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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 4 • Date Aug 1996

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Displaying Results 1 - 16 of 16
  • Size effect and statistical characteristics of dc and pulsed breakdown of liquid helium

    Publication Year: 1996 , Page(s): 507 - 514
    Cited by:  Papers (8)
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    Breakdown time lag in liquid helium is measured over a wide range of electrode sizes and pulsed electric field strengths. The breakdown time lag and dc breakdown strength are statistically analyzed by using the Weibull distribution function and weak link theory. It is found that the time lag depends on both electrical stress and the electrode surface area stressed above a critical level. It is supposed that breakdown triggering electrons are generated by field emission phenomena at small protrusion tips on the cathode surface. In higher external electric fields, a less sharper protrusion emits initial electrons with a shorter time lag and may become responsible for liquid breakdown. A theoretical equation is proposed to predict the electrode size and electrical stress dependency of the breakdown time lag, based on Fowler and Nordheim theory. It is shown that the equation is consistent with the Weibull distribution function under multiple stress of electric field and stressing time View full abstract»

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  • Influence of humidity on current waveform and light emission of a low-frequency discharge controlled by a dielectric barrier

    Publication Year: 1996 , Page(s): 537 - 543
    Cited by:  Papers (4)
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    The aim of this work is to determine the effect of humidity on a silent discharge in air. A point to plane configuration has been used. Current and emission spectra of the discharge have been recorded in dry and humid air for short and long inter-electrode gaps and with the plane electrode coated by a silicone rubber plate alone or under a polypropylene film. Results show the typical corona discharge regimes. For a long gap, humidity decreases the number of breakdown streamer pulses. For a short gap, the behavior observed depends on the surface conduction of the dielectric in contact with the discharge. These results show that the presence of humidity in the discharge cell not only modifies the gas but also the surface states of the dielectrics coating the electrodes. The surface charge decay appears as a major parameter, since it is related to the local field value which results from the applied field and the reverse field induced by charge accumulation during previous discharges View full abstract»

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  • Absolute vs. comparative end-of-life age

    Publication Year: 1996 , Page(s): 567 - 576
    Cited by:  Papers (2)
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    Conventional insulation statistics typically use test averages of a normal, log normal, or Weibull distribution to compare the quality of different materials or constructions. None of these numbers characterizes an age at end of life with a near 100% reliability, which is necessary to predict when failure occurs. This is a major fundamental and misleading aspect in the manner in which life and age data is reported. The distributed nature of insulation aging and failure requires taking into account the length of samples to report age at end of life. This implies that probability of failure at any time should be reported as a function of cable length. Simply reporting an average probability of failure can endanger human life and have negative economic consequences. This paper shows test data and a method of calculating end of life that provides high reliability operation of aircraft wire that is aging from root-cause stresses. The authors believe this paper treats a major refinement which is essential for realistic prediction of insulation end of life, for instance in the control of reliable operation of aircraft electrical wiring. This technique is now being used in maintenance scheduling of active aircraft and is hoped to influence future revisions of standards View full abstract»

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  • Hydrophobic property of XLPE filled with calcium carbonate

    Publication Year: 1996 , Page(s): 577 - 586
    Cited by:  Papers (7)
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    The aging of crosslinked polyethylene (XLPE) is often initiated by a process of thermal oxidation that gradually degrades the insulation until failure occurs. Oxidation on the surface causes the loss of hydrophobicity which is accelerated by increasing temperature and moisture. After the hydrophobicity on the surface has diminished, water can permeate into the bulk more readily. To investigate the process of wettability of the surface and its dependence on the level of calcium carbonate filler, XLPE specimens were immersed in saline water solutions at levels of salinity from 5.5 to 10000 μS/cm, at temperatures from 0 to 98°C and durations from 5 to 150 h. It was found that at a fixed salinity level the contact angle decreased with increasing temperature and was more pronounced for 15 and 25% filler levels than when no filler was used. The contact angle decreased slightly with increasing salinity of the water from 5.5 to 1000 μS/cm and rapidly from 1000 to 10000 μS/cm for all three filler levels. The contact angle decreased rapidly with increasing time of immersion up to 50 h and slowly thereafter up to 150 h for all filler levels. The contact angle was found to be constant at 86° after exposure to air for 50 h at five temperatures from 0 to 98°C. On the other hand after immersing in a saline solution in the range 5.5 to 10000 μS/cm for 50 h, the contact angle decreased with increasing time of immersion and increasing the temperature from 0 to 98°C for all filler levels. The recovery of hydrophobicity depends on filler concentration, temperature and salinity level of the aqueous solution. Mechanisms and reactions of oxidation responsible for the observed phenomena are suggested View full abstract»

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  • Accurate measurements over wide parameter ranges

    Publication Year: 1996 , Page(s): 473 - 481
    Cited by:  Papers (1)
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    The SI units of physical quantities can be accurately realized and reproduced. The application of macroscopic quantum effects is an option for a highly improved reproduction of units. However, the measurement uncertainty over wide scale ranges easily can become unacceptably high. Proving traceability which is a general demand in establishing quality management systems according to ISO 9000, may then become a very difficult task. This is of particular significance to industrial metrology and research where multiples and submultiples of the respective SI units occur. Typical examples of this in the characterization of electrical insulation are gigaohm, megavolt, micrometer, nanoampere or even picocoulomb. The evaluation and improvement of the measurement accuracy on scales are therefore topical research subjects in metrology View full abstract»

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  • dc field effects on streaming electrification in insulating oils

    Publication Year: 1996 , Page(s): 499 - 506
    Cited by:  Papers (8)
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    The results of an experimental investigation into the influence of impurities on electrification in insulating liquids in a tube model under an external dc field effect is described. Both conduction and streaming currents were measured as a function of temperature and flow rate using a paper-pressboard model with concentric cylindrical electrode geometry. Positive or negative dc voltage was applied to the inner electrode. The streaming current was measured from a shielded test chamber and the conduction current was measured from the ground electrode. Energization by positive dc voltage always enhanced the streaming electrification; however, when the applied voltage was negative, the polarity of streaming current was field strength dependent. At low fields (<0.52 kV/mm), the streaming current with negative dc field Isnc increased with increased field and with field strength >0.52 kV/mm, polarity reversal was observed. The field at which current crossed the zero value has been defined as the zero cross over point (ZCOP). Flow rate did not have any influence on the ZCOP; however, the additive concentration, moisture content and the temperature affected this ZCOP. In the presence of the corrosion inhibitor carbazole, the streaming current with negative dc field, Isn attained zero at low fields. The conduction currents corresponding to fields where streaming currents were zero were also relatively low. It is therefore possible that by applying a moderate dc field of appropriate polarity the streaming current can be reduced to very low values by adjusting the carbazole concentration in the test liquid View full abstract»

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  • Factors affecting corona on twin-point gaps under dc and ac HV

    Publication Year: 1996 , Page(s): 544 - 553
    Cited by:  Papers (4)
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    The factors affecting corona formation on twin-point/plane gaps are investigated both theoretically and experimentally. In the theoretical analysis, an electrostatic field computation program based on the charge simulation method was used to get the electric field profile on the plate for different gap lengths and interspacings between the two points. Also, the effect of voltage level, air pressure and gap length on the number of axial streamers occurring across the critical field line was introduced for a constant time duration. The variation of the integral of the ionization coefficient with the axial length from the point was studied. Finally, a study of the effect of air pressure and gap length on both the corona inception voltage and critical length was presented. Corona formation in single and twin-point/plane gaps also was investigated experimentally under both high direct and alternating voltages. There are many factors affecting such corona formation. The investigated factors were the interspacing between the two points, gap length between the point and the plane, ambient temperature, relative humidity, polarity or frequency of the applied voltage, and electrode material. In order to study and explain such phenomena, a photographic investigation together with a simultaneous measurement of both the applied voltage and the corona current were introduced. Curve fitting of the dc corona current vs. gap length and voltage for both single and twin-point/plane gaps gave the conventional relationship, but in case of twin point the power of the gap length varies between 1.2 and 1.3 View full abstract»

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  • Arc propagation over electrolytic surfaces under power frequency voltage

    Publication Year: 1996 , Page(s): 529 - 536
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (632 KB)  

    Flashover of wet polluted insulation is caused by the propagation of arcs along the electrolytic surface. Several factors affect this flashover process under actual operating conditions. These include the variation in pollution resistance due to arc root movement and thermal effects. The difference between the discharge gradient and the effective pollution gradient is an important factor in determining the growth of the discharge. Moreover, the distribution of instantaneous power along the wet surface also affect the arc propagation significantly. This paper describes the different physical criteria controlling are propagation, over electrolytic surfaces under ac voltage application. Contrary to the existing concept of a critical point, from where the discharge takes the final leap to complete flashover, the results obtained show that there are critical zones, referred to as the growth period lying between 40 to 63% of the total pollution length, which depends on the degree of surface pollution and the nature of pollutants View full abstract»

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  • Current pulses during water treeing detection system

    Publication Year: 1996 , Page(s): 515 - 522
    Cited by:  Papers (1)
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    A detection system has been developed to seek signals associated with water treeing. In this paper, Part 1 of two parts, the test system is found able to detect fast pulses generated by charges as small as 1.2 fC and slow pulses with current amplitudes as small as 66 nA, with 27 MHz bandwidth. Such pulses are studied in the following paper. The detection system functions in real time and non-invasively, using the common configuration of a dielectric between electrodes. Very low noise, wide bandwidth transimpedance (current in, voltage out) preamplifiers detect very small and fast current pulses in two ground electrodes which monitor the same sample respectively at an `active' (treeing) region, and a `reference' (no treeing) region. The currents in the sample are observed by their image currents in the electrodes. Noise that appears identically in both channels (e.g. corona) can be ignored by a fast logic circuit that selects data to be recorded. Noise components that are different in the two channels and their effects also are studied; they are small enough to be defeated primarily by a fast discriminator. An averaging mode is sometimes used to characterize signals and further reduce noise when one signal type is recurrent; noise artifacts in those averages are identified. A means is developed to observe the phase of the 60 Hz HV when short events are recorded. The detection system is studied carefully in this paper for confidence in the ultimate conclusions. The use of the system to study events during water treeing is described in the following paper View full abstract»

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  • Silicon-based inorganic electrets for application in micromachined devices

    Publication Year: 1996 , Page(s): 494 - 498
    Cited by:  Papers (9)  |  Patents (2)
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    Layers of SiO2 and Si3N4 have good mechanical properties for application in micro-machined electret capacitor microphones. These materials are investigated as single or double layers in terms of chargeability and long-term charge stability. The main emphasis is put on the miniaturization of electret layers. The lateral dimensions of the electrets are reduced to 2 mm and the charge decay characteristics under different environmental conditions are described. According to the experimental data, multilayer samples of silicon dioxide and nitride possess good chargeability and higher charge stability compared to the well investigated single layers. Typically, the double layers loose ~10% of their surface potential when annealed for 200 min at 300°C and show a peak of the thermally stimulated current at 430°C. It can also be shown that the miniaturization of samples does not necessarily cause a faster charge decay View full abstract»

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  • Short-term thermal endurance characterization of polymeric cable insulating materials. Use of oxidative stability measurements

    Publication Year: 1996 , Page(s): 561 - 566
    Cited by:  Papers (10)
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    In this paper, a method which provides thermal endurance characterization in significantly shorter duration than that required by IEC 216 Standard is presented. The method proposed resorts to measurements of oxidative stability to estimate the activation energy of the degradation process which is deemed to prevail in the range of test and service temperatures, as well as to a conventional thermal life test of suitable length. By this way, slope and location of the thermal endurance line are determined. Comparisons between the results provided by the proposed method and characterizations made according to IEC 216 are presented, referring to the most common insulating materials for HV cables, that is, crosslinked polyethylene and ethylene-propylene rubber. It is shown that, considering different diagnostic properties, both temperature index and halving interval obtained by the proposed method are very close to the values derived by applying IEC 216 View full abstract»

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  • Current pulses during water treeing procedures and results

    Publication Year: 1996 , Page(s): 523 - 528
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (504 KB)  

    The procedures and results are described for an experiment to seek electrical current signals during water treeing, and to characterize any that exist. The apparatus was described in the companion paper Part 1 (see ibid., vol.3, no.4, pp.515-22, 1996), where the sensitivity characteristics, and noise effects were assessed carefully because such signals had not been detected before. In this Part 2, the use of the several modes of the apparatus is described for various conditions of signal, noise, and phase resolution. Methods of identifying and eliminating noise artifacts are described. A method is described for resolving the HV phase existing when a fast event occurs. Four types of pulsed currents are studied; two are noise (1-channel and 2-channel noise) and two are signal (fast and slow pulses). Each has unique characteristics as to the channel(s) in which it occurs, amplitude, shape, and phase of the 60 Hz HV when it occurs. It is observed that two types of pulsed signals originate only in the region where water treeing is occurring; any conclusion that they are produced by the water treeing process is inferred from that observation. The `fast pulse' signal lasts only a few days, is <25 ns wide, contains 1 to 4 fC charge, and occurs only at the water treeing region and only when the 60 Hz HV waveform is zero and falling. The `slow pulse' signal appears after a few days, is 35 to 60 μs wide, has 70 to 125 nA peak, and occurs only at the water treeing region and only when the 60 Hz HV waveform is at the negative minimum View full abstract»

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  • Existence of partial discharges in low-voltage induction machines supplied by PWM drives

    Publication Year: 1996 , Page(s): 554 - 560
    Cited by:  Papers (19)
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    When standard low voltage induction motors are supplied by pulse-width modulated (PWM) adjustable speed drives, some of the traditional sinusoidal system concepts cannot be applied. This paper reports the results of an investigation of the effect of PWM drives on the insulation of the stator winding. The characteristics of the employed dielectric material and the magnitude of the applied voltages led to the hypothesis of the possible existence of partial discharges within the insulation. Practical experiments were carried out and high frequency signals which could be associated with such a phenomenon were observed. These measurements also showed that the interturn insulation undergoes some high electrical stresses and that the nominal rotation speed (50 Hz) appears to be a point of high disturbance in the working of the motor. Further work is required, since most of the points identified here need to be understood fully View full abstract»

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  • Multiscale compression of BEM equations for electrostatic systems

    Publication Year: 1996 , Page(s): 482 - 493
    Cited by:  Papers (2)
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    This paper describes the use of wavelet bases to create a sparse approximation of the fully populated matrix that one obtains using an integral formulation like charge simulation or surface charge simulation for numerically solving Laplace's equation with mixed boundary conditions. The sparse approximation is formed by a similarity transform of the N×N coefficient matrix, and the cost of the one employed here is of optimal order N2. We must emphasize that benefits of computing with a sparse matrix typically do not justify the costs of the transformation, unless the problem has multiple right hand sides, i.e. one wants to simulate multiple excitation modes. The special orthogonal matrices we need for the similarity transform are built from wavelet bases. Wavelets are a well studied and mature topic in pure and applied mathematics, however, the fundamental ideas are probably new to many researchers interested in electrostatic field computation. Towards this end an important purpose of this paper is to describe some of the basic concepts of multiresolutional analysis using wavelet bases View full abstract»

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  • The influence of chemical structure on the dielectric behavior of polypropylene

    Publication Year: 1996 , Page(s): 594 - 598
    Cited by:  Papers (4)
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    An initial study has been made of radiation-induced oxidation of polypropylene (PP) in both its additive-free form and one containing discrete amounts of stabilizer (HALS). Samples of PP were irradiated in a 60Co γ source to various doses. This took place in either room air to give in-source oxidation or in vacuum at 77 K to render post-irradiation oxidation when the material was exposed to room air following irradiation. Studies were made of the chemical structure of the material and its concomitant dielectric behavior in terms of loss at low frequencies and thermally-stimulated discharge current (TSDC). The polar functional groups induced via irradiation were observed to affect the dielectric response of the material, with the introduction of HALS reducing the concentration of gamma-induced oxidation products and the magnitude of the corresponding electrical signals View full abstract»

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  • Electric field and energy computation on wet insulating surfaces

    Publication Year: 1996 , Page(s): 587 - 593
    Cited by:  Papers (11)  |  Patents (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1188 KB)  

    The paper presents the results of electric field and energy computation on insulating surfaces covered by water in the form ranging from small discrete droplets to large wet patches. The goal of this study was to understand the differences in the flash-over performance of outdoor insulators, in particular nonceramic insulators, under contaminated conditions as a function of surface wettability. The computational algorithm is based on the charge simulation method. A decrease in the maximum electric field at the water droplet tip is obtained as the droplet size increases, however, the electrostatic potential energy increases with the size of the water droplet, which might increase the surface discharge intensity. The role of the coalescence of small water droplets into larger droplets and filaments on the electric field and energy has been investigated. The location of such larger water bodies with respect to the electrodes has been examined. These computations shed some light not only on the role of hydrophobic surfaces but also on the location of hydrophobic surfaces along the insulator on the flashover performance of outdoor insulators. It has been shown that these computations provide quantitative data that compliments information obtained from simpler techniques such as visual observation and surface resistance measurements, for the purpose of assessing the performance of insulators in service 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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