Showing papers on "Insulator (electricity) published in 1989"

Surface flashover of insulators

Abstract: The author reviews surface flashover (i.e. voltage breakdown along the surface of insulators), primarily in vacuum. He discusses theories and models relating to surface flashover and pertinent experimental results. Surface flashover of insulators in vacuum generally is initiated by the emission of electrons from the cathode triple junction (the region where the electrode, insulator, and vacuum meet). These electrons usually then multiply as they traverse the insulator surface, either as a surface secondary-electron-emission avalanche or as an electron cascade in a thin surface layer, causing desorption of gas which had been adsorbed on the insulator surface. This desorbed gas is then ionized, which leads to surface flashover of the insulator. The theory and modeling of this phenomena and experimental studies of surface charging, the applied voltage waveform, prestressing, conditioning, discharge delay and speed, insulator geometry AMD material, surface treatment, surface gases, temperature, and pressure are reviewed. Some suggestions are made regarding how to choose the material geometry and processing when selecting an insulator for a particular application. >

Mechanism and effect of DC charge accumulation on SF/sub 6/ gas insulated spacers

Abstract: Mechanism and effect of DC charge accumulation on the surface of a solid insulating support (spacer) have been studied in compressed SF/sub 6/ gas using various cylindrical model spacers. The distribution of surface charged is closely related to the normal component (gas side) E/sub n/ of electric field on the spacer surface. The ,maximum charge density can be estimated from the condition of E/sub n/=0. When voltage is applied in a polarity opposite to prestressed DC, surface charge increases the maximum field strength in the arrangement, resulting in the reduction of the insulating ability. It is possible to estimate the lowest flashover voltage due to surface charges only from numerical fields calculations. An anticharging spacer shaped along electric lines of force is proposed and studied. >

Strip lighting assembly

Abstract: A low voltage strip lighting assembly has an elongated flexible insulator strip having a base including two sides and a top surface having elongated slits that retain flat electrical conductors. A miniature wire terminal lamp bulbs are adjacent to the top surface of the insulator strip with terminals extending from the top surface of the insulator strip into a slit in the insulator strip to electrically contact one side surface of a respective flat conductor at any point along the length of the insulator strip, whereby the application of electrical power from an external source to each of the respective conductors will light the bulbs. A generally U-shaped lens covers the light bulbs and has legs engaging the sides of the insulator and inwardly-biased and secured to the insulator by the engagement of ribs into matching grooves in the insulator.

Light emitting apparatus

Abstract: An apparatus is disclosed that includes a plurality of conductors arranged in parallel, and a plurality of multi-light emitting diodes. One terminal of each of the multi-light emitting diodes is conected to one of the conductors, and the other terminal of each is connected to another conductor. The multi-light emitting diodes include a plurality of light emitting diodes which are connected in series or in parallel and are molded integrally, and are spaced at intervals in the longitudinal direction of the conductors. A flexible insulator covers the conductive portions of the multi-light emitting diodes. The conductors enable an electric current to pass to the multi-light emitting diodes to emit light. The apparatus is very flexible and compact, and can be powered by a portable power source.

Electrical properties of electrode/polyethylene/electrode structures

Abstract: The pressure-wave propagation (PWP) method allows for the nondestructive measurement of charge distributions in dielectric materials. This method has been used to study the electrical properties of electrode/polyethylene/electrode structures such as those involved in high-voltage cables. The influence of the composition of the insulating resin itself and that of the electrodes are analyzed in polyethylene samples. According to the chosen combination, charge transfer at the interfaces, migration of ionizable impurities, or a strong decrease of both is observed. This application of the PWP method is of particular interest, since it allows for a suitable choice of the materials and structures involved in insulator/conductor interfaces. >

Hygroscopic properties of pollutants on HV insulators

Abstract: The mechanisms of moisture absorption by an insulator surface contaminated with different salts is described A list of some salts depending on their hygroscopicity is given Measurements of surface conductivity and leakage current on flat insulator models at different relative humidities and with different types of salt were carried out At natural atmospheric conditions, the flashover voltage of cap and pin insulators subjected to 250/2500 mu s switching impulses was evaluated It is shown that the intensity of moisture absorption on insulator surfaces depends upon the chemical constitution of the pollution The effect of intense and continuous moisture absorption can cause long-lasting surface discharges at operating voltage or can be a reason for flashovers a short time after the voltage has been switched on to a line >

A study on the profile of HVDC insulators-DC flashover performance

Abstract: The results of flashover tests on insulator models with various geometrical configurations under DC field are presented. Dimensional parameters are used to describe the influence of the insulator profile on the flashover performance. The effects of both the surface contamination level and the geometrical shape on the arc propagation characteristics are investigated. It is shown that the flashover voltage of an insulator model under DC field is significantly influenced by the relative width and depth of the insulator underrib. It is also shown that the arc path under DC field is a function of both the geometry and surface contamination distribution. >

Equipment to carry out maintenanace operations, particularly washing, on insulator chains of high voltage electric lines

Abstract: The invention relates to an equipment for the maintenance and particularly for the washing of insulator chains supporting high voltage electric lines, which allows operation on a live line The washing of the insulator chain is performed through jets of washing fluid sprayed in close proximity to and over the whole contour of the insulators The equipment comprises a guide body movable along the insulator chain, a plurality of washing liquid spraying nozzles mounted on said guide body, and moving means to guide said body along the insulator chain This moving means consists of at least one track, rotating on two transmission rollers, at least one of these rollers being motor-driven and the active section of the track being in contact with the peripheral edge of the insulators of the chain According to the invention, the guide body surrounds the insulator chain, at a short distance from the same insulators; the track presents a plurality of tongue teeth, projecting perpendicularly from its outer surface The assembly of guide body and track has a plurality of contact points with the periphery of each insulator, distributed over the whole contour of the same insulators

Electrical cables and serpentine pattern shielding tape therefor

Abstract: An electrical cable is provided wherein a conducting member is surrounded by an insulator. Around the insulator is a shielding element which is configured such that a pressure sensitive or heat fusible adhesive is placed on a top surface of the shielding tape in a parallel, nested serpentine pattern. The shielding tape is positioned so that it surrounds the insulator and has an overlap of its top surface with its bottom surface. Due to the parallel serpentine adhesive pattern between the top surface and the bottom surface in the overlap, an effective seal against moisture is achieved, independent of the width of the individual adhesive strips. Similarly, contact between the metallic portion of the top surface and the metallic bottom surface achieves a barrier to electrical and electromagnetic interference within the electrical cable. The serpentine adhesive layer also functions to securely connect the shielding tape to the insulator.

Fault current limiter

Abstract: A current limiter according to the invention has two resistors, the first one consisting of a thin layer of a material which may become superconducting and the second one is a resistance layer applied on first resistor, the latter resistor layer being dimensioned such that its resistance becomes considerably lower than the resistance of the superconducting layer when it is not superconducting, however with a resistance of such a magnitude that, in the current circuit in question, it limits the current to a permissible value. For mechanical support of the resistors, an insulator is included in the current limiter, and the superconducting material is applied on insulator. The resistors and the insulator are arranged immersed into a cryotank.

Breakdown phenomena in high-power klystrons

Abstract: In the course of developing new high-peak-power klystrons, high electric fields in several regions of these devices have become an important source of vacuum breakdown. In addition, a renewed interest in breakdown phenomena for nanosecond-pulse, megavolt-per-centimeter fields has been sparked by recent work in the area of gigawatt RF sources. The most important regions of electrical breakdown are in the output cavity gap area, the RF ceramic windows, and the gun ceramic insulator. The experiments and results on the breakdown in these regions are discussed, as well as the solutions to alleviate this breakdown problem. >

Effects of spark conditioning, insulator angle and length on surface flashover in vacuum

Abstract: A flashover test was conducted in a vacuum gap, bridged by an insulator in the shape of a frustum of a cone. The test gap was subjected to a lightning impulse voltage of 2/50 mu s. The flashover voltages were observed for various cone angles and insulator lengths. Traces on the cathode due to flashovers were observed. A spark conditioning using voltage pulses of reverse polarity is shown to reduce the number of shots needed to obtain a stable flashover voltage. The flashover voltages depend strongly on the cone angle even in the case of the lightning impulse. The flashover fields depend strongly on the insulator length. The traces are distributed not only at the triple junction but also over a wide area of the cathode surface when the gap distance is shorter than 15 mm. This fact indicates the existence of two different flashover processes. >

Trap-dominated breakdown processes in an insulator bridged vacuum gap

Abstract: Measurements of coordinated, time‐resolved breakdown current and luminosity phenomena associated with a pulsed surface flashover event in an insulator bridged vacuum gap are presented. It was observed that the luminosity and current waveforms differ vastly in their temporal character. The luminosity profile has a sharp pulse with no counterpart in the current waveform. A significant afterglow activity is also observed after the cessation of the breakdown current. Further, the profiles of the luminosity and current waveforms changed with successive breakdowns. The rise times and decay times of the luminosity waveform, the time delay between the onset of the luminosity and current waveforms, and the rise time of the current waveform changed with successive breakdowns showing regular trends. The above modifications in the temporal profiles of luminosity with successive breakdowns are analyzed on the basis of carrier trapping and recombination processes within the localized levels of the forbidden gap associated with the insulator‐vacuum interface. The experimental results reported here qualitatively support the surface flashover model based on carrier trapping for low mobility, large band‐gap insulators. The analysis of the results is suggestive of a breakdown model in which hot‐electron generation culminates in impact ionization‐induced breakdown in the subsurface layers of the insulator.

Multiple core coaxial cable

Abstract: A multicore coaxial cable includes a conductor layer, an insulator layer surrounding the conductor layer, plastic tape surrounding the insulator and a metal shield surrounding the plastic tape. The exterior surface of the plastic tape is coated with a metal layer to contact the metal shield to reduce the electrostatic noises in the cable.

Optical fiber composite insulator

Abstract: An optical fiber composite insulator which including an insulator body having a through-hole formed in its axis portion and at least one optical fiber extending through the through-hole and sealed in the through-hole by an inorganic vitreous sealing material at both end portions of the through-hole. The sealing material has substantially the same thermal expansion coefficient as that of the insulator body, has high resistances against breakage in the power frequency voltage test and in the lightning impulse voltage test, against crack formation in the cooling and heating test, heat proof test and heat cycle test, and against the light-transmission loss.

The influence of surface phenomena on the initiation of discharges in vacuum

Abstract: Breakdown phenomena are initiated by surface conditions in a variety of systems ranging from high-voltage vacuum tubes and friction devices to distillation columns containing unstable molecules. Vacuum tube flashover is a particular case of breakdown attributed to an electron cascade occurring at the insulator surface and causing an electron-stimulated desorption. This model fails to provide an overview of surface-initiated breakdown phenomena or a satisfactory account of the conditioning and deconditioning processes on which flashover reproducibility depends. To further understanding of these phenomena, the authors examine the electrical and mechanical energy dissipation processes in insulators. The dissipation processes arise from the atomic, electronic, and vibrational characteristics of the insulating surfaces and illustrate the influence of point and linear defects on material properties. The authors propose a flashover model based on an electron excitation cascade developing in the surface layers where the defects are concentrated. Physical measurements for breakdown determination and surface treatments for improving dielectric resistance to breakdown under voltage are discussed. >

Electric breakdown under quasi-uniform field conditions and effect of emission shields in polyethylene

Abstract: Electric breakdown strengths were measured using a quasi-uniform field spherical electrode assembly for polyethylene with and without an emission shield between the electrode and the polyethylene surface. The results show that the emission shield suppresses carrier injection from the electrodes and enhances the breakdown strength. The short-circuit breakdown strength is always lower than the normal breakdown strength under DC voltage conditions. With the carrier injection from one electrode suppressed by an emission shield, polarity effects during breakdown were observed. These effects, for measurements using a quasi-uniform field electrode assembly, are similar to those measured using an extremely nonuniform field needle-plate electrode assembly. The AC breakdown strength measured under quasi-uniform field conditions is about the same as that measured under extremely nonuniform field conditions, and it is also not much affected by the DC bias voltage. All of the breakdown phenomena can be interpreted on the basis of K.C. Kao's et al. (1982) electrical discharge and breakdown model. One significant finding is that a suitable emission shield and a suitable method of reducing the energy released from nonradiative transition due to trapping and recombination may greatly improve the breakdown behaviour for high voltage polymeric-insulated cables. >

Investigations of pulsed surface flashovers for the triggering of pseudospark high-power switches

Abstract: Surface discharges over insulators in vacuum under rectangular high-voltage pulses are investigated. These discharges are among the methods used to trigger pseudospark switches. Low breakdown voltage and long lifetime are the requirements these insulators have to meet to be used as trigger materials. For this purpose the breakdown behavior of certain ceramics stressed by high-voltage pulses was investigated in an oil-free vacuum of 10/sup -5/ Pa. The erosion of the insulator by the discharge plasma was investigated with a scanning electron microscope. An evaluation of the measurement and a comparison of the properties of the investigated insulators show that SiN, Al/sub 2/O/sub 3/, and CaTiO/sub 3/ have low breakdown voltages and exhibit the lowest erosion rates. First results obtained with these materials as trigger insulators in pseudospark switches are promising with regard to the achieved lifetimes. A lifetime of four million triggered discharges under 90% switch current reversal was reached at a peak switch current of 25 kA without a serious reduction in trigger performance. >

Electrical measuring device

Abstract: The disclosed porcelain insulator has built-in optical transmission media extending therethrough from a base metal secured at one end thereof to a conductor-holding fixture at the opposite end thereof. The conductor-holding fixture may carry one or both of two type sensing units: namely, a sensing unit having a current transformer (CT) coupled to the phase conductor, a load resistor connected to the CT, and an electro-optic element coupled to both said load resistor and said optical transmission medium; and another sensing unit having a capacitive potential transformer (PT) coupled to said conductor and an electro-optic element coupled to both said PT and said optical transmission medium. An electric measuring device is formed by applying light to the electro-optic element through the optical transmission medium of the insulator, modulating the incident light by the electro-optic element depending on the line current or voltage, receiving the modulated light through the optical transmission medium of the insulator, converting the modulation of the received light into an electric signal. Due to information detection right by the line conductor and optical transmissioin of the information, ill effects such as electromagnetic interference and insulation deterioration at the spot of detection and along the information transmission path are eliminated. Thus, when used in transmission and distribution lines the device improves the safety and accuracy and maintains the improved level of safety and accuracy.

Current distribution during the breakdown in a coaxial electrode system

Abstract: The breakdown stage in a coaxial electrode system (similar to Mather‐type plasma focus) was studied in hydrogen for a wide range of pressures through the fast discharge of the energy stored in a parallel plate capacitor. A peak current Iep of 18.5 kA was reached in 60 ns for a 20 kV capacitor charge voltage, giving the typical initial current rise time obtained in normal plasma focus discharges. Thin current sheaths (≲2 mm thick) in the insulator region with current density ≳10 kA/cm2 were measured. These current sheaths always showed a well‐defined filamentary structure with the number of filaments depending linearly on Iep , with an observed current limit per filament If depending on the pressure P as I2f∼1/P.

Optical fiber built-in type composite insulator and method of producing the same

Abstract: An excellent optical fiber built-in type composite insulator including at least two insulator bodies each having a penetration bore, at least one optical fiber inserted in the penetration bores, and sealing structures for the penetration bores of the insulator bodies and for a joining layer of opposing end surfaces of adjacent insulator bodies, is provided, which effectively prevents leakage of inner silicone grease, bending and breakage of the optical fiber, leakage of electric current along the penetration bores, short circuited trouble, and destruction of the insulator bodies, improves joining strength of the opposing end surfaces of the insulator bodies, and maintains the joining strength for a long period, affords a change of numbers of the insulator bodies, and facilitates the production. A method of producing such composite insulator is also provided.

A study on the profile of HVDC insulators-mathematical modeling and design considerations

Abstract: The effects of underrib configuration on the flashover voltage of contaminated HVDC (high-voltage direct-current) insulators are analyzed. Mathematical models are developed to characterize the behavior of multiple discharges sustained along a contaminated surface. The effectiveness of the insulator leakage length is investigated by predicting the discharge path between neighboring underrib tips. An empirical equation is developed to estimate the effectiveness of the creepage length of a profile segment in terms of its physical dimensions. It is confirmed that the effective leakage length as a direct influence on the critical flashover voltage. A method for application of the effectiveness of the insulator leakage length for profile design considerations has been developed. The method yields the optimum number of underribs required for a given insulating space. >

High voltage insulator

Abstract: A high voltage porcelain insulator has a plurality of sheds extending laterally thereof at spaced apart locations therealong. The creepage path length of the shedded insulator is extended by mounting polymeric insulating creepage extenders on the porcelain sheds. The creepage extenders do not extend completely around the periphery of the sheds but leave a gap. Preferably the gaps of adjacent creepage extenders along the insulator are not aligned.

State-of-the-art insulator and electrode materials for use in high current high energy switching

Abstract: An investigation into the failure of ceramic insulators that are used in a surface discharge switch (SDS) was conducted. The materials analyzed are Al/sub 2/O/sub 3/-25% SiC, MTF (modified alumina titanate), and CZA 500 (zirconia-alumina composite) ceramics. These insulators were subjected to high-current ( approximately 300 kA) surface discharges in atmospheric air and nitrogen. Energy-dispersive X-ray surface analysis was performed on the insulator surfaces in order to determine the contaminants that are present and the possible failure modes associated with the plasma arc environments mentioned above. Electrode erosion rates have been measured as a function of total charge transfer (up to 50 C/shot) for several in-situ materials including Cu-Nb, Cu-Nb+LaB/sub 6/, and Cu-Ta. Results from comparisons with standard Cu and CuW materials indicate that the in-situ materials represent an efficient method of retaining the copper in the bulk until it vaporizes and thus yield significantly lower erosion rates at high Coulomb transfer rates. >

The influence of the insulator surface in the plasma focus behavior

Abstract: The insulator (Pyrex glass pipe in our system) surface alteration suffered due to successive plasma focus discharges was found to be responsible for the improvement in stabilization in the plasma focus behavior. The development of microscopic conductive sites (∼1 μm in size), observed on the insulator surface due to the accumulation of successive discharges, increases the efficiency, by a metal‐insulator‐metal process, on current sheath buildup with an increment in current density during the breakdown. The influence of the surface in the early stage of the discharge, and its correlation with the intensity of the pinch, was studied by analyzing the Si concentration on the surface of the targets of AISI 304 exposed to the plasmas and ion beams generated in the discharge for different experimental situations.

Particle-initiated corona and breakdown in GITL systems

Abstract: The authors discuss corona and breakdown in compressed sulfur hexafluoride (SF/sub 6/) gas-insulated transmission line (GITL) equipment due to free conducting particles. For particle-contaminated conditions, dynamic models of particle movement and particle-initiated corona and breakdown have been developed. Moreover, a method is proposed to utilize externally measured quantities (for example, time to first corona, corona magnitude, and repetition frequency) during testing and commissioning for identifying the size of contaminating particles. >

Ageing of rotating machines insulation-mechanisms, measurement techniques

Abstract: The long-term ageing of insulator models until breakdown was investigated with repetitive diagnostic measurements. The physical mechanisms in models with isolated defects were examined, and improved diagnostic tools were developed on the basis of the ageing experiments. Experimental results on the properties of a mica paper, glass fiber, and epoxy resin composite are presented. The authors discuss the effect of temperature on ageing and especially on defect induction by thermomechanical forces. >

Low voltage operation a-Si:H thin film transistors with very thin PECVD a-SiO2 gate dielectric

Abstract: In the present paper are shown the characteristics of devices succesfully employing low temperature plasma deposited a-SiO2 as gate insulator with thickness of 40 and 18 nm. They show a very sharp field effect conductivity onset and an on/off current ratio of about 8 orders of magnitude. Due to the very low insulator thickness it is possible to operate these devices in the range between -2 and 5V with on current of the order of 10−5A.

Optical fiber composite insulator and method of producing the same

Abstract: An optical fiber composite insulator comprises a hollow insulator body having an axial through-hole and at least one optical fiber extended through the through-hole and hermetically sealed to the inner surface of the through-hole by a sealing material. A relationship between a condition of the inner surface of the through-hole and the sealing material, a relationship between the inner diameters of the through-hole and the outer diameter of the hollow insulator body or a coating material on the optical fiber are selected to provide high insulating property, mechanical strength and airtight property of the optical fiber composite insulator.