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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 3 • Date June 2007

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Displaying Results 1 - 25 of 38
  • Table of contents - June 2007, Vol 14 No 3

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

    Page(s): c2
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  • Editorial: Vacuum insulation, fundamentals and applications

    Page(s): 531
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  • Effect of contact surface on non-sustained disruptive discharge phenomena after current interruption in vacuum

    Page(s): 532 - 537
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    Non-sustained disruptive discharge (NSDD) sometimes occurs when the voltage is applied for a certain time between vacuum gaps after the current is interrupted. However the mechanism of NSDD has not yet been clarified. We conducted a study to clarify this phenomenon using the contact materials CuCr, CuW, and AgWC. We applied a voltage (20[kV]-30[kV]) of AC 50(Hz) after current interruption (1 (kArms|-8 |kArms]), and checked the generating frequency of NSDD. After the current interruption test, we measured the contact surface roughness and field emission current. The result showed that surface roughness and field emission current are not related to this phenomenon and also that CuCr (melt) showed the best performance in all tests. View full abstract»

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  • Effect of electrode surface roughness on breakdown conditioning under non-uniform electric field in vacuum

    Page(s): 538 - 543
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    Breakdown (BD) characteristics in vacuum are strongly dependent on the electrode surface conditions, such as surface roughness. However, there is little known concerning the details of the relationship between the surface roughness and BD conditioning effect. In practical application, it is important to clarify how the surface roughness affects the breakdown conditioning characteristics, especially for the non-uniform field configuration. This paper discusses the effect of surface roughness on breakdown conditioning characteristics under non-uniform electric field in vacuum under applications of negative standard lightning impulse voltage. For this purpose, we examined the BD conditioning of a rod-to-plane electrode made of SUS304 and Cu-Cr. The surface roughness of Ra is controlled from 0.3 to 2.5 mum. Experimental results revealed that the enhancement of surface roughness of electrodes increases the number of BD to complete the conditioning effect. We explained the results from the observed results of the electrode surface. Consequently, we could clarify the effect of surface roughness on the conditioning effect under non-uniform electric field in vacuum quantitatively. View full abstract»

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  • On the variety of triggering mechanisms of high-voltage breakdown in vacuum

    Page(s): 544 - 552
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    The paper is devoted to study of the mechanisms responsible for vacuum breakdown as a whole and the total voltage effect in particular. Experiments at DC and pulsed voltages were carried out. It has been shown that there is no manifestation of the total voltage effect at DC voltages up to 20 kV. The strong dependence of hold-off on anode temperature was recognized at DC voltages while pulsed hold-off turned out to be almost the same with heating the electrodes. This gives a basis to consider gas desorption as an insufficient factor in the initiation of pulsed breakdown. An attempt to enhance hold-off with electrostatic removing of loosely bound particles with assistance of electron flow from a thermionic cathode was undertaken in the work. The approach turned out to be ineffective. View full abstract»

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  • Partial vacuum breakdown characteristics of helium at 20 kHz for inhomogeneous field gap

    Page(s): 553 - 559
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    In general, power devices and systems operating in vacuum or space environment are more susceptible to partial discharges, corona, or volume discharges due to the partial vacuum conditions. Additionally, high frequency operation of a power system is a contributing factor in lowering the breakdown voltage of insulation. In this paper we present our studies on the breakdown characteristics of helium operating in DC and 20 kHz AC field in partial vacuum, for a point-to-point and point-to-plane electrode configurations. Breakdown voltage as a function of pressure in the range of 27 to 400 Pa (0.2 to 3 torr) for both the DC and 20 kHz AC cases is presented. Voltage and current waveforms along with the optical emission waveform of the breakdown events are also presented. A variable DC power supply for DC and an in-house built variable DC-offset-AC power supply for the high frequency breakdown experiments are used. A high voltage probe and a Pearson current sensor are used for the voltage and current detection, and a photo-multiplier-tube with a digital pico-ammeter and a video camera are used for the optical signal detection of this set-up. The breakdown voltage as a function of pressure for both the AC and DC experiments, along with voltage breakdown waveforms for both electrodes are presented. View full abstract»

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  • Gas discharge caused by triboelectricity around a contact during friction between insulators

    Page(s): 560 - 565
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    Gas breakdown caused by triboelectricity during friction between two insulators was observed in measuring two-dimensional spatial distributions of luminescence from gas discharge plasma. The insulators were chosen among diamond, quartz, sapphire, MgO single crystal, and quartz glass. The discharge between identical materials was also observed. The patterns of the gas discharge luminescence had a shape of a ring for all frictions between insulators. By using a gold-coated diamond pin for friction with a quartz disk, the discharge pattern differed from the ring pattern. In the case of insulator pin, electric field induced by a localized charge at the off-track area near the tip of the pin during friction accounts for formation of the ring shape of the gas discharge. The gold coating makes it possible to monitor an amount of charge transferred from the quartz surface during friction. The negative charge density at the frictional track on the quartz disk is calculated to be -2.2times10-4 C/m2, which leads the gas discharge in a micro-gap near the contact between the diamond pin and the quartz disk. View full abstract»

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  • Field enhancement at a triple junction in arrangements consisting of three media

    Page(s): 566 - 571
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    The field singularity occurs at an edge point where wedge-like interfaces of dielectrics meet. This phenomenon is well known as "Takagi-effect" or "triple-junction effect" in the cases of two dielectrics. Its significance is twofold; (1) the negative phenomena of possible discharge or breakdown inception, and (2) the positive application of the field enhancement to field emission or electron beam generation in vacuum. This paper describes the analytical expression which gives the exponent of the field singularity in three dielectric media, and applications of this expression in various simple conditions. The analytical expression is also compared with the value given from the numerical field calculation. View full abstract»

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  • Correlation between voltage holding capability and light emission in a 500 keV electrostatic accelerator utilized for fusion application

    Page(s): 572 - 576
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    Voltage holding capability in a 500 keV electrostatic accelerator with large FRP insulators was examined without the beam acceleration. When high voltage was applied, the light with a broad peak at a wavelength of 420 nm was mostly emitted inside the accelerator even though breakdown did not occur. The voltage holding capability has a strong correlation with the light intensity. Stable voltage holding was realized under the conditions in which light emission was well suppressed. View full abstract»

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  • Breakdown conditioning characteristics of long gap electrodes in a vacuum

    Page(s): 577 - 582
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    An improvement in dielectric strength is required in vacuum circuit breakers (VCBs) intended for use in higher voltage systems. In order to develop higher voltage VCBs, it is important to improve the dielectric strength in a vacuum based on consideration of the vacuum breakdown mechanism. Particularly for gaps longer than 10 mm, little is known about the breakdown mechanisms and their quantitative analyses in a vacuum. This paper discusses the breakdown conditioning characteristics of long gap electrodes, under a non-uniform electric field in a vacuum. We treat gap lengths of up to 50 mm in this paper. The conditioning characteristics are investigated under impulse voltage applications. A negative standard lightning impulse voltage was applied to rod-plane electrodes made of Cu-Cr and SUS304 for different tip radii and gap distances until the conditioning effect was completed. We observed illumination spots on electrodes at each breakdown during the conditioning process and calculated the corresponding breakdown field strengths. Experimental results revealed that the tendency of breakdowns associated with long gaps is different from that for the short gaps. As a result, we clarified that the breakdown field strengths are nearly constant at 110-120 kV/mm at the distances longer than 10 mm, and the breakdown field strength is at its maximum when the gap distance is about 5 mm. View full abstract»

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  • Secondary electron emission from alumina RF windows

    Page(s): 583 - 592
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    The development of alumina RF windows for use at the output section of high-power RF sources is an important issue for accelerators. The breakdown of RF windows is caused by 1. the multipactor effect (electron multiplication on the surface) and/or 2. the discharge of accumulated charges (due to the multipactor effect). Measurements of secondary electron emission (SEE) and surface charging were carried out to determine the durability of various alumina ceramics in this application. Since excessive surface heating takes place during surface discharge, SEE yields at high temperature were also measured. The results indicate that SEE yields become lower at high temperature, and that higher purity alumina ceramics show higher SEE yields. High-power tests using a resonant ring were conducted in order to observe surface charging and electron accumulation. Higher purity alumina ceramics showed superior performance in the high-power tests, probably due to lower surface charging. View full abstract»

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  • Determination of gas pressure in vacuum interrupter based on partial discharge

    Page(s): 593 - 599
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    Partial discharge characteristics in air under a low vacuum region were studied to develop a diagnosis technique to determine the gas pressure in a vacuum interrupter. The pressures were set at from 1.3 Pa to 2.6 kPa in order to simulate the leakage of gas into the vacuum interrupter. The structure of the vacuum interrupter, the measurement and the circuit construction for the experimental setup were described. The measurement of partial discharge occurring inside the vacuum interrupter was performed with a current transformer, intensified charge coupled device (1CCD) camera and photomultiplier tube. The measurement of partial discharge light intensity with a photomultiplier tube was more sensitive compared with that measured with a current transformer. From this result, an attempt has been made to distinguish the pressure below and above 260 Pa on the basis of the rise time and peak intensity of discharge light pulses. A relatively longer rise time (2 mus) with a smaller magnitude (less than 0.5 mA) was attributed to a Townsend-like discharge at pressures below 260 Pa, while a sharper rise time ~(10-100 ns) with a larger magnitude (greater than 1 mA) was characterized as a streamer-like discharge above 260 Pa. In addition, the estimation of gas pressure in a vacuum interrupter was made based on phase-resolved discharge characteristics utilizing an artificial neural network. View full abstract»

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  • Dielectric strength and statistical property of single and triple-break vacuum interrupters in series

    Page(s): 600 - 605
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    The maximum possible improvement factor of vacuum circuit breaker (VCB) with multiple breaks compared with one-break, which had the same total gap length, was deduced. Based on the research on the breakdown weak points in high voltage vacuum gaps, a theoretic model was set up for describing the statistical property of multi-break VCB. Dielectric experiments were carried out on a commercial vacuum interrupter arrangements with single break or triple breaks in series under lightning impulse voltage. It shows that the more numbers in series are, the higher the breakdown voltage improvement factor is. The statistical results show that the triple-break vacuum interrupters in series have lower breakdown probability compared with a single-break one. It can be demonstrated in a preliminary study that the experimental results confirm the previous theoretical studies. View full abstract»

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  • Depression of insulator charging in vacuum by partial mechanical processing

    Page(s): 606 - 612
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    It has been clarified by researchers that insulators having contact angles, e.g. plusmn45 deg with regard to the plane electrodes, is hard to acquire charge when they are exposed to high voltages in vacuum. For example, an insulator in the shape of a truncated frustum is free from charging. However, since such insulator has inevitably large volume, it obstructs the compactness. On the other hand, roughening insulator surface is also effective to prevent the surface from charging. In this case, the cost for roughening the entire surface may be high when the insulator is large. In this paper we review our previous investigation results and describe new experimental results aimed at developing feasible and compact insulators used in vacuum. Surface charging characteristics of partially and mechanically processed insulators have been studied for this purpose. That is, the charging phenomena when such insulators are exposed to a ramped DC or AC voltage have been observed by using a capacitive probe embedded in the cathode. Firstly, we describe of a partially roughened cylindrical insulator, where the height of the roughened surface measured from the cathode is varied keeping the total length constant. Secondly, we describe a cylindrical insulator having a truncated conical frustum or a chamfer at its cathode-side end. Based on these experimental and simulation results we prove that the partial mechanical processing is effective to control and suppress the insulator charging in vacuum. View full abstract»

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  • Compact high-voltage vacuum circuit breaker, a feasibility study

    Page(s): 613 - 619
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    This study focuses on a compact vacuum circuit breaker (CB) for nominal voltage of 72.5 kV, nominal current of 2000 A and short circuit current of 31.5 kA. The dielectric conception of the vacuum interrupter (VI) is based on a multiple floating shield configuration which reduces the risk of total breakdown due to partial breakdown between the contacts and shield. An optimised electrical field distribution on the contact surface with the maximum field pointing towards the floating shield is arranged and hence reducing the breakdown effective area of the contacts facing each. All tests have been performed on a specially designed single-phase circuit breaker pole-unit equipped with a low energy spring type mechanism. Interruption performance was confirmed in direct tests. The circuit-breaker pole was proved to be restrike-free with respect to cable charging class C2. Dielectric tests in new condition and after interruption tests confirmed the announced ratings. These tests confirm the feasibility of this HV-CB with 25% more compact VTs based on the outlined dielectric conception. View full abstract»

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  • Effect of mechanical finishes on secondary electron emission of alumina ceramics

    Page(s): 620 - 626
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    The effect of surface roughness of insulator on the secondary electron emission (SEE) coefficients was investigated with changing the incident angle of primary electrons. The SEE coefficients were measured using a scanning electron microscope with a single-pulse electron beam (100 pA, 1 ms). As a result, the SEE coefficients increased with the incident angle for smooth surface, while those of rough surface almost did not change with the incident angle. The SEE coefficients of commercial alumina ceramics with three different surface finishes, i.e. as-sintered, as-ground and mirror-finished samples, were also measured before and after annealing treatments. This treatment was carried out in air at 1400 degC for 1 h. After annealing, the SEE coefficients were higher than those of unannealed samples. This increase is proposed to be due to the defect recoveries as well as neutralizations of charging, which significantly influence the SEE. For relative low purity (95% or less), the SEE coefficients of annealed alumina became lower after mechanical grinding operations and lower again after mirror-finished operations. The SEE coefficients of 99.7% purity alumina were almost unaffected by the mechanical finishes. View full abstract»

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  • Surface charges on alumina in vacuum with varying surface roughness and electric field distribution

    Page(s): 627 - 633
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    We investigated the charging characteristics of alumina in vacuum with varying surface roughness, and the electric field distribution. The charging on the alumina surface in vacuum is, in general, strongly influenced by field electron emission (FEE) and secondary electron emission avalanche (SEEA). We varied the surface roughness and the electric field distribution on the alumina surface in order to control the FEE and the SEEA. Under these conditions, we measured the 2-dimensional distribution of surface charging potential on the alumina surface. From the measurement results, we quantitatively discussed charge polarity and charge density on the surface for various types of electric field distributions. Finally, we successfully propose a concept of a charging control technique on the alumina insulator in vacuum for the improvement of electrical insulation performance on the alumina surface. View full abstract»

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  • Experimental investigation of surface flashover in vacuum using nanosecond pulses

    Page(s): 634 - 642
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    Based on a Marx generator and a coaxial pulse forming line, an experimental investigation of surface flashover characteristics in vacuum is conducted by using nanosecond pulses of 10 ns rise time and 30 ns full width at half maximum (FWHM). Insulator dielectrics chosen for this investigation are Teflon, PMMA and Nylon. The tested factors include gas pressure, cone angle of conical frustum, diameter and length of cylindrical insulator, material and shape of electrode, and contact style between insulator and electrodes. The effects of these parameters on the surface flashover characteristics are described and analyzed in this paper. In addition, the character of flashover time lag in the nanosecond range, and surface flashover theory in vacuum charged by nanosecond pulses are also discussed. View full abstract»

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  • Contact behavior in vacuum under capacitive switching duty

    Page(s): 643 - 648
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    According to the relevant IEC standards vacuum circuit-breakers have to meet various needs, e.g. the interruption capability, making operations, and dielectric strength. Besides the interruption of short-circuit currents, switching of capacitive currents causes high stress of the circuit-breaker. Switching of capacitor banks, overhead lines, or cables leads to very small currents in comparison with short circuit currents. After current interruption the circuit-breaker must withstand twice the peak value of the system voltage. Furthermore, restrikes can lead to voltage multiplication. This conjunction of relatively small breaking currents with high voltage stress must be considered in detail. This work introduces a test arrangement for combined tests of making operation, current interruption, and dielectric stress of a vacuum gap under capacitive switching condition. A test vessel permits investigations of various contact materials and designs. It is connected to a synthetic test circuit which provides the appropriate test currents and capacitive voltage. During the test sequence the contacts are stressed by inrush-currents up to 4.5 kA peak, followed by a breaking operation at 500 A peak and a subsequent capacitive voltage up to 50 kV peak. Both the appearance of pre-ignitions at contact closing and the behavior under capacitive voltage stress after breaking are indications of the contact surface conditions. View full abstract»

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  • A utility optimization method of a high voltage vacuum interrupter design

    Page(s): 649 - 652
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    In this paper, a new method of electric field design of high voltage vacuum interrupters is proposed. According to the multi-gap insulation effect and the vulnerable breakdown spots theory, the symmetrical voltage shields are proved to be effective. So the initial model of a high voltage vacuum interrupter is designed. Next with finite element methods and optimization theory, a mathematical model of electric field intensity in interrupters is given. What's more, by optimizing this model with finite element software, a more symmetrical electric field in vacuum interrupters is obtained and its peak value appear as in the second auxiliary gaps (the two gaps between the center floating shield and the symmetrical voltage shields). The result shows that with this method, the vacuum interrupters can withstand higher static breakdown. View full abstract»

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  • Effect of endothermic reaction associated with glass transition on the breakdown strength of biodegradable polymer films

    Page(s): 653 - 655
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    Effects of endothermic reaction associated with glass transition on the impulse, DC, and AC breakdown strengths of poly-L-lactic acid (PLLA) and polyethylene terephthalate succinate (PETS) were studied experimentally. Impulse breakdown strength at first decreases monotonically with an increase in temperature for both PLLA and PETS. However, it increases and shows a hump when the temperature approaches a certain temperature that is slightly lower than the glass transition temperature. Then it decreases again. Moreover, it is shown that the degree of increment in impulse breakdown strength at this hump depends upon the crystallinity in PLLA. In the case of DC and AC breakdown, a similar local maximum as seen in impulse breakdown is not observed. Two possibilities, namely energy absorption and structural change both due to glass transition are discussed to explain the experimental results. View full abstract»

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  • Time evolution of current through insulating paper under DC voltage stress

    Page(s): 656 - 662
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    The use of paper in electrical insulation, especially in power transformers, remains a topical subject. The interest for the insulating paper, despite the emergence of many synthetic materials is ascribed to its low cost price and its ability to an oil impregnation. However its strong hydrophilic character remains the main difficulty for its implementation in electrical insulation. This affinity to water absorption and/or adsorption shows that a good understanding of the moisture effect becomes imperative. Several studies were carried out on the subject but a systematic study of the moisture effect on overall behavior of paper has not been achieved. This work is meant to be a contribution for this systematic investigation. The first step of this study, which is the object of the present article, concerns an examination of time variation of the electrical currents, under DC stress, for different moisture levels. A particular attention is given to the polarization and depolarization currents. These currents are examined during alternate charges and discharges sequences. An increase of the current, during the application of continuous voltage was noticed. This unexpected phenomenon would be related to an electro-dissociation and/or polarization processes. View full abstract»

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  • Biological effects of narrow band pulsed electric fields

    Page(s): 663 - 668
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    This paper describes the process of narrow band pulsed electric fields (NPEFs) and its effect on mammalian cells. The NPEF consists of a pulse modulated sinusoidal wave (PMSW), which allows delivery of well-defined electric fields in terms of frequency, field strength and deposition energy to the biological systems. 100 mus long sinusoidal electric fields with a frequency of 0.02, 2 or 50 MHz and field strengths of up to 2 kV/cm are applied to CHO cells with variation in the DNA density in the cells investigated by means of Acridine Orange assay. The experiments indicate that 50 MHz fields cause DNA degradation without cell membrane defects, while 0.02 MHz fields lead to an increase in membrane permeability which is similar to the effect known as electroporation. The intermediate frequency of 2 MHz influences both the membrane and DNA. It is demonstrated that the MHz range narrowband electric fields with the amplitude level of 1 kV/cm cause intracellular effects in mammalian cells. View full abstract»

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  • SVM classification of contaminating particles in liquid dielectrics using higher order statistics of electrical and acoustic PD measurements

    Page(s): 669 - 678
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    Electrical and acoustic partial discharge (PD) measurement and pattern recognition procedures are described for detecting and identifying contaminating particles in transformer mineral oils. This work introduces the use of support vector machines (SVM), a nonlinear non-parametric automatable machine learning algorithm, for the purpose of classifying the size and composition of such particles. The training and validation of acoustic and electrical PD measurement data, which are contaminated by time varying noise, are first filtered adaptively using wavelet decomposition. Statistics of a particle's impact upon collision with the walls of a tank, containing the electrode test assembly and the inter arrival time between collisions constitute the features for the SVM classifier. These statistics include higher order moments and the entropy of the estimated density function of the features. Results based on experimental training and testing data indicate that fusing of the acoustic and electric PD information at the features level provides a nearly perfect classification success rate. These observations demonstrate that, while electrical and acoustic PD data are correlated, they contain individually independent and complementary information regarding the state and condition of transformer type mineral oils. View full abstract»

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Aims & Scope

IEEE Transactions on Dielectrics and Electrical Insulation contains topics concerned with dielectric phenomena and measurements with development and characterization of gaseous, vacuum, liquid and solid electrical insulating materials and systems.

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Editor-in-Chief
Reuben Hackam