By Topic

Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 3 • Date Jun 1998

Filter Results

Displaying Results 1 - 22 of 22
  • Kerr electro-optic theory and measurements of electric fields with magnitude and direction varying along the light path

    Page(s): 421 - 442
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1928 KB)  

    Differential equations that govern light propagation in Kerr media are derived when the applied electric field direction and magnitude vary along the light path. Case studies predict Kerr electro-optic fringe patterns for the specific case of point/plane electrodes. We apply the characteristic directions theory of photoelasticity to understand these fringes. We also study birefringent media with small Kerr constant, in particular transformer oil. For this case we show that simplifications in the characteristic parameter theory is possible, resulting in simple integral relationships between the characteristic parameters and the applied electric field. We use these simple relationships to extend the ac modulation method to measure the characteristic parameters for small Kerr constant media. Measurements of the characteristic parameters using the ac modulation method are presented for point/plane electrodes in transformer oil. The measurements agree reasonably well with space charge free theory for infinite extent electrodes for which analytical expressions are available. We finally employ the `onion peeling' method to reconstruct the axisymmetric electric field magnitude and direction from the measured characteristic parameters and compare the results to the analytically obtained electric field View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Transport of electrons in atomic liquids in high electric fields

    Page(s): 450 - 457
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (780 KB)  

    To describe the transport properties of hot electrons in high electric fields, a theory for the scattering of excess electrons in atomic liquids with high atomic polarizability is proposed. The theory is based on the variable phase method and it does not require information about the short-range part of the electron-atom potential. The scattering of electrons by density fluctuations in a liquid of a given macroscopic density (the liquid density) is taken into account in the framework of the theory. Mobility, mean and characteristic energies of electrons as functions of the electric field strength are calculated for different liquid densities. The results are presented for liquid Ar at the triple point, and at the density at which the scattering of electrons by density fluctuations is predominant View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Propagation of electrical tree structures in solid polymeric insulation [comment and reply]

    Page(s): 458 - 460
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (384 KB)  

    For original paper see L.A. Dissado et al., ibid., vol.4, pp.259-79, 1997. The authors of the original paper presented a review of two theoretical approaches to electrical tree propagation and pointed out the significance of space charge and its spatial arrangement in the formation and propagation of treeing structures. This comment raises some questions and presents some observations on the original paper. A reply by the original authors is included View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Spectral analysis of the light emitted by streamers in hydrocarbon liquids

    Page(s): 382 - 387
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (632 KB)  

    This paper presents the results of a spectroscopic study from 200 to 850 nm of the light emitted by streamers initiated in cyclohexane and n-pentane under step voltage in point-plane geometry. Experimental spectra of the light emitted by bush-like and filamentary streamers are composed of the Hα-Balmer line, the C2 Swan band system and a background continuum. The diagnostic method we used for evaluating rotational and vibrational temperatures of excited C2 in streamers was first tested on high-pressure corona discharges in nitrogen. For streamers in cyclohexane and n-pentane, it was impossible to determine the rotational temperature of C2 and consequently the effective temperature of molecules in the streamer. Moreover, we found that vibrational populations of excited C2 do not follow Boltzmann statistics. This indicates that excitation processes are due to chemical reactions. Electron densities deduced of the Stark broadening of Hα are in the range 4×10 16 to 7×1016 cm-3 for filamentary streamers and 2 to 6×1017 cm-3 during the breakdown phase. For slow bush-like streamers, the electron density is not measurable View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • The behavior of the Debye rotator

    Page(s): 316 - 320
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (400 KB)  

    When an electric field is created in a liquid containing polar molecules, the resulting buildup of polarization is hindered by the chaotic thermal motion of the molecules of the environment of the dipoles. The dipoles are represented as being embedded in hard spheres, sometimes called Debye rotators, and these spheres are subjected to a frictional torque somewhat analogous to that which would be exerted on a rotating sphere in a viscous liquid, but which is in fact caused by collisions between the rotator and other molecules, and on that account is named `inner friction'. In addition, the environmental thermal energy greatly reduces the response of the rotators to the applied field, so that only a very slight degree of order is imposed on the random dipole orientation. This effect is often taken into account by the inclusion of a Brownian motion term in the dynamic equation describing the motion of the rotators. The system can be seen as one consisting of orderly processes, namely the field driven and friction retarded motion of the rotators, against a background of the disorderly thermal behavior of the molecules. The relative importance of these factors may vary from system to system, and in principle it is of interest to know what would be the outcome of the operation of the orderly processes alone. That has been examined in what follows, and while it turns out that what is predicted, though as expected is not in accordance with predictions when a Brownian motion term is included, nevertheless has more resemblance to them than might have been anticipated. However, an interesting difference is that the present analysis leads to the expectation of a distribution of relaxation times View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Static electrification of pressboard/oil interface and transient phenomena

    Page(s): 443 - 449
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (520 KB)  

    The static electrification phenomenon of insulating materials used in power transformers is investigated through two devices. The first is a cell with a rotating disk covered on both sides with a given pressboard and immersed in a metallic tank containing ~41 of oil; and second device enables us to measure the electrostatic charge tendency of oils. The electrostatic charge tendency (ECT) of insulating oils and the leakage current generated by the charge concentration gradient at the oil/pressboard interface are analyzed as function of temperature, water content and aging of oils, the nature of the pressboard, and the rotating speed of the disk. The influence of the surface roughness of the pressboard and antistatic additives on the aging and ECT also are considered. A correlation between ECT and the physico-chemical and electrical characteristics of the oil is established. It is shown that the temperature gradients and the water migration phenomena at the oil/pressboard interface play an important role in the charge separation. Transient currents are strongly affected by the presence of air in the pressboard and the aging of the oil View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Dielectric liquids

    Page(s): 305
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (56 KB)  

    First Page of the Article
    View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • The growth of prebreakdown cavities in silicone fluids and the frequency of the accompanying discharge pulses

    Page(s): 344 - 350
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (576 KB)  

    Measurements have been made of prebreakdown cavities in silicone fluids, and of the current pulses that accompany cavity growth. These experiments were carried out in silicone fluids of 0.65, 10, 100 and 1000 cS viscosity. Cavity growth, driven by the electrostatic field, is limited at low viscosities by inertia, and at high viscosities by viscous drag. The electrostatic force on the cavity wall is related to the local field and to the space charge density in the liquid adjacent to the cavity. We are concerned with the relationship between the electrostatic force and the cavity growth, and with the discharges that accompany cavity growth. Discharges occur in well-defined pulse trains: the first pulse in a train generates the cavity, and subsequent pulses are due to discharges within the cavity. Knowing the scaling laws for cavity growth we can use the time between the first and second pulses to estimate the cavity size when the first cavity discharge occurs; this gives a cavity diameter of ~5 to 7 μm. The next pulse cannot occur until the charge from the previous discharge has dispersed. We find that the time between pulses Δt is strongly viscosity dependent; at high viscosities the average time between pulses at is proportional to fluid-viscosity, but in the low viscosity limit the dependence approaches η1/3. To explain this viscosity dependence we consider three mechanisms: (1) a decrease in charge density due to increase in cavity size; (2) ion detrapping from the cavity wall and drift in the applied field; and (3) diffusion of an impurity species to the cavity surface, charge exchange to create a mobile ion, and its subsequent drift in the field. Our experimental results are consistent with the cavity expansion model, but there is evidence of diffusion effects in low viscosity liquids, and with ion-drift at high viscosities View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • A phenomenological aging model for combined thermal and electrical stress [comment and reply]

    Page(s): 463 - 465
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (208 KB)  

    For original paper see A.C. Gjaerde, IEEE DEIS, vol.4, no.6, pp.674-80, 1997. In the original paper, Gjaerde proposed a life model for insulating materials under combined stress, where the combined effect of temperature and PD is considered. The latter is taken into account by means of measurements of void gas pressure. The purpose of this comment is not to contest Gjaerde's model from the point of view of its inability to fit all experimental results, but to show that the model can be derived from the well-known model proposed by this author, but with several inadmissible simplifications and errors. A reply by Gjaerde to the comment is included View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Propagation of positive and negative streamers in oil with and without pressboard interfaces

    Page(s): 388 - 395
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1036 KB)  

    Inception and propagation of streamers in a point-plane gap, with and without pressboard interface parallel to the field, have been studied. Current and light emission have been recorded. Positive streamers propagate at ~2 to 4 mm/μs at voltages to 2× the minimum breakdown voltage. Above this voltage, a fast event (>100 mm/μs) may occur, preceded by streamers propagating with velocities in the 10 to 20 mm/μs range. The negative streamers have ~2× the breakdown voltage as the positive ones. Even negative streamers may develop into a slower kind of fast event. Solid pressboard parallel to the field does not change the breakdown voltage, but makes inception of fast events easier. The breakdown process in oil shows several similarities to a gas breakdown View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Optimization of multi-wavelength interdigital dielectrometry instrumentation and algorithms

    Page(s): 408 - 420
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1332 KB)  

    Interdigital frequency-wavelength dielectrometry can be used to measure the dielectric permittivity and conductivity of insulating materials. The complex dielectric permittivity is directly related to other material properties, such as moisture content, temperature, concentration of impurities and additives, density, aging status, etc. The analysis of spatial and temporal variations of these properties lends valuable insights into physical phenomena which take place in electrical equipment, provides instrumentation for system monitoring and diagnostics, and can be used for optimization of design and performance of electrical apparatus. The optimization of various aspects of this technology is described in this paper. Improvement of performance is achieved through variation of geometrical design, materials, manufacturing processes, and electronic circuitry. Accumulated effects of non-ideal geometry of the experimental setup and the sensor itself are accounted for through empirical measurements, calibration, and use of finite-element calculations. Three distinct operating modes are developed: floating voltage with grounded backplane, floating voltage with guarded backplane, and short circuit current. Measurements reveal that the interfacial contact quality has a strong influence on the sensor's response. Gain/phase measurements over the frequency range 5 mHz to 10 kHz agree well with theoretical calculations on the interfacial contact quality. Full-frequency measurements for several liquid and solid dielectrics are shown to have a good match with theoretical predictions View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Full text access may be available. Click article title to sign in or learn about subscription options.
  • Full text access may be available. Click article title to sign in or learn about subscription options.
  • The effect of tip curvature on the prebreakdown streamer structure in cyclohexane

    Page(s): 396 - 401
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (748 KB)  

    In this paper the effect of tip curvature on the generation and propagation of prebreakdown density change streamer in cyclohexane was investigated. The streamer inception voltage increased with tip curvature, which was higher for a positive point than a negative one. The initial structure of the streamer was a single filament for both polarities in cyclohexane. For a negative point, the streamer structure changed from sphere to hemisphere, pagoda and bush-like form with increasing tip curvature and applied voltage. A slim channel connecting the streamer with needle tip was clearly observed for tip radii >5 μm, while it was not for those <3 μm. For positive streamers, on the other hand, it changed from slow bush-like form to fast filamentary with increasing tip curvature and applied voltage. Both magnitude and propagating speed of the streamer at its inception voltage increased with a tip curvature. After the growth stopped, they broke up into small bubbles and contracted with damped oscillation. From the spatial distribution of the field strength between the point and plane calculated by the finite element method, it was found that there exists a specific distance from the tip where the field strength is equal under any tip radii used in this study for both polarities. This distance and field strength may considered to be the minimum length and field for an electron avalanche in liquid View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • A new model for the primary process of electrical breakdown in liquids

    Page(s): 306 - 315
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1148 KB)  

    A new model for the generation of electrical streamers in insulating liquids is proposed, It is based on the mechanical stress generated by the electric field and its influence on the cohesive properties of the liquid. In fields of 108 to 109 V m-1 the stress is sufficient to enhance significantly the thermal generation of sub-microscopic rupture surfaces (holes) in the liquid which has solid-like properties in the short time of streamer development. Using the well-known Griffith concept of mechanically-generated crack propagation in solids, it is then argued that, when the population of sub-microscopic holes becomes sufficiently large, the same stress encourages macro-crack development which has all the hallmarks of streamer growth, In this model electrical discharges do not have a traditional primary role although they will have an important secondary role once macro-cracks have developed View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • A molecular model to evaluate the impact of aging on space charges in polymer dielectrics [comment and reply]

    Page(s): 461 - 462
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (176 KB)  

    For original paper see J.P. Crine, ibid., vol.4, pp.487-95, 1997. The original paper by Crine provides further consideration and experimental data in support of the already well-known model for the electrical/thermal/mechanical aging of polymeric insulation proposed by the same author some years ago. This paper comments on certain aspects of Crine's model. A reply by Crine is included View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Nonlinear electrohydrodynamics of free surfaces

    Page(s): 334 - 343
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (832 KB)  

    Electrical stresses acting at electrified interfaces govern the dynamics of fluids in many situations of practical relevance. In order to understand the electrohydrodynamics of these systems, a knowledge of the forces acting at interfaces is necessary. We will review the balance equations for the forces and the charge density. The relevant nondimensional numbers will be derived and illustrated with some particular cases. The problems of shallow horizontal layers and inclined thin films of perfectly conducting liquids will be examined in some detail View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Positive streamer propagation in large oil gaps: electrical properties of streamers

    Page(s): 371 - 381
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (884 KB)  

    This paper presents experimental data and calculations concerning the electrical properties of positive streamers in mineral oil at large gaps and HV. The experiments concern the measurement of charge, electric field, and the determination of the potential drop along streamers, either in the liquid alone, or for streamers guided within insulating tubes. Calculations of charge and field distribution around streamers are carried out by charge simulation. To do this, streamers are represented by objects with simple shapes (spheres or cylinders) equivalent to their macroscopic aspect. These models lead to a correct agreement with measured streamer charge and field on the plane electrode. Qualitative correlations are established between calculated field distributions and streamer behavior such as velocity, transitions between propagation modes. It is also concluded that the potential drop in streamers and branching both act as regulating mechanisms that help to keep the streamer tip field, and hence the velocity, constant over a wide voltage range View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Positive streamer propagation in large oil gaps: experimental characterization of propagation modes

    Page(s): 360 - 370
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1184 KB)  

    This paper presents an experimental study of positive streamer propagation in mineral oil, in large point-plane gaps under impulse voltage. A systematic investigation was done concerning the influence of gap distance (⩽35 cm), and voltage from streamer inception up to large overvoltages. The measurements presented concern breakdown voltage measurements, time to breakdown, visualization of streamers (streak and still photographs), electrical measurements (transient current, charge), light emission intensity. Streamers are characterized and then classified into different modes (2nd, 3rd and 4th modes) according to their propagation velocities up to more than 100 km/s. According to the applied voltage, transitions between modes are observed, and correlations are established between charge, shape, and streamer velocity. The physical mechanisms as well as practical consequences are then discussed View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Simultaneous ultrawide and narrowband detection of PD pulses in dielectric liquids

    Page(s): 402 - 407
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (592 KB)  

    Simultaneous narrow band (300 kHz) and wide band (500 MHz) measurements were carried out to obtain estimates of the apparent charge transfer associated with discrete isolated discharge pulses and discharge pulse bursts, which are observed to occur in dielectric liquids when wide band detection systems are utilized. The integrated apparent charge transfers determined with the narrow band system ranged from 12 to 95 pC, which represents typically the lower range of values normally encountered with PD activity in dielectric liquid-filled or impregnated equipment. While the discharge phenomena in perfluoro polyether liquids was found to be typified by sporadic appearances of single isolated pulses involving charge transfers between 33 and 38 pC, the discharge events in mineral oils assumed most frequently the form of pulse bursts, displaying the usual pulse sequences of quasi-ascending amplitudes; the overall integrated apparent charge transfer of the observed pulse bursts extended commonly from 12 to 16 pC. The charge release, occurring with the first (initiating) smallest discharge pulse within the pulse bursts, was estimated to be of the order of ~1 pC View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Streamer propagation and breakdown under ac voltage in very large oil gaps

    Page(s): 351 - 359
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (876 KB)  

    This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps (⩽80 cm). The investigations presented concern the measurement of breakdown voltages together with the recording of streamers in rod-plane and sphere-plane electrode systems, at different gap sizes. A breakdown mode specific to ac voltage is found, in which `bursts' composed of streamers initiated at each half cycle appear. This mode leads to the lowest breakdown fields recorded under ac. Values of the average stress required for burst or direct breakdown modes are deduced from the experiments. Positive streamers are responsible for breakdown in large gaps: their propagation is easy and does not depend on the gap geometry and on the presence of particles. On the other hand, the propagation of negative streamers is quenched when not very divergent fields are used View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.
  • Breakdown probability and size effect in liquid helium

    Page(s): 321 - 333
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1356 KB)  

    Breakdown statistics in liquid helium have been evaluated in order to correlate with size effect degradation, i.e. the area and the volume effect, respectively. Reprocessing large number of data sets yielded the popular two parameter Weibull distribution not being representative; more sophisticated functions, e.g. the Gompertz function had to be used in order to fit the data. A considerable amount of data were found to be not homogeneous, and a sum of 2-parameter Weibull subdistributions had to be defined. These results are discussed in detail. No formal straightforward correlation with the known size effect could be established. Special emphasis has been attributed to the low breakdown probability limit in order to meet engineering design needs. There seems to be a kind of threshold, but size degradation cannot be ruled out, and long term performance has still to be proven View full abstract»

    Full text access may be available. Click article title to sign in or learn about subscription options.

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.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Reuben Hackam