Flashover phenomena in positive rod-to-plane air gaps under impulse and DC voltages

Discharge phenomena in positive rod-to-plane air gaps under lightning impulse and dc voltages were investigated. Under impulse voltages, the 50 percent flashover voltages agree with the 50 percent corona inception voltages only in a gap shorter than a certain value d_c which increases with the rod diameter φ. The 50 percent flashover voltage at d_c rises at a rate of about 14 kV/cm with d_c in the range of d_c < 2.8 cm (φ ? 1.5 cm), and then it rises at a rate of about 5 kV/cm. Experimental results show that the flashover process in the former region (termed the Gp process) is different from that in the latter region (the Lp process). Under dc voltages, the critical gap length, at which the first corona may lead to flashover without any succeeding streamer corona, corresponds to d_c under impulse voltage. In a gap shorter than d_c flashover occurs through the Gp process (φ ? 1 cm) or Lp process (φ ≥ 2 cm), but in gap longer than d_c flashover may occur through another process which cannot be seen under impulse voltages. In this paper, those flashover characteristics and processes are described on the basis of experimental results and photographic observations with an image converter camera.     

Estimation of dc breakdown mechanisms in air gaps containingfloating metallic particles

The characteristics of breakdown voltages against the position of floating metallic particles in atmospheric air are investigated experimentally. The breakdown mechanism is estimated on the basis of electric field calculation and visual observation of discharges at different voltages and a set of flow charts for calculation of breakdown voltage is proposed. The studied gaps are needle-to-plane and sphere-to-sphere electrodes of 120 mm in length forming Poissonian and Laplacian fields respectively and the shape of tested floating objects is a needle or sphere. The theoretical breakdown voltages obtained by the flow charts agree reasonably with the experimental ones     

Breakdown characteristics in air gaps with artificial floatingmetals under DC voltage

The DC breakdown characteristics of a sphere-to-sphere gap with conducting objects supported by an insulator rod in atmospheric air are investigated experimentally to provide fundamental parameters determining the breakdown voltage in the presence of floating objects around DC power apparatus by an air insulation. The shape of tested floating objects made of metal is plate, needle or sphere. Experimental results show that the main factors affecting the breakdown voltage are the shape and size of floating objects, the object location and the gap length. If the object is placed at the location giving the lowest breakdown voltage, any object lowers the breakdown voltage below that without the relevant objects     

Impulse breakdown of insulators and air gaps of similar electrodeconfigurations: effects of temperature

Comparative measurements are reported on the breakdown, under lightning and switching impulses, of a concentric electrode arrangement in which the inter-electrode space is spanned either by an insulator surface or by air. Temperature in the system has been varied in the range 10m, where 0.81.0 for negative polarity at the highly stressed electrode. Variations are the same, in the presence or absence of the insulator surface, but absolute positive flashover voltages are lower across the surface, than in air, while negative flashover voltages are higher. Times to breakdown decrease with increasing temperature     

Electrical breakdown of small gaps in vacuum

The comparison of the characteristics of an electric arc breakdown in vacuum between two silver or silver-nickel alloy convex electrodes, enabled us to show experimentally that the process initiating discharge in vacuum is the electronic emission at the micrometer scale. Increased content of nickel in the alloy improves the insulation and achieves high resistance to erosion for low power arcs. Heating in vacuum demonstrates the multiple layer structure of studied alloys. Secondary electron microscopy and energy dispersing spectroscopy of the electrode surfaces after breakdown, as well as the measurements of fall and delay times, enabled us to evidence the origin of the electric arc     

Influence of sand/dust contamination on the breakdown ofasymmetrical air gaps under switching impulses

This paper discusses the influence of airborne and surface-adhering sand/dust contamination on the breakdown characteristics of asymmetrical air gaps when subjected to standard switching impulses of both polarities. Extensive measurements of breakdown voltages and breakdown times are carried out under clean and polluted conditions using rod-plane and sphere-plane gaps. When positive switching impulses are applied to gaps ⩽1.8 m, the presence of sand/dust particles ⩽150 μm diameter and ⩽2 g/m³ concentrations have insignificant influence on the breakdown characteristics of the air gaps. However, the presence of such particles can greatly affect the breakdown characteristics for negative switching impulses. Under such conditions, the particles can decrease V₅₀ by ⩽40% for gaps with field factor f ⩽3.5. For gaps with 3.550 by ⩽80%. For more nonuniform field gaps, V₅₀ values for clean and contaminated conditions are similar. The particles can also significantly influence the breakdown time lag characteristics under negative switching impulses. The observed effects are primarily caused by sand/dust particles which settle on the cathode, while the particles present in the interelectrode gap or on the anode have very little influence     

Impulse partial discharge and breakdown characteristics of rod–plane gaps in air and N2 gases

In response to growing environmental concerns, we attempted to develop switchgear without using SF₆ gas. In our research, we used compressed air and pure N₂ as an electrical insulation gas, because of their low global warming potential. In this paper, we examined the impulse breakdown and impulse partial discharge characteristics under various conditions related to nonuniformity of the electric field. The experimental results show that the breakdown voltage (BDV) of air is higher than that of pure N₂ gas under highly nonuniform field conditions in the rod–plane gap. On the other hand, the discharge inception voltage of air and N₂ were almost the same. Furthermore, first partial discharge (PD), leader discharge, and its transition to the breakdown were successfully observed through the measurement of discharge current and light emissions under impulse voltage application. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(3): 36–43, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10277     

Negative DC corona discharge effects on the ultraviolet photon count in rod to plane air gaps

In order to investigate the effects of negative DC corona discharge on ultraviolet (UV) photon count, a corona discharge measurement system based on rod to plane air gaps was established. The variations of positive ion, negative ion, and electron densities were calculated with a fluid model, and the generation process of photons during negative corona discharge was investigated. The differences of photon count and the variation of charged particles between negative and positive corona were also compared. The corona current, Trichel pulses, and corona‐generated photons were measured with increasing applied voltage. An approximate parabolic relationship found to exist between the photon count and the corona current, and also an ideal quadratic function was found to exist between the photon count and the frequency of Trichel pulses. These results provide a solid foundation in the application of UV imaging detection of negative DC corona discharge in power equipment. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Conducting particle motion and particle-initiated breakdown in dcelectric field between diverging conducting plates in atmospheric air

This paper deals with the analysis of spherical conducting particle motion as well as particle initiated breakdown in electric fields between diverging conducting plates with dc voltage in atmospheric air. Motion of spherical particle was estimated by solving the motion equation numerically, and the results agreed well with the experimental ones. It was found that when the particle is placed on the horizontal electrode surface where the electric field is nearly equal to the lifting field, the particle progresses towards a higher electric field region by the effect of Coulomb force and electrical gradient force. This is true for a nonspherical conducting particle. The obtained results for particle motion suggest that much attention should be paid in the design of GIS particle traps to forces acting on the particle directly towards the higher electric field region. Moreover, the particle-initiated breakdown is discussed on the basis of the experimental and theoretical results. It was found that one of the reasons for reduction of the breakdown voltage is the effect of microdischarge between the particle and an oppositely charged electrode     

Formation mechanism of surface corona on dielectric plates under negative impulse voltage in atmospheric air

The formation mechanism of the surface corona on dielectric plates under negative impulse voltages has been investigated with a high-speed gated image intensifier in atmospheric air. An acrylic plate was inserted perpendicularly to the axis of a rod-plane electrode system, and as a backside electrode, a translucent electric conductive film was used to observe the luminosity of the surface corona. The size and shape of the negative surface corona were measured using acrylic plates of 1, 2, 5 and 10 mm thick and rod electrode with diameters of 10, 20, 50 and 125 mm. Experiments show that the negative surface corona occurred just after the impulse voltage application and formed a ring-like luminescence. The diameter of the ring increased with the thickness of the acrylic plate and the diameter of rod electrode. The formation mechanism of the negative surface corona is studied using a numerical analysis of the electron avalanche developing along the electric line of force around the rod electrode. As a result, it is found that the occurrence positions of the surface corona agree with the theoretical computed points of the maximum linear density of positive ion.     

Comparison of the breakdown of rod-plane gaps with floatingelectrode

The possible influence of a metal body in a lightning protection system was determined by comparing the electrical breakdown probability of two symmetrical sphere-rod gaps, one conventional and the other with a floating electrode, i.e. a rod with a series gap introduced in its conducting path. 50 switching voltage impulses were applied to the arrangement for each series-gap length. The breakdown probability was obtained for each rod as a function of the series-gap length. The results show that the rod with a series gap shows the same breakdown probability as the conventional rod for a certain limited range of the series-gap length. Beyond this range the conventional rod has always a higher breakdown probability. The analysis of the actual breakdown voltage in different configurations has shown some differences in the physical process that lead to breakdown in different arrangements     

Comparisons of direct-current breakdown characteristics in dry air and SF6 gaps using a parallel-plane arrangement with variable height protrusion

Breakdown voltages in uniform and quasi-uniform field gaps are sensitive to the presence of small protrusion on the electrode surface in SF₆ at high pressures. The aim of the present work is to study direct breakdown and corona stabilized breakdown for the transitive region from uniform to nonuniform gap in dry air and SF₆ at low pressures up to a critical pressure when direct breakdown takes place by a leader discharge crossing the gap in SF₆. In a parallel-plane gap with a variable-height protrusion subjected to the dc voltage, corona onset voltage is remarkably controlled by the protrusion height. The present electrode arrangement has the advantage of directly measuring the minimum critical guiding field strength for the propagation of a streamer discharge at corona onset. The experimental observations have been explained qualitatively on the basis of a streamer model and precise electric field calculations of gap.     

Thermionic arc breakdown in small discharge gaps: model andapplications

A model for thermionically induced breakdown is developed which predicts the voltage levels experienced in low-voltage discharges. Initially, electron emission across an active discharge minigap must be established. Prior to breakdown, emitted electrons populate the cathode space-charge region and the remaining gap is unionized. Breakdown criteria and a means for estimating the reduced breakdown voltage in the bulk gas are derived in terms of conventional Townsend ionization mechanisms, wherein multistep or cumulative ionization must be active. Since the cathode space-charge region is replete with emitted electrons, positive ions traversing it release to the bulk gas considerably more electrons than would be available with the conventional Townsend γ mechanism at the cathode surface. This equivalently higher γ significantly shifts the Paschen curve toward lower minima. One practical thermionic assisting device is a tungsten light-bulb-grade filament, which shunts the primary discharge electrodes and, within the filament's minigaps, breakdown occurs. Thermionic arc breakdown has been implemented in the starting of arc devices for lamps and in thermionic power converters; an application to ionization chambers in electrostatic thrusters is also discussed     

Streamer propagation and breakdown under ac voltage in very largeoil gaps

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