polymer had a strong influence on its electrical properties. Electric conduction resulted from non-activated or thermally activated hopping with some contribution of the Poole-Frenkel effect. Electrode contact phenomena, such as thermionic emission and field emission (tunneling), are very likely responsible for charge carrier injection into the investigated material. The obtained activation energies range from 0.002 to N 0.3 eV
 
Article
SF<sub>6</sub> gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative means from the environmental perspective. One of the alternative candidates is CO<sub>2</sub> gas, which has a lower global warming potential. In order to use this CO<sub>2</sub> gas for actual GIS, the insulation specification must be rationalized by lowering the lightning impulse withstand voltage because the dielectric strength of CO<sub>2</sub> gas is lower than that of SF<sub>6</sub> gas. To lower the lightning impulse withstand voltage of GIS while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms (non-standard-LIWs) that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform (standard-LIW) quantitatively. In this paper, with single-frequency oscillations as typical examples of non-standard-LIWs, the insulation characteristics for the CO<sub>2</sub> gas gap were experimentally obtained while changing the frequency and damping rate. Consequently, even if the frequency and damping rate were changed, the dielectric breakdown voltage varied little, consistently remaining higher than that with standard-LIW at a level of 1.08 to 1.17 times. Accordingly, it was found that, for GIS using CO<sub>2</sub> gas, the insulation specification could be rationalized by approximately 10% by converting non-standard-LIWs to equivalent standard-LIWs. In addition, the influence of gas pressure and gap length was examined and it emerged that the results obtained under basic experimental conditions were likely to be applicable to the conditions close to actual GIS conditions.
 
Graph of delay versus percent of self-breakdown voltage. This graph lists both the voltage at which the switch was triggered and the voltage at which the switch actually broke down. Most breakdowns occurred in a narrow range from 85 to 93% of the switch's self-breakdown voltage. 
"Time sequence" of laser-triggered water switch breakdown process assembled from shadowgrams of four different shots. Laser enters through hole in bottom electrode. 
Article
Multiple water switches are used in the self-breaking mode in many large pulsed power systems. We are studying laser-triggering of water switches at voltages of up to 1.6 MV to see whether we can lower the command jitter of water switches. We have previously reported studies of 170 kV water switching with command jitters as low as plusmn2 ns. The 1.6 MV triggering experiments reported here are performed on a water switch in the middle of a 1.8 m long 7.8 Omega coaxial water line that is directly charged by a 65 kJ Marx generator. The 10 to 90% rise-time of the sinusoidal pulse impressed across the water switch is 250 ns. To trigger the switch, we transport a green laser beam (0.4 J, 7 ns pulsewidth) radially inward though the water of the coaxial line to a 'dry box' inside the inner coax line. There, the laser beam is turned 90 degrees and focused through a hole in one electrode to a breakdown arc in the water between the switch electrodes. Best results, of plusmn8.3 ns jitter and 100 ns delay at 60% of the self-break voltage, have been achieved using an axicon lens to focus the beam to a long narrow chain of point breakdowns between the switch electrodes.
 
Article
A method is introduced for testing insulators under icing conditions. The method is based on and developed from those described in the standard of IEC60-1, IEC 60507 and IEEE Task Force on Insulator Icing Test Methods, which give instructions to test insulators under pollution. Then, the paper explores the effects of various factors, including ice thickness, pollution severity on the surface of insulators before ice accretion, atmospheric pressure, and shed profiles, on the flashover performance of short samples of two different types of silicone rubber (SIR) composite long rod insulators intended for ultra high voltage (UHV) ac transmission lines. The experiments were carried out in the multi-function artificial climate chamber in the High Voltage and Insulation Technological Laboratory of Chongqing University, China. The experimental results reveal the regularity of the effects of ice thickness, pollution severity and atmospheric pressure on the average flashover voltage of two types of composite insulators. Those can provide reference for the outdoor insulation design for UHV ac transmission lines.
 
Article
Flashover occurring under repetitive pulsed voltage perhaps may not happen at the first pulse of applied repetitive pulses. Thus, studies of flashover stressing time (FST) are important to comprehend the flashover mechanism under nanosecond-pulsed voltage. In our experiments, the pulsed power source was SPG200, based on semiconductor opening switch (SOS). Solid dielectrics employed were polymethyl methacrylate (PMMA) and polyamide 1010, and liquid dielectric was transformer oil. The dispersion of FST was statistically large at invariable or variable voltage amplitude and frequency of repetitive pulses. Furthermore, the dispersion of FST decreased, and concurrently, the odds of that flashover arising at the first pulse of repetitive pulses increased, with the increase in the voltage amplitude of repetitive pulses whose frequency was invariably retained. The flashover tended to occur at the tail of the pulse waveform, and the mean value of FST was found to gradually reduce. Moreover, the extent of reduction was prominently decreased after about 200 Hz, when the pulse frequency increased, which simultaneously restricted the variation of the voltage amplitude of the applied repetitive pulses. Thus, it was demonstrated that longer FST is the ultimate factor, which induces lower flashover field strength under repetitive nanosecond pulses.
 
Article
The diminishing trend of reliability owing to possible power system failures has become a serious concern for the power system as a whole. It has necessitated the development of advanced protection methodologies employing advanced information technology and substation monitoring system (SMS) constitutes an integral part of such methodologies. The proposed scheme for condition monitoring of insulators, which serves as an augmented feature of SMS, aims at alleviating overall system reliability as well as power quality because cracked insulators cause disruption of power, thereby incurring heavy loss to the power system utilities. The image processing based video surveillance (VS) is incorporated to dispense with the cumbersome and time consuming conventional manual on-site detection using discrete orthogonal S-transform (DOST) in conjunction with some intelligent classification algorithms to ascertain the condition of the insulators.
 
Article
Ferroelectric polyamide 11 films were prepared by melt-quenching, cold-drawing and electrical poling. Their ferroelectricity was studied by means of dielectric-hysteresis measurements. A remnant polarisation of up to 35 mC/m<sup>2</sup> and a coercive field of 75 MV/m were obtained. The piezoelectric d<sub>33</sub> coefficient and the pyroelectric coefficient of the films are reduced by annealing just below the melting region, but remain at about 3 pC/N and 8 μC/(m<sup>2</sup>K), respectively, during further heat treatment. Differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarisation (TSD) were applied for investigating the conformational changes induced by melt-quenching, cold-drawing and annealing. The results indicate that the cold-drawn film mainly consists of a rigid amorphous phase which exhibits considerably lower conductivity, no glass transition and consequently no dielectric α relaxation. Instead, an α<sub>r</sub> relaxation is found, which is related to chain motions in regions of the rigid amorphous phase where the amide-group dipoles are not perfectly ordered. Annealing removes imperfectly ordered structures, but does not affect the ferroelectric polarisation. Therefore, it may be concluded that essentially the α<sub>r</sub> relaxation causes the thermally nonstable part of the piezo- and pyroelectricity in polyamide 11.
 
Article
This paper deals with two-dimensional EHD flow occurring between a blade and a plate electrode a distance H apart. This type of EHD flow, known as electrohydrodynamic plumes, arises when sharp metallic contours submerged in nonconducting liquids support high electrostatic potential resulting in charge injection. The aim of this paper is to analyze, from a numerical point of view, the evolution of velocity profiles, flow pattern and other relevant parameters characterizing plumes flows. Numerical results are compared with those obtained from experimental or theoretical works found in literature and show that the numerical technique used is suitable to study EHD plumes.
 
Article
This paper reports and discusses qualification and type test procedures for random wound motors as specified in IEC Technical Specification 60034-18-41. Since laboratory tests dealing with off-line partial discharges measurements followed by life tests showed that motors qualified as good can fail in very short times, an extensive investigation on the causes of such discrepancy and, in particular, on the differences existing between the stresses applied during off-line tests and in service is described. In addition, the results of partial discharge measurements performed on crossed pairs are presented, highlighting how the differences in the applied voltage waveform can influence the Repetitive Partial Discharge Inception Voltage. Suggestions to improve the off-line test procedure and related equipment are eventually discussed.
 
Article
The author briefly outlines his recollections of the Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), mentioning the changes in direction that have occurred and the changes in personnel during the review period.
 
Article
An account is given of the time when the International Symposium on Electrical Insulation (ISEI) was inaugurated, the Group on Electrical Insulation was elevated to Society status, Dr. van Roggen was appointed as Editor of the Transactions upon the retirement of E. J. McMahon, the Scientific Achievement award was established in honor of Dr. Dakin, and when, much to the consternation of many, discussions began on the transfer of the Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) from it aegis of the NAS/NRC to a more humble abode, namely that of the Electrical Insulation Society (EIS).
 
Block diagram of experimental arrangement with Impedance Analyzer, Hot stage, Central Processor and Computer for dielectric measurements.
shows that the plots of dielectric loss with frequency for smectic A at 379.0 K, smectic C U at 363.0 K 1070-9878r r r r r1r r r r r$17.00 2004 IEEE
and 3 in smectic X phase is below 10 kHz for 1MC1EPOPB and 0.4 kHz for 1BC1EPOPB giving loss  
Article
Dielectric measurements have been carried out using an impedance analyzer for two ferroelectric liquid crystals R-4' (1-methoxycarbonyl-1-ethoxy) phenyl 4-(4-octyloxy phenyl) benzoate (1MC1EPOPB) and R-4' (1-butoxycarbonyl-1-ethoxy) phenyl 4-(4-octyloxy phenyl) benzoate (1BC1EPOPB). The two types have large spontaneous polarization, +1700 μC/m<sup>2</sup> for 1MC1EPOPB and +2400 μC/m<sup>2</sup> for 1BC1EPOPB. The permittivity and dielectric loss have been measured at different temperatures in the range 343.0 K to 383.0 K for 1MC1EPOPB and 318.0 K to 353.0 K for 1BC1EPOPB in the frequency range 2 Hz to 2 MHz. Both of the ferroelectric liquid crystals, 1MC1EPOPB and 1BC1EPOPB show a new phase smectic X along with smectic C* and smectic A phases. The work reported in this paper is new and is very useful in understanding their application in switching devices.
 
Article
A small test ion chamber has been constructed using 2,2,4,4 Tetramethylpentane as the ionizing liquid in direct contact with an array of lead plates used as the high voltage and ground planes. The transient response resulting from the passage of cosmic ray muons through the ion chamber was monitored for a period of 54 weeks to measure possible degradation of the signal due to contamination of the ionizing liquid that may have come from the lead plates. Over the first 17 weeks of the experiment, the loss of pulse height was measured to be less than 11% per year at 95 confidence. During the following 37 weeks, a more sensitive measurement was consistent with zero loss, or a limit of less than 2.9% per year at 95% confidence
 
Article
Microwave cavity spectrometer and time domain reflectometer is used to measure the permittivity and dielectric loss at different temperatures in 2,4-dimethyl substituted pyridine. The observed data of the width of resonance profile and the shift in the resonance frequency have been analyzed using Slater perturbation equations for cavity spectrometer. The dielectric parameters measured from the time domain reflectometer as a function of time at different temperatures have been transformed to frequency domain to obtain the permittivity and dielectric loss. The observed values of the permittivity and dielectric loss at 9.0 GHz are fitted in Slater's perturbation equations to obtain the form factor, which represents interactions. The relaxation time has been evaluated at different temperatures using the ratio of width and twice of frequency shift and thermodynamical parameters have been determined. This experimental study provides fruitful information about the bulk properties of 2,4-dimethyl substituted pyridine.
 
Article
The presence of antioxidant is the key factor in controlling the oxidation of an insulating oil. Any monitoring program set up to keep track of its concentration could result in substantial savings by prolonging the oil service life and slowing down the transformer aging process. This paper describes a method for measuring the amount of the antioxidant 2,6-di-tert-butyl-para-cresol (DBPC) in new or service-aged insulating oils by high-performance liquid chromatography (HPLC) using a Hamilton PRP-1 column (10 μm) and an ultraviolet detector (283 nm). The mobile phase is a mixture of acetonitrile-water (97.5/2.5, v/v) with a flow rate of 1 cm<sup>3</sup>/min. The byproducts from DBPC, oil oxidation and oil components, whose retention time in HPLC is identical to that of DBPC, were eliminated by filtering the oil on a Sep-Pak Classic silica cartridge. The proposed method has a detection limit of 2.0 ppm and a precision better than 1.7%
 
Article
Poly(ethylene naphthalate 2,6 dicarboxylate), thanks to its heat resistance, may be used as insulating material in motors and laminated chip capacitors. In these specific applications the occurrence of partial or corona discharges leads to insulation aging and possible breakdown. An experimental investigation has been carried out to determine the changes in dielectric properties as function of the aging conditions, e.g. the temperature of irradiation. Dielectric loss values have been determined in the 10<sup>2</sup> to 10<sup>6</sup> Hz frequency range from 150 to 200°C. Three relaxation peaks were found α,β*,β, in the order of decreasing temperature
 
Article
SF<sub>6</sub> gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative means from the environmental perspective. The authors are focusing on CO<sub>2</sub> gas, which has a lower global warming potential (GWP), as one of its potential alternatives. In order to use this CO<sub>2</sub> gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (called non-standard lightning impulse waveforms) must be obtained. For this purpose, the preceding study experimentally obtained and evaluated the insulation characteristics for relatively low frequency oscillation waveforms in disconnector switching surges generated in the actual field. In this paper, the insulation characteristics of the CO<sub>2</sub> gas gap for higher frequency oscillation waveforms were experimentally obtained while changing the frequency and damping rate. Consequently, in the high frequency area, if the frequency was raised or the damping rate increased, the dielectric breakdown voltage tended to increase, consistently remaining higher than that for standard lightning impulse waveforms at a level of 1.08 to 1.36 times. Accordingly, it was found that the insulation specification could be rationalized by approximately 10% by converting non-standard lightning impulse waveforms to equivalent standard lightning impulse waveforms. In addition, the influence of gas pressure and gap length was examined and it emerged that the results obtained under basic experimental conditions were likely to be applicable to the conditions close to actual GIS conditions.
 
Article
The paper provides an example of a test programme, which aims to add to the evaluation of composite insulators after they have being supplied. It is to be considered as a tool to prove the product quality versus the submitted reports from international standards. The proposed tests are not selected to discriminate against any particular material or manufacturer but rather to consider well accepted and recently developed test philosophies, in order to safeguard the quality of the supplied insulators, in particular for bulk volume contracts.
 
Article
Polymers are extensively used as insulating material in high voltage devices, such as underground power cables, power capacitors and transformers. During normal operation the polymeric insulation of a power device is not only subjected to electrical stresses, but could also be subjected to other stresses that can cause the degradation and ultimately lead to insulation failure. It has been well established that electroluminescence and charge injection, which gives rise to space charge in the polymeric insulation, occur at ac, dc and impulse field above a certain threshold value. Space charge can cause dissipative energetic processes such as photon and phonon emission, increase the local electric field and reduce the withstand voltage of the insulation. This paper describes the characteristics of electroluminescence in polymeric insulation subjected to ac voltage and shows its relevance to space charge injection in the material. It is shown that insulation subjected to high voltage could emit various types of light but electroluminescence emission which is related to space charge injection can be clearly distinguished from these other types of light emission. The electroluminescence technique is a valuable tool to evaluate the dielectric properties of novel insulating materials, such as nanodielectrics.
 
Article
Various methods of characterizing insulating materials by their ability to take up charge, retain it, and release it, are reviewed critically in search of measurable quantities that could be used to predict material behavior under stress up to failure conditions. Space charge characterization data on different types of materials from polymers to inorganic single crystals and ceramics are surveyed. The charging behavior is found to be influenced by many details such as surface condition and residual stresses. The traditional approach of linking dielectric breakdown to an intrinsic critical field for the material is tested against the newly emerging view that breakdown could be linked to space charge trapping at defect sites and to the attendant energetics of the mechanically strained lattice. The characterization process thus requires more care than was previously thought necessary, but after more research should become more predictive
 
Article
The direct voltage electric strengths of several polymers are measured using a new type of environmental chamber in which a new specimen of the material can be introduced without the necessity of cooling and reheating after every voltage application. The range of temperatures at which investigations are carried out is 23°C-250°C. The plots of the dielectric strength on Weibull paper demonstrates departure from the two parameter Weibull distribution, particularly at low field strengths. In each case the method of determining the more favorable distributions are discussed with the view of obtaining consistent results for the dielectric strength. The lower voltage breakdown cannot be ignored if high reliability is desired and the evaluated dielectric strengths on this basis are also presented. The polymers studied are aromatic polyamide, PTFE, aromatic polyimide and polyamide-polyester-polyamide composites. Thickness effects are presented for selected polymers.
 
Article
The breaking capability of two vacuum interrupters in series and their behaviour with added grading capacitors has already been reported in preliminary publications. In the first part of this paper the dependence of breaking capability of the series arrangement on arcing time, recovery voltage and test current is compared with the performance of a single tube. Within these investigations the influence of the test current on the breaking capability of the double arrangement is shown. The second part of the work deals with the performance of the series arrangement during the re-ignition of one of the tubes. Tests with simultaneous openings of the contacts are presented which demonstrate that even one of the two tubes is able to withstand the total transient recovery voltage (TRV) in case of re-ignition of the other tube. Immediately after arc-extinguishing the previous re-ignited tube is able to recover and to take its part of the total TRV again. As a result the double arrangement is able to interrupt the test current.
 
Article
This paper deals with the breaking capability of a series design consisting of two 24-kV vacuum circuit breakers (VCB) with grading capacitors. The investigations were done under worst-case conditions with regard to the rate of rise of the transient recovery voltage (TRV) exceeding the values given in the relevant IEC standard. In previous publications it was shown that the breaking capability of the series arrangement is higher than doubling the voltage of a single tube. In addition to these results it is shown that in some cases grading capacitors can increase the breaking capacity even more. Tests with simultaneous opening of the contacts of both tubes are presented with the application of different grading capacitors as well as the choice of different arcing times. In the case of reignition of one of the tubes the current circulating between the two tubes is shown. As a result of the tests the breaking capability of the arrangement depending on the arcing time and the value of the grading capacitors can be classified into three zones: area of reignition of the arrangement, scatter area and area of successful arc quenching.
 
Article
This paper details the construction of a sub-nanosecond pulse generator capable of delivering 250 kV into a high impedance load. The pulse width is approximately 600ps with a voltage rise of up to 1 MV/ns. The pulse rise-time can be adjusted by manipulation of a peaking gap, whereas the pulse-width can be changed by adjusting a novel tail-cut switch located close to the load.
 
Article
The fundamental characteristics of motion of a droplet ejected from a nozzle with 26 μm aperture have been carefully investigated. Droplets ejected are periodically charged and deflected by the deflection field. The behavior of charged and uncharged droplets in air is examined by a CCD camera, and from its images velocities and deflection distances are calculated. The results indicate that droplet train is decelerated linearly by the aerodynamic resistance, which is much smaller than the Stokes resistance for a single particle. It was found that there are two types of coalescence: one is due to Coulomb force between a charged and an uncharged droplet and the other is due to purely aerodynamic effect which is caused by the vacancy of the train. Deflected charge droplets showed complicated motion because of Coulomb and aerodynamic effects when they are proceeding in air. A factor to the Stokes resistance is shown to be between 0.5 and 0.6. Thus, the aerodynamic resistance for the train of droplets is much smaller than for the single particle.
 
Article
The newly constructed Qinghai-Tibet Railway connecting Golumd and Lhasa is the highest railway in the world, which is located on the hinterland of Qinghai- Tibet Plateau with the average altitude of above 4000 m. Therefore, the external insulation of electric equipments will be affected by the atmospheric conditions in the high altitude regions. In contrast to many test investigations in the artificial climate chamber, there are few ones at the high altitude sites of 3000 m and above. In the paper, the AC artificial pollution tests of four types of composite insulators are carried out at three high altitude sites of 2820 m, 3575 m and 4484 m along the Qinghai-Tibet Railway. The test results show that the AC pollution flashover voltages in high altitude regions are affected not only by the pollution grades but also by the atmospheric parameters. The pollution flashover voltages will decrease with the increase of air pressure but increase with the decreases of ambient temperature. The exponent characterizing the influence of equivalent salt deposit density is 0.26 to 0.32 and the exponent characterizing the influence of non-soluble deposit density is 0.12 to 0.16, the influence of non-soluble deposit density cannot be neglected. The exponent characterizing the influence of air pressure is 0.507 to 0.587. The exponent characterizing the influence of ambient temperature is about 0.20. Considering the combined influence of the air pressure and ambient temperature, the AC flashover voltage of the polluted composite insulators will decrease by 4.0% to 6.4% for the altitude increased per 1000 m.
 
Article
This paper explores the influence of air pressure, temperature and humidity on the switching impulse (SI) flashover performance of different types of clean and dry insulators at different high altitude sites of 2820 m to 5050 m along the Qinghai-Tibet Railway, the highest elevation railway in the world. Seven types of insulators are used in the flashover tests: one is a composite insulator, two are glass insulators and four are porcelain insulators. At the test sites, the ambient temperature varies between 6.7 degC to 28.5 degC and the absolute humidity is between 4.11 g/m<sup>3</sup> to 9.69 g/m<sup>3</sup>. The test results show that the 50% switching impulse flashover voltage of different types of insulators at high altitude sites is independent of the materials and sheds of insulators but has a multi-factor non-linear relation with the shortest arc distance, the dry air pressure, the absolute humidity and the ambient temperature. The relative flashover voltage decreased with the ambient temperature is dependent on the voltage polarity, with 0.152% to 0.165% /degC for negative SI and 0.099% to 0.108% /degC for positive SI at the ambient temperature of from 6.7 degC to 28.5 degC. The relative flashover voltage increased with the absolute humidity and is dependent on the air pressure. The higher the altitude is, the more the influence is of the absolute humidity on the flashover voltage of insulators.
 
Article
The structure and topography of surface tracking patterns generated on the surface of unfilled and filled samples of polyester resin using the international standard procedure (IEC 587, Inclined-plane Tracking Test) have been studied. The effect of contaminant flow rate, applied voltage and the percentage content of particulate zinc oxide on tracking behavior has been determined. Three alternative mathematical algorithms have been used to establish the fractal dimensions of the tracking patterns as a function of the above three parameters. To model the surface tracking patterns,two methods have been applied. Firstly, a resistive network has been used in which the insulator surface is assumed to consist of imaginary vertically and horizontally placed resistors. This model is capable of producing several types of trees observed in insulating materials. However, the surface tracking patterns are mostly unbranched and it is not possible to produce realistic images with this model. The second method, Brownian motion, is mainly a recursive technique and does not take Laplacian field values into account. The resolution of the images is high, hence the simulated patterns are almost indistinguishable from the real images
 
Article
This paper presents three models for the induction voltage adder (IVA) from a novel perspective. Firstly, it proposes a specific model based on the concept of transformer from analyzing the principle of the IVA injector. In a certain condition, the specific model can be predigested to a lumped element model. When the electrical transit time of the IVA is comparable to, or even longer than the pulse duration, a distribution element model must be applied.
 
Article
High current density (>100 A/cm<sup>2</sup>) electron beam diodes operating beyond 250 kV/cm in a 1 cm gap configuration are described. The principal features of the electron source are the high current density and high field without the onset of vacuum arcs or arc collapse prior to gap closure. The electron beam diode was controlled by a variable pulse-width output Marx generator. Fields to 600 kV/cm were applied for 25 to 30 ns, and to 300 kV/cm during space-charge limited current conduction at pulse lengths of 100 to 120 ns. Evidence of the transition to space-charge limited unipolar flow and transition to bipolar space-charge limited flow was obtained. Traces of the typical behavior and the different transitions are shown. Beam uniformity was measured by using a set of Faraday cups. The Faraday cup setup was then used to demonstrate suppression of electron emission from surfaces coated with a dielectric film
 
Article
A residual plasma at 3000 K and under diatomic equilibrium is left inside an axially blown gas blast SF<sub>6</sub> circuit breaker after the thermal extinction of an arc. The breakdown of such a residual plasma depends on the electron generation and loss mechanisms. Altogether six ionization and chemical reactions relevant to the time scale of breakdown have been taken into account. It has been found that the critical field strength for breakdown is proportional to the pressure and is equal to 2.0 V/(m.Pa). This field strength is in agreement with the experimental results
 
Theoretical (a) and experimental (b) interface charge hysteresis at V G =±3 kV for a 3layer sandwich with blocking layers d F of 12.5 m thickness and different porous layer thickness d P as indicated.
Remanent interface charge rem versus poling voltage for two 3layer sandwiches with blocking layer d F of 12.5 m and different porous layer thickness d P as indicated. The symbols correspond to the experimental data for rem , while the solid lines are model calculations.
Dependence of the piezocoefficient d 33 of the FEP/ePTFE/FEP sandwiches on thickness of the porous layer at different thicknesses of the blocking layers (in m) as indicated by the numbers near the curves, calculated using equations (16) and (4a) and considering the thickness dependence of the breakdown field presented in Figure 5. Young's modulus of the porous layer is assumed as equal to 0.3 MPa.
Article
Electrical and mechanical properties of corona poled three layer FEP/ePTFE/FEP sandwiches were studied theoretically and experimentally. Modeling of the properties has been performed, as well as their experimental verification. The piezoelectric d<sub>33</sub> coefficient has been studied with the objective of its possible optimization. It has been shown that the maximum value of the d<sub>33</sub> piezocoefficient depends on interrelation between solid and porous layers thicknesses, the intrinsic Young modulus of the porous layer and dependence of the breakdown field on the thickness of the porous layer. The best geometry of the sample can be designed considering the obtained results.
 
Article
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.
 
Article
The aim of this study is to determine the effect of Nb<sup>5+</sup> doping on PZT (65/35) based bulk materials in their structure, micro structure and electrical properties. The Nb content was chosen 0-9 mole%. These materials were prepared by conventional mixed oxide method. X ray diffraction studies suggest the compound to be of rhombohedral perovskite phase. Excess addition of Nb result in pyrochlore and fluorite phase develops in PZT (65/35) sample. Detailed studies of dielectric constant as a function of temperature (40degC to 500degC) and frequency (100 Hz to 1 MHz) suggest that the compounds undergo diffuse type of phase transition. Maximum dielectric constant and resistivity were found to be strongly dependent on doping and measuring frequencies. Using complex impedance analysis micro structural parameters such as bulk and grain boundary resistance, bulk charge carrier concentration and relaxation time were determined
 
Article
This paper presents corona ring optimization for 380 kV V-insulator string composed of glass insulator units. The influence of the corona ring design parameters are examined not only with regard to 3D simulations but also laboratory tests. The purpose of the simulations is to find reasonable corona ring design parameters such as corona ring diameter, corona tube diameter and installation height on the point of effective field regulation around the critical region of the string. Indoor laboratory tests including radio interference voltage levels, corona inception voltages and ac flashover voltages are conducted on the centre phase of a full scaled tower both for experimental based corona ring design as well as verifications of the simulations. After both simulations and experimental based investigations, appropriate corona ring design parameters for the string are determined for effective field regulation, minimum interference level, ac flashover performance and economic point of view.
 
Article
A new method of measuring the temperature distribution due to high frequency electromagnetic field (HF-EMF) power absorption is established. The method is applied to the measurement of the spatial distribution of specific absorption rate (SAR) in tissue exposed to HF-EMF. A microencapsulated thermotropic liquid crystal (MTLC) is employed as the temperature probe because of high resolution and high sensitivity. A high-molecular gel phantom with a high transparency and a high viscosity is developed to realize visualization technique with MTLC. The complex permittivity of the phantom is adjusted to that of muscle at 900 MHz and 1.45 GHz. This method is proven to be efficient in visualizing HF-EMF power absorption by exposure experiment
 
Article
In this paper, two non-destructive thermal methods are used in order to determine, with a high degree of accuracy, three-dimensional polarization distributions in thin films (12 mum) of poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE). The techniques are the frequency-domain Focused Laser Intensity Modulation Method (FLIMM) and time-domain Thermal-Pulse Tomography (TPT). Samples were first metalized with grid-shaped electrode and poled. 3D polarization mapping yielded profiles which reproduce the electrode-grid shape. The polarization is not uniform across the sample thickness. Significant polarization values are found only at depths beyond 0.5 mum from the sample surface. Both methods provide similar results, TPT method being faster, whereas the FLIMM technique has a better lateral resolution.
 
Article
The secondary arc extinction is the main concern in single-phase auto reclosure (SPAR) switching studies, since it dictates whether the reclosure will be successful or not. In this context, a function that describes the arc length variation over time (l(t)) is an important information not only for dynamic response but also for the analysis of transient arc processes. Given that purely electrotechnical approach only gives a profile of the curve l(t), the application of techniques based on image analysis has been considered for obtaining such function. In this paper, a new approach based on 3D-reconstruction and the tracking of an electrical arc is proposed for performing a computer-based evaluation of variations in its length. It was applied a three-dimensional active contour named 3D-snake that is geometrically represented by a B-spline, which evolves in 3D space. This new model is much less dependent on the determination of homologous points than the ad hoc approaches found in the literature for recovering 3D geometry of electrical discharges based on image analysis. In addition, the proposal presented is capable of tracking the evolution of the electrical discharge taking into account the time dependence between consecutive pairs of frames in two videos providing the required function l(t).
 
Article
This paper shows how to use the solution of electrostatic field problems by means of the basis set of solutions for the determination of the three dimensional partial capacitances between the electrodes of a given configuration. The Surface Charge Method and the Boundary Element Method are used to carry out the field calculation and determine the charge distributions on the electrode surfaces. The integration of the charge densities for the basis vectors of potentials allows the direct determination of the capacitance matrix of the problem. The difference between integrating surface charge densities and adding up fictitious charges for the determination of capacitances is also shown. Special attention is given to the computation in the presence of electrodes at floating potential.
 
Results of EDX chemical analysis. 
The ratios of polymeric group to hydrophilic groups. 
Raman spectra of samples. 
A comparison of FTIR and Raman ratios of Si-O-Si to SiOH peak height from Tables 3 and 4. 
Article
Surface analysis of 400 kV silicone rubber composite insulators is presented. These insulators have experienced 15 years of service on a coastal 400 kV transmission line. Inspection of the insulators shows low levels of degradation through cracking and oxidation of title surface. This has been quantified by microscopy, energy dispersive X- ray analysis (EDX), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The analysis shows non-uniform ageing over each shed and ageing on the south side more advanced than on the north side. It is believed that non-uniform ageing was due to environmental factors including natural UV radiation and prevailing wind direction, and the resultant pattern of growth of organic species. EDX and FTIR were found to be the most useful and effective tools for analysis of these polymeric insulators.
 
Article
To guarantee the insulation strength of gas insulated substations (GIS), a number of different voltage waveforms, e.g., switching, lightning and AC have to be applied to the GIS after installation. Because of very huge dimensions of GIS for nominal high voltages, it is not possible to carry out these tests in the factory and parts of the whole system have to be delivered and put together to build the complete GIS. As the result, all insulation tests have to be performed on site. Even if different parts of the system are tested in the factory, because some of the problems occur during the transportation and installation, the insulation strength of the whole GIS can be degraded. In this paper, a novel test set-up and the measurement results of a 400 kV GIS have been installed for the Mobarakeh steel industries, Isfahan, Iran, are presented. This system has a length of about 100 m, which corresponds to a total capacitance of about 10 nF per phase. Because of this relatively large capacitance, the power ratings of the test voltage sources have to be very high. For achieving such a high power, a two step cascade voltage transformer (each 800 V/300 kV with a maximum output current of 2 A) fed through an autotransformer enhanced with a number of inductors to compensate the capacitive current and to minimize the input current of the test transformers have been used to apply the necessary 515 kV to perform the AC tests of the whole GIS. The measurements carried out on the system showed that the first two phases passed the test successfully; however the third phase could not withstand the applied voltage because of the pollution near one of the spacers. After replacing the faulty spacer, the insulation strength of the third phase has been recovered.
 
400 kV Insulator Details.
Mean contact angles and standard deviations on the core.
Mean contact angles and standard deviations on the top surface.
Mean contact angles and standard deviations on the outer underside.
Mean contact angles and standard deviations on the inner underside.
Article
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
 
Article
This paper presents the electrical and material characteristics of field-aged silicone rubber composite insulators, which have been deployed for 15 years on a 400 kV transmission line in a coastal region of the UK. There were no indications of reduced performance in service. Observations indicate non-uniform aging of the insulators on the different surfaces of the insulator sheds and core, along the insulator string length and in the different compass orientations. A uniquely large number of contact angle measurements, made on each of the insulators' different surfaces, confirm the visual aspect of nonuniform aging. Electrical investigations of the insulators have been performed in terms of leakage current analysis, ac flashover / withstand and switching-surge impulse flashover. A correlation is seen between the ac leakage current and the hydrophobicity measurements. Energy dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR) analysis of the changes of the materials' surface chemistry is presented. The role of solar radiation and organic growth appears critical to the observed non-uniform aging and must be considered if the longer term operation of the insulator is to be forecast. It is proposed that a reduction of the difference between positive and negative wet-flashover voltages may be a good indicator of early insulator aging.
 
Article
High-energy proton (3 MeV) irradiation with dosages ranging from 43 to 200 Mrad have been carried out to investigate the potential for modifying both the structure and property of vinylidene fluoride-trifluoroethylene 56/44 mol% copolymer. The structural and transitional behavior of the irradiated copolymer was studied by X-ray diffraction and differential scanning calorimetry. The polarization hysteresis, relative permittivity properties and electrostrictive strain response of these copolymers were also measured. It was found that the ferroelectric copolymer could be successfully converted to a relaxor at a low proton dosage of about 75 Mrad at ambient temperature. A slim polarization hysteresis loop and a frequency dispersion of the relative permittivity observed in the irradiated copolymer imply that the high-energy protons break up the coherent polarization domains in the ferroelectric copolymer into nanosized regions. In addition, the irradiation leads to a significant change in the ferroelectric-to-paraelectric phase transition behavior. X-ray diffraction measurements show that the crystalline region in the copolymer is converted into a nonpolar phase upon irradiation, and the lattice spacing increases significantly. The electric field induced phase transformation of the nanosized regions between the nonpolar and polar phase leads to a high electrostrictive strain observed in the irradiated copolymer.
 
Top-cited authors
Gian Carlo Montanari
  • University of Bologna
George Chen
  • University of Southampton
Boxue Du
  • Tianjin University
Stanislaw M. Gubanski
  • Chalmers University of Technology
Yoshimichi Ohki
  • Waseda University