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Electrical Insulation, IEEE Transactions on

Issue 4 • Date Aug 1993

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Displaying Results 1 - 25 of 32
  • Vacuum breakdown strength of vacuum-degassed oxygen-free copper electrodes

    Publication Year: 1993 , Page(s): 500 - 506
    Cited by:  Papers (2)
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    Using an in situ electrode surface cleaning system, the vacuum breakdown strength of vacuum degassed oxygen free copper (VDOFC) electrodes satisfying ASTM-F-68 Class 1, 3, and 5 purity specifications was measured. Breakdown experiments reveal that the breakdown field (breakdown voltage/gap length) depends on the purity of the VDOFC electrodes. The initial breakdown strengths are 1.96×107 V/m for Class 1 copper, 1.89×107 V/m for Class 3, and 1.85×107 V/m for Class 5. After 500 breakdowns, these figures were improved to 18.3×107 V/m for Class 1, 17.5×107 V/m for Class 3, and 11.8×107 V/m for Class 5. It is concluded that the gas content in the electrodes is an important factor relating to the vacuum breakdown strength, provided the electrode surface is properly cleaned View full abstract»

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  • Breakdown characteristics of He gas at cryogenic temperature and low pressure

    Publication Year: 1993 , Page(s): 507 - 511
    Cited by:  Papers (1)
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    The breakdown of He between plane electrodes was investigated under conditions similar to those of the operation of superconducting magnets at room temperature and 4 K. It was found that the breakdown voltage in the Paschen minimum was higher than expected at room temperature, which was explained by the presence of impurity. An additional effect observed, especially at low pressures, is that the measured pressure in the vacuum vessel does not seem to agree with the pressure of the electrode gap. Furthermore a helium leak of 1 Pa 1/s was introduced between the electrodes. No measurable influence of this leak was found within the measuring accuracy at low temperatures View full abstract»

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  • Radiation-triggered breakdown phenomena in high-energy e+e- colliders

    Publication Year: 1993 , Page(s): 444 - 453
    Cited by:  Papers (5)
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    The frequency of breakdown phenomena in a HV system under ultrahigh vacuum, can be increased by orders of magnitude by the presence of radiation, either in the form of electromagnetic waves (laser and UV light, synchrotron light, X-rays, γ rays) or charged particles and ions. Both types of radiation are abundant in high-energy e+e- colliders, and studies of the effect of a particular radiation source in this difficult environment are subject to many limitations. Thus, the principal strategy is to reduce the radiation flux incident on critical devices by absorbing the electromagnetic radiation or by trapping the charged particles. However, in the case of the `pretzel' separation project for CERN's large electron positron (LEP) collider, this strategy failed. As a result many experiments have been carried out in order to achieve a better understanding of radiation-triggered breakdown phenomena. Observations made in some other e+e- colliders are also reported View full abstract»

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  • Investigation of high-current interruption of vacuum circuit breakers

    Publication Year: 1993 , Page(s): 607 - 620
    Cited by:  Papers (14)
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    The physical processes occurring during recovery of a vacuum interrupter after high-current interruption are surveyed. New results have been obtained by the application of modern diagnostic techniques like laser shadow imaging, laser-induced fluorescence of atoms and of metal vapor ions, Mie scattering of droplets, and high-resolution recording of voltage and current on digital oscilloscopes. The density of metal vapor produced by the arc determines the kind of breakdown processes occurring after current zero. Liquid droplets and residual plasma play a minor role. The commonly used contact material, i.e., copper-chromium, is compared with pure copper, and its advantage is explained by the surface structure of the melt View full abstract»

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  • Optimization of test procedure to improve insulator performance under high electric stress

    Publication Year: 1993 , Page(s): 545 - 554
    Cited by:  Papers (4)
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    Surface flashover properties in relation to test procedures were investigated, using single-cycle steady step, multiple-cycle steady-step, multiple-shot, and hill-valley tests. In general, the test procedures were found to influence significantly the surface flashover properties of large-bandgap insulators such as polycrystalline alumina and signal-crystal quartz in vacuum. The steady-step tests increased the degree of conditioning to a higher level during the second day. However, the samples subjected to these tests displayed a higher degree of deconditioning during the second and third day. A simultaneous increase of conditioning and decrease of deconditioning was observed for the multiple-shot and hill-valley tests. The surface flashover performances of 6-μm-finish alumina specimens for different test procedures are also compared and discussed. The behavior of the insulators under a given test procedure is explained on the basis of surface electronic defects and adsorbed gaseous atoms or molecules View full abstract»

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  • Effect of X-irradiation and friction on the properties of insulators

    Publication Year: 1993 , Page(s): 528 - 534
    Cited by:  Papers (2)
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    The study of ceramics is tackled from the space-charge physics point of view. The role of the polarization and relaxation mechanisms is demonstrated for friction coefficient and wear, as well as for breakdown voltage; therefore these characteristics depend on the permittivity. This result is based on experiments performed with pure or X-irradiated single crystal alumina (sapphire) or polycrystalline alumina. A method called the mirror method is presented, in which the possibility of creating trapped electrical charges in a given material is measured using the electron beam of a scanning electron microscope. This method is based on the fact that X-ray irradiation changes permittivity, Young modulus, and charging properties. Therefore, when a dielectric material is under irradiation, the trap and the bond energies will be modified and consequently changes in electrical and mechanical properties of the material are expected. It is shown that these changes can modify both irradiated and nonirradiated areas View full abstract»

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  • Flashover of insulators in vacuum: review of the phenomena and techniques to improved holdoff voltage

    Publication Year: 1993 , Page(s): 512 - 527
    Cited by:  Papers (110)
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    Reviews surface flashover of insulator, primarily in vacuum, although some comments are made about the effect of ambient gases on surface flashover. It presents theoretical mechanisms of surface flashover and pertinent experimental results. The holdoff voltage of insulators depends upon many insulator parameters, such as material, geometry, surface finish, and attachments to electrodes, but also on the applied voltage waveform (duration, single pulse or repetitive), the process history of the insulator operating environment, and previous applications of voltage. Several suggestions are made regarding choice of the material, geometry, and processing when selecting an insulator for a particular application. Some specific techniques for improving the holdoff voltage of insulators are recommended View full abstract»

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  • Field electron emission from large-area Y1Ba2Cu3O7-x superconducting films in vacuum

    Publication Year: 1993 , Page(s): 494 - 499
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    Thin films of Y1Ba2Cu3O7-x deposited on copper and stainless steel electrodes by a flame spray method were tested in vacuum at electric fields ranging from 10 to 100 kV/cm. Field electron emission current was measured from the surfaces before and after the application of the film. Metallic surfaces, of 20 cm2 in surface, cooled to LN2 (~90 K) temperatures showed a 1.75 to 2×increase in current. The coated surfaces showed a 5 to 6× increment in current when compared to room temperature metallic surfaces. Stainless steel electrodes with a surface area of 20 cm2, which were coated by a flame spray process and tested under vacuum, showed a similar behavior. Coating procedures and experimental results are shown View full abstract»

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  • The influence of a cylindrical shield electrode on the dielectric strength of large vacuum gaps

    Publication Year: 1993 , Page(s): 473 - 480
    Cited by:  Papers (1)
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    Breakdown voltages of three-electrode arrangements have been determined. The shield electrode shows a significant influence on the breakdown behavior of the electrode arrangement at the investigated gap distances between 10 and 20 mm and at shield diameters of 100 to 120 mm. Confirming former investigations, the breakdown voltages increase according to the material in the order: copper, stainless steel, aluminium. The amplitudes of the AC prebreakdown conditioning voltages are of great importance if the measured breakdown voltages are compared. The electrical strength of three-electrode arrangements can be increased by 30% if field-optimized electrodes are used. The breakdown of a three-electrode gap is normally not included by a charging or discharging of the shield electrode View full abstract»

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  • Vacuum breakdown with microsecond delay time [interrupters]

    Publication Year: 1993 , Page(s): 461 - 467
    Cited by:  Papers (7)
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    The delay time of voltage breakdown has been analyzed as a function of the time interval to preceding interruption of high currents. With intervals >1 ms the behavior of the gap corresponded to cold electrodes. For this case, in a separate experiment, the dependence of the delay time on various parameters has been studied. At constant voltage a steep increased with the gap distance was found. This leads to the conclusion that field emission rather than a clump process is responsible for late breakdowns View full abstract»

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  • Spark conditioning procedures for vacuum interrupters in circuit breakers

    Publication Year: 1993 , Page(s): 621 - 627
    Cited by:  Papers (10)
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    The breakdown voltage between the open contacts of vacuum circuit breakers (VCBs) immediately after manufacturing is low and subject to high scatter. To attain a sufficiently high breakdown voltage, these unknown and undefined surface conditions must be improved by conditioning. Several usual conditioning procedures applied to a large number of 12-kV circuit breakers of a standard design are compared. A large number of conditioning experiments have been made and statistically evaluated. Additional studies have been performed in a vacuum test chamber in which the contacts could be interchanged easily. The aim was to find the most important conditioning parameters and to explain the underlying physical processes. The studies show that the conditioning procedure can be optimized View full abstract»

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  • Time/current characteristics and breaking capacity of experimental vacuum fuses

    Publication Year: 1993 , Page(s): 642 - 649
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    Conventional HV fuses, which are filled with quartz sand, have an excellent current-limiting capability, but they have a zone of uncertain operation in the low overcurrent range. To overcome this disadvantage, substituting of the quartz sand by a vacuum was considered. In particular, a vacuum fuse can be designed by inserting a fusible element between the fixed butt contacts (electrodes) of a vacuum chamber. Melting curves and the arc quenching capability at current zero are investigated for a simply designed vacuum fuse with flat contacts. Variables are the current amplitude, the electrode material, the diameter of the fuse wire, and the distance between the electrodes. These results show that it is possible to design an operable vacuum fuse with a simplified electrode configuration. The time/current characteristic is strongly influenced by the length of the fuse wire View full abstract»

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  • Surface flashover in silicon-vacuum systems

    Publication Year: 1993 , Page(s): 555 - 565
    Cited by:  Papers (3)
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    The properties of the prebreakdown response of silicon-vacuum systems under HV excitation are presented. The most frequent case of system breakdown by surface flashover is treated. The particular properties of the system response in the high-field pulsed regime demonstrate the essential differences between the silicon-vacuum and solid-insulator-vacuum systems. The main ideas of a new physical model of surface flashover in silicon-vacuum systems are presented. The properties of the surface flashover response are discussed in terms of the proposed model. A concept called system surface flashover sensitivity is introduced to provide a better understanding of the surface flashover physical process in silicon-vacuum systems View full abstract»

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  • The properties of triggered vacuum gaps

    Publication Year: 1993 , Page(s): 650 - 656
    Cited by:  Papers (4)
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    The reaction of the trigger plasma of triggered vacuum gaps (TVGs) to the main gap voltage changes with the main gap polarity. The trigger polarity has no influence when the main gap voltage exceeds a certain value. The high current-chopping instability in positive TVG mode is investigated in detail, with the influence of the loop parameters and electrode contamination. In the negative mode, a history effect is found in TVG triggering, associated with the surface condition of the main gap cathode. The trigger delay time in the negative mode depends on the trigger plasma density and the plasma buildup. The general TVG triggering is explained, and the reaction of the trigger plasma to the main gap polarity determines the main-gap polarity dependence of TVG View full abstract»

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  • Effect of low-temperature electrode baking on breakdown in vacuum

    Publication Year: 1993 , Page(s): 574 - 579
    Cited by:  Papers (2)
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    Effects of electrode baking at a comparatively low temperature in vacuum have been examined. The temperature of the electrode is controlled well below the melting point of organic insulators, if installed. Currents accompanied by the microdischarges and the conditioning process during breakdown test are observed. It is shown that low temperature baking effectively suppresses the microdischarge, and that baking of the anode is more effective than baking of the cathode. It is also shown that baking reduces the number of voltage applications which are necessary for the spark conditioning View full abstract»

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  • Dependence of the surface flashover properties of alumina on polishing abrasive parameters

    Publication Year: 1993 , Page(s): 535 - 544
    Cited by:  Papers (9)
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    Systematic investigations of the relationship between the surface finish, the polishing medium, and surface flashover properties of high purity polycrystalline alumina ceramics are reported. Samples polished with SiC abrasives displayed a linear behavior, with the breakdown strength increasing with decreasing surface roughness. The breakdown properties of the diamond polished samples were influenced significantly by the polishing sequence adopted; samples polished from coarser to finer finish exhibited erratic surface breakdown properties, whereas the breakdown properties of samples polished from finer to coarser finish varied linearly with respect to the particle size of the abrasive used in the final polishing. These results are discussed in terms of surface and subsurface defects that are formed during diamond polishing. The breakdown properties of the samples in relation to the abrasive type are also analyzed View full abstract»

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  • New approach to flashover in dielectrics based on a polarization energy relaxation mechanism

    Publication Year: 1993 , Page(s): 437 - 443
    Cited by:  Papers (19)
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    The polaron concept is used to describe trapping of charge in a dielectric medium. It is shown that trapping is associated with defects identified as being due to a local decrease of the electronic polarizability. The polarization energy around a charge is estimated, using a self-consistent calculation of the local field. In nonpolar dielectrics this energy is 5 eV per charge, in the limit of validity of the Clausius-Mosotti relation. In polar dielectrics it can be much higher. A flashover process is proposed as resulting from the destabilization of the space charge by a variety of perturbations (electrical, mechanical, thermal), followed by the mechanical relaxation of the lattice, initially polarized by the charge View full abstract»

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  • Relation of breakdown voltage and prebreakdown microdischarge parameters in vacuum

    Publication Year: 1993 , Page(s): 481 - 487
    Cited by:  Papers (5)
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    Some selected parameters describing microdischarges are examined. The statistical distribution of charge transferred by microdischarges, obtained by means of multichannel pulse-height analysis, is presented. The relations of breakdown voltage to the parameters characterizing microdischarges are defined. The parameters considered are the number N of pulses in count time, maximum charge Qmx of a single microdischarge, total charge Q transferred by microdischarges in count time, and summary charge Q30 transferred by the most intensive microdischarges with magnitude of 0.7Qmx to 1.0Qmx View full abstract»

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  • Monte Carlo simulation of surface charge on angled insulators in vacuum

    Publication Year: 1993 , Page(s): 706 - 712
    Cited by:  Papers (20)  |  Patents (1)
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    A two-dimensional computer analysis of the surface charging on angled insulators has been performed by using the Monte Carlo simulation method. Results are compared to the charge in equilibrium model, originally proposed by C.H. de Tourreil and K.D. Srivastava (1973) View full abstract»

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  • Electrical methods for verifying internal pressure of vacuum interrupters after long-time service

    Publication Year: 1993 , Page(s): 635 - 641
    Cited by:  Papers (12)
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    Results of recent investigations dealing with the fundamentals of two new methods to check the vacuum inside the chamber, using only electrical parameters, are presented. With increasing pressure, the prebreakdown current changes in such a way that not only do the power frequency emission current patterns change, but also high-frequency current pulses, with a frequency range of ~30 to 300 kHz, appear superimposed on the emission current. It is reported under which conditions these pulse currents appear, how they can be detected, and how they correlate with the pressure. When the current is interrupted at a frequency range of ~10 to 90 kHz, the current switching capability and the reignition voltages change significantly with increasing pressure inside the vacuum chamber. The vacuum chamber loses its high-frequency current switching capability when the internal pressure is higher than ~10-2 to 10-1 Pa. The breakdown voltage and reignition voltages do not appreciably decrease even when the pressure rises to ~1 Pa View full abstract»

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  • The effect of insulator charging on breakdown and conditioning

    Publication Year: 1993 , Page(s): 681 - 691
    Cited by:  Papers (16)  |  Patents (1)
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    As part of a study on HV design concepts for microwave tubes, a number of different insulator designs have been studied. Analysis of the measured DC current, partial discharge activity and breakdown voltage shows that surface charging of insulators is a key mechanism in the breakdown process and in the conditioning process. Insulator parameters are not only the breakdown voltage, but also the conditioning speed and the sensitivity to gas exposure or charge leakage. In all these respects insulators with a field enhancement at the anode are superior. Field enhancements at the cathode are less harmful if stepped insulator shapes are chosen. Effective conditioning requires at least a limited number of breakdowns. With sufficient conditioning breakdowns, all insulator geometries tested reached a breakdown field exceeding 12 kV/mm View full abstract»

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  • Mechanical shocks as cause of late discharges in vacuum circuit breakers

    Publication Year: 1993 , Page(s): 468 - 472
    Cited by:  Papers (11)
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    The time dependence of late discharges in high-voltage vacuum circuit breakers after current interruption has been measured as a function of the mechanical and electrical stress exerted on the vacuum circuit breaker bottle. A correlation between the occurrence of strong, shocklike acceleration peaks of >104 m/s2 and the increase of the probability of late discharges has been found. A reduction of the shock wave amplitudes by simple mechanical damping results in a reduction of the probability of late strikes by as much as 50%. It is concluded that most of the late discharges observed in these experiments are caused by particles which adhere loosely to the contact and vapor shield surfaces, and which are released by the strong mechanical shocks View full abstract»

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  • Insulation characteristics and welding behavior of vacuum switch contacts made from various CuCr alloys

    Publication Year: 1993 , Page(s): 628 - 634
    Cited by:  Papers (9)
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    CuCr is generally accepted and applied as contact material in vacuum switches. Although CuCr has many well-known advantages it has, compared to CuBi, the disadvantage of welding more easily at high current-closing operations of circuit breakers. Thus, a currentless separation of the welds occurring after closing operations affects the dielectric properties of the open contact gap. In this study welding forces as well as field enhancement factors β and breakdown voltages of the contacts after currentless opening are investigated and used as diagnostic tools in order to study the influence of different CuCr compositions and manufacturing processes on the performance of butt-type contacts installed in a model vacuum breaker chamber View full abstract»

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  • X-ray emission accompanying cathode microdischarge

    Publication Year: 1993 , Page(s): 488 - 493
    Cited by:  Papers (1)
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    Electrical discharge in vacuum is accompanied by X-ray radiation. Knowledge of the generation mechanism of radiation may lead to its practical use. The analysis of this phenomenon indicates that it is only at the discharge development phase until the moment of the anode plasma generation that the proper conditions for X-ray generation are satisfied. The intensity of radiation is dependent on the field current intensity on the cathode surface as well as on the value of the electron current supplying the anode. The anode current at the vacuum discharge development phase has a pulse character, and the pulse amplitude is dependent on the cathode plasma density. The radiation intensity may be increased by increasing the current amplitude, e.g. by artificial plasma generation in the trigger system View full abstract»

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  • Long-delayed breakdown in vacuum gaps under DC pulses

    Publication Year: 1993 , Page(s): 454 - 460
    Cited by:  Papers (5)
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    Concerning HV insulation in vacuum, there is still some uncertainty about the processes leading to breakdown, especially in the case of long time lags and large contact spacing. The present measurements show that mechanical shocks cause breakdown of the gap. Breakdown probability is highest for virgin contacts and decreases after heavy arcing. The role of nonmetallic inclusions and organic impurities is highlighted. In comparison with a demountable vacuum device industrial vacuum interrupters do not show any sensitivity to mechanical shocks. Besides breakdown probabilities and time lags, the temporal development of voltage and current during breakdown was measured. For a contact spacing of >14 mm and for breakdown sites located on the side face of the contacts, HV discharges were observed lasting for ~500 μs. In this case, current is limited by a negative space charge layer in front of the anode View full abstract»

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Aims & Scope

This Transactions ceased production in 1993. The current retitled publication is  IEEE Transactions on Dielectrics and Electrical Insulation.

Full Aims & Scope