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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 4 • Date Aug. 2002

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Displaying Results 1 - 17 of 17
  • Pattern classification of impulse faults in transformers by wavelet analysis

    Page(s): 555 - 561
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (409 KB) |  | HTML iconHTML  

    One of the tests carried out on a transformer after assembly is the lightning impulse test, for assessment of the integrity of its winding insulation. In the case of a fault, it has been well established that the pattern of the fault currents contains a typical signature of the nature and location of the insulation failure involved. This paper describes a new approach using the 'wavelet transforms' to classify the patterns inherent in different fault currents. Whereas conventional frequency-response analysis based techniques fail to identify the time-localization of a particular frequency component in a time-dependent signal, the wavelets are not only localized in frequency, but also in time. The 'time-frequency localization' feature of wavelet transform is employed for pattern classification of impulse fault currents of transformers. Results for simulated models of 3 5 and 7 MVA transformers are presented to illustrate the ability of this approach to classify insulation failures. View full abstract»

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  • Comparison of on-line partial discharge detection methods for HV cable joints

    Page(s): 604 - 615
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (772 KB) |  | HTML iconHTML  

    The capacitive coupler, acoustic emission (AE) sensor and radio frequency current transducer (RFCT) have been used to detect partial discharge (PD) activity within a 132 kV prefabricated cable joint containing a known defect. Although each of the three methods has been applied individually under different situations, a comprehensive investigation and comparison of these three methods has not yet been made. Results obtained were compared to measurements made using the conventional electrical detection method. The quantification of the capacitive coupler measured signal in mV with a discharge apparent quantity in pC has been investigated and an on-line quantification method proposed. The capacitive coupler has good detection sensitivity and PD location can be realised by studying the time of Right between signals from two sensors. The difference in pulse shape, spectrum and time of flight between an internal discharge and external interference has been investigated. The AE approach has the advantage of being free from electrical interference. However, investigation indicated that AE signals were significantly attenuated within the cable joint. RFCTs were used to detect the discharge current flowing through the cable sheath. Where the detection sensitivity was low, a wavelet de-noising method was applied to process the RFCT signals and proved to tie effective in increasing the measurement signal to noise ratio. View full abstract»

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  • Measurements of equivalent salt deposit density (ESDD) on a suspension insulator

    Page(s): 562 - 568
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (431 KB) |  | HTML iconHTML  

    Equivalent salt deposit density (ESDD) is widely used to describe the state of contaminated insulators. In this paper, a new method of ESDD on-site measurement is studied. Two types of sensors are designed, which can not only carry enough water for the contamination on the insulator surface to dissolve and cover the insulator surface with conducting solution, but also measure the conductance of the solution. The design of the sensors and the determination of the desired parameter are introduced in detail. Experiments prove that the conductance of the contamination solution is linear with the square root of ESDD of the insulator. This relationship can also be deduced by Debye-Huckel-Onsager theory. According to this relationship, the conductance measured by the sensor can be converted to ESDD. Therefore, Measuring ESDD on-site can be realized. View full abstract»

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  • Maximum likelihood parameter estimation in the extended Weibull distribution and its applications to breakdown voltage estimation

    Page(s): 524 - 536
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (632 KB) |  | HTML iconHTML  

    Although the Weibull distribution is widely used in a variety of reliability applications, difficulties in its treatment, particularly in three parameter cases in the maximum likelihood estimation, hinder us from using the distribution. The extended Weibull distribution proposed by Marshall and Olkin (1997) can avoid the difficulties which appear in the conventional Weibull distribution models. This paper shows the maximum likelihood estimation method in the extended Weibull distribution model. The paper also illustrates some typical applications for breakdown voltage estimation in which the extended models are superior to the conventional Weibull models. The central discussion is whether the shape parameters in the extended model accomplish the mass shifting effect of the distribution. View full abstract»

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  • Electric field behavior near a zero-angle contact point in the presence of surface conductivity

    Page(s): 537 - 543
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (457 KB) |  | HTML iconHTML  

    The paper describes the electric field behavior near a contact point in various arrangements with a zero contact angle when surface conductivity is present on a solid surface. Electric field distributions are calculated for arrangements with three contact conditions: point, line, and surface contact. We focus on the effect of surface conductivity on the electric field. It is found that the presence of surface conductivity results in electric field intensification. Similar to the effect of volume conductivity, when the surface conductivity is higher than a certain value, a change in the position of the peak electric field takes place. The effect of the surface conductivity is noticeable for values as low as 0.8 nS. View full abstract»

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  • Charge acquired by a spherical ball bouncing on an electrode: comparison between theory and experiment

    Page(s): 589 - 595
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (477 KB) |  | HTML iconHTML  

    A conducting ball immersed in a poorly conducting liquid between two horizontal plates acquires a charge when subjected to a dc voltage. When the applied voltage is high enough for the electric force to overcome gravity, the ball rises, moves up through the liquid and falls down as its charge leaks away. The threshold of voltage, as well as the time of flight between contacts, depends on the charge acquired by the ball during the contact with the electrode. Experiments have been done using liquids of different conductivities and it was shown that the conductivity does not affect the lift-off voltage. When the ball gets close to the electrode a discharge occurs and a current pulse is registered by the external circuit. The charge carried by the pulse is an order of magnitude smaller than the charge transferred to the ball from the electrode. We have made a detailed study, with different balls and liquids, of the charge transferred to the external circuit. The dependence on the different parameters of the ratio charge transferred to the circuit-charge acquired by the ball is explained using the coefficients of capacity of the electrostatic system. View full abstract»

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  • Understanding electrical trees in solids: from experiment to theory

    Page(s): 483 - 497
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1033 KB) |  | HTML iconHTML  

    A review of recent developments made in the understanding of the electrical tree mechanism is presented. The life of the tree is covered from initiation, through propagation, to long-term changes in shape. The initiation process is examined in terms of the injection of space charge and its ability to transfer energy to the polymer to create damage. Theoretical models for the processes involved are assessed in terms of the experimental data and an outline for the sequence of events in tree initiation developed. The inter-relationship between tree discharges, tree propagation, and tree shape is discussed. Theoretical models for these processes are evaluated in terms of their ability to reproduce experimental data, especially tree shapes and discharge sequences in time and space. The chaotic nature of the tree propagation mechanism is discussed both through experimental data and the results of a completely deterministic theoretical model. Some special features of electrical trees such as the existence of conducting trees, acceleration at long times and slow growth in thick insulation are briefly touched upon. Finally a summary of the state of the art is presented. View full abstract»

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  • Experience with development and evaluation of corona-suppression systems for HV rotating machines

    Page(s): 569 - 576
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (545 KB) |  | HTML iconHTML  

    This paper presents the results of a project aimed at the development of reliable corona-suppression systems for high voltage rotating machines. These systems are based on both conducting and semi-conducting dry and B-stage tapes as well as paints of different resistivity. Three groups of anti-corona systems were developed; namely, paint (on the slot portion) - paint (on the slot exit) system, paint (on the slot portion) - tape (on the slot exit) system, and tape (on the slot portion) - tape (on the slot exit) system. Sample results of standard evaluation and acceptance tests are presented. The accelerated life-aging test was run on test coil samples at the National Electric Coil HV testing facility. A numerical 2-dimensional finite difference model for the electric potential and field analysis along the end-turn zone was developed and the model results are used as guidelines through the design process of the grading system. Laboratory test results on randomly selected sample coils were confirmed by an independent testing facility. The proposed corona-suppression systems were applied on thousands of production coils. View full abstract»

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  • Effect of semicon-dielectric interface on conductivity and electric field distribution

    Page(s): 596 - 603
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (552 KB) |  | HTML iconHTML  

    Development of solid dielectric dc transmission class cable has become a priority throughout much of the world. Interdiffusion between the semiconducting electrode materials and the dielectric inevitably causes variations in conductivity of the dielectric near the semicon which results in distortion of the electric field and space charge formation under dc conditions. Analytical approximations and numerical computations provide a basis for analyzing space charge measurements, and based on such space charge measurements and the analysis, we estimate the field distortion for several material systems. View full abstract»

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  • Pulsed electrical discharges assisted by dielectric pellets/catalysts for diesel engine exhaust treatment

    Page(s): 616 - 626
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (720 KB) |  | HTML iconHTML  

    This paper reports the study on gaseous pollutants (NOx, CO and HCs) using a combination of electric discharge plasma and catalysts. In this study, catalysts being used in conventional catalytic converters of gasoline engine were tried, for the first time, in diesel engine exhaust treatment assisted by electric discharge plasma. Initial studies investigate the effect of packed dielectric materials and catalysts on NOx removal. Both conventional and non-conventional catalysts were used in the studies. With plasma alone, the removal efficiency of oxides of nitrogen was around 75% and with a suitable combination of catalytic reactor the removal efficiency was as high as 90%. Among the catalysts studied, a new catalyst (CuMnAlO4) was found to be as effective as the conventional expensive catalyst. The formation of byproducts like N2O and HNO3 have been studied and the results are discussed. View full abstract»

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  • Life model based on space-charge quantities for HVDC polymeric cables subjected to voltage-polarity inversions

    Page(s): 514 - 523
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (693 KB) |  | HTML iconHTML  

    This paper illustrates a new model that enables the estimation of life of polymeric insulation subjected to DC stress, both in the presence and in the absence of voltage polarity inversions. The derivation of the model parameters is based on the results of space charge measurements and accelerated life tests realized at constant DC stress, which allows the characterization of the performance of insulating materials under DC electrical stress to be carried out in short times. The expected life of a cable in service can be evaluated by means of this model, provided that the rate of voltage inversions expected during its life is known or estimated. The model is applied here to results of tests carried out on polyethylene-based materials for cable insulation, and model parameters are calculated resorting to two different approaches. A satisfactory fitting of life test results is achieved, thus confirming the validity of the proposed model. View full abstract»

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  • A quantitative approach to estimate the life expectancy of motor insulation systems

    Page(s): 627 - 628
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (221 KB) |  | HTML iconHTML  

    H. Oraee (ibid. vol. 7, pp. 790-796, 2000) presented a timely analysis. However, since I originated this research and guided the author in his initial approach, I feel that I must share some observations with readers who want to learn more about induction motor thermal aging caused by voltage distortion and imbalance. The following observations and comments are meant to help engineers interested in the continuation of this work. View full abstract»

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  • Charging and behavior of a spherically conducting particle on a dielectrically coated electrode in the presence of electrical gradient force in atmospheric air

    Page(s): 577 - 588
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (843 KB) |  | HTML iconHTML  

    This paper discusses the charging and behavior of a spherically conducting particle on a dielectrically coated electrode in non-uniform dc and ac electric fields between non-parallel plane electrodes in atmospheric air. The charging mechanism of a particle resting on a dielectrically coated electrode is investigated by observing light emissions from partial discharges and by utilizing the dust figure technique. The charge acquired by a particle is estimated from the results of particle motion onset voltage measurement and direct measurement with a Faraday cage. The basic particle movement as well as particle-triggered breakdown characteristics under non-uniform electric field distribution along the electrode surface is also investigated. The results suggest that the effect of the electrical gradient force/dielectro-phoretic force on particle motion on a dielectrically coated electrode should be adequately considered in the design of gas insulated systems (GIS). This is because a particle can move laterally on the coated electrode toward increasing electric field regions by the action of the electrical gradient force which is independent of the charge on the particle. Also the possibility of the particle charging and lifting at high field regions or reaching and adhering onto an insulating spacer further increases the risk of breakdown. View full abstract»

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  • Dielectric behavior of aged polyethylene under UV radiation

    Page(s): 507 - 513
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (655 KB) |  | HTML iconHTML  

    The present paper reports the results of a study of dielectric aging of low-density polyethylene (LDPE) by UV radiation. The extruded polymer contained carbon black particles, approximately 3% in weight. The aging studies were made for polymer without and with carbon black. LDPE flat specimens were aged under UV radiation for up to 300 h, resorting to dielectric spectroscopy measurements in the frequency range from 10-5 Hz to 105 Hz. The dielectric susceptibility behaviors indicate the presence of relaxation peaks obeying the asymptotic forms of Dissado and Hill susceptibility functions. As the aging progresses, quasi-DC and flat loss processes may be observed in the low frequency range, for the LDPE specimens without and with carbon black, respectively. In addition, dielectric spectroscopy results have been compared with other results obtained with additional techniques such as Fourier transform infrared spectroscopy and scanning electron microscopy. View full abstract»

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  • Parallel streamer discharges between wire and plane electrodes in water

    Page(s): 498 - 506
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (624 KB) |  | HTML iconHTML  

    Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm/μs, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process. View full abstract»

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  • Gel electrolyte system based on polyvinyl chloride with addition of LiClO4/propylene carbonate: structure, morphology, thermal and conductivity properties

    Page(s): 551 - 554
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (301 KB) |  | HTML iconHTML  

    X-ray scattering, calorimetric, and quasi-elastic neutron scattering (QENS) investigations of the polymer gel electrolytes based on 20% PVC, containing 80% of propylene carbonate (PC)-LiClO4 solution as a liquid phase were made. It was established that after incorporating PC into the micropores of PVC gels the glass transition of PC increased by 3 K. This is, evidently, a reflection of an effect of confining liquid PC in the pores of PVC microphase-separated morphology. Addition of salt to the PVC-PC system results in raising Tg of both the pure PC and PVC-PC gel by 10 K. The SAXS data of pure PVC and PVC-PC and PVC-PC-LiClO4 indicate that the gels based on PVC consist of microdomains of a different composition. Single-particle and collective dynamics of solvent molecules in free state and in the pores of polymer gels based on PVC were studied by quasi-elastic neutron scattering technique. View full abstract»

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  • Impulse partial discharge and breakdown characteristics of rod-plane gaps in N2/SF6 gas mixtures

    Page(s): 544 - 550
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (483 KB) |  | HTML iconHTML  

    Impulse partial discharge (PD) and breakdown (BD) characteristics of rod-plane gaps in N2/SF6 gas mixtures were investigated for different gap geometry, gas pressure and SF6 gas content. Experimental results revealed that the 50% probability breakdown voltage increased with gas pressure, which agreed with the theoretical values with consideration of discharge time lag for impulse voltage application. For the calculation of discharge inception voltage, the volume-time theory was successfully applied to the N2/SF6 gas mixture. Furthermore, impulse PD light emission image was observed together with PD current pulse and light intensity waveforms. The long discharge with stepwise propagation and double-peak PD pulses could be observed, which corresponded to the streamer/leader transition leading to breakdown. 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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Reuben Hackam