Surface flashover in compressed-gas insulated systems is a much studied, but poorly explained phenomenon. In this paper we review the literature of surface flashover with primary emphasis on the understanding of physical processes leading to discharge initiation and insulator flashover under high voltage excitation. The flashover models presently in vogue will first be discussed, followed by the results of some recent experiments which are likely to have an impact on further modeling. Included in this context are phenomena such as ionization, surface charging, partial discharges, optical activity, and gas/dielectric interactions. Finally, the influence of system parameters such as insulator size, shape, surface condition, triple junction geometry, voltage waveform, gas formulation and particle contamination are discussed with regard to their effect on the flashover characteristics. Mechanisms are suggested in an effort to provide a physical explanation for the observed phenomena. Although the physics of the discharge initiation and propagation processes are presently not well understood, and the present models only account for a few of the mechanisms known to be important in the discharge development, all the work points to an interaction between the spacer and the various electron/photon processes in the surrounding gas volume. This interaction has not been accounted for in the discharge models proposed to date. Further modeling work should incorporate these interactions and the intrinsic properties of the dielectric ric which are related to these interactions. More basic research is suggested to provide a better understanding of the physics of the discharge initiation and breakdown phenomena.

Mechanisms of Surface Flashover Along Solid Dielectrics in Compressed Gases: a Review

This report intends to reveal the role of electron migration and its effects in triggering direct current (DC) surface flashover under temperature gradient conditions when using epoxy-based insulating composites. The surface potential and the surface flashover voltage are both measured using insulators that are bridged between two thermo-regulated electrodes. The space charge injection and migration properties under different temperature are detected. The results show that the surface potential rises significantly because of electron migration near the high voltage (HV) electrode under high temperature conditions, thus creating an "analogous ineffective region". The expansion of this "analogous ineffective region" results in most of the voltage drop occurring near the ground electrode, which serves as an important factor triggering positive streamers across the insulation surface. This work is helpful in understanding of DC surface flashover mechanism from a new perspective and also has important significance in design of a suitable DC insulator to avoid surface flashover problem.

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The potentially neglected culprit of DC surface flashover: electron migration under temperature gradients.

TITLE UV RADIATION TRIGGERED RAIL-GAP SWITCH INVENTIORSRoderick S. Taylor Kurt E. Leopold A. John Alcock ABSTRACT OF THE DISCLOSUREThe UV radiation triggered rail-gap switch applies a high volt-age from a low impedance source to a low impedance load. The switch includes first and second parallel elongated electrodes spaced to form a uniform gap along their length. The first electrode is adapted to be connected to the high voltage source, while the second electrode is adapted to be connected to the low impedance load. When a voltage from the source is applied to the electrodes, one electrode will be positive relative to the other electrode. In addition, the cross-section of each of the electrodes is sufficiently smooth to prevent points of high field concentration between the electrode. This cross-section is defined by the field enhancement factor of each electrode which is preferably less than 1.5. The switch also includes an enclosure in which the electrodes are located and which contains a gas mixture for maintaining the break-down threshold between the electrodes. A preferred gas mixture includes Ar, N2, and SF6 at a gas pressure selected to prevent breakdown in the switch until triggered by a UV radiation source. The W radiation source directs a beam of radiation substantially parallel to the pair of electrodes, preferably near the positive electrode, for initiating multi-channel, sub-nanosecond jitter, breakdown in the gap between the elec-trodes. The UV radiation may be obtained from either an incoherent radiation source or an UV laser source, but should be of short duration in the form of a narrow beam which is uniform along its cross-section.

Uv radiation triggered rail-gap switch

Flashover is a crucial issue in both high-voltage engineering and surface physics. It not only challenges the existing theories about its dynamic evolution, but also inhibits the clean energy revolution by limiting the accessible voltage rating of power equipment. It is of significance to elucidate the microscopic process along the interface to improve the flashover performance. In the present study, the synergic effect of adsorbed gas and surface charging is investigated, which reveals a long ignored factor for determining the flashover voltage. Depending on the relative amount of adsorbed gas, the flashover voltage varies, which exhibit different behavior from the bulk breakdown of the same gas. The amount of N2 gas adsorbed on epoxy resin (EP) surface is much larger than that on Al2O3 ceramic surface, corresponding to the observed higher flashover voltage on EP. It is proposed that the adsorbed gas molecules not only modify the local surface charging state via their interaction with the trapped charges, but also capture free electrons due to the distortion of their electronic distribution. Both effects suppress the free path length of electrons in the gas-solid interface. This work explores another possibility to improve the surface flashover performance.

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Synergic effect of adsorbed gas and charging on surface flashover.

The effect of a dielectric plate on characteristics of discharges in air from a point electrode was investigated experimentally Experiments have been carried out with either 250/2500 mu s switching impulse, or DC voltage applied to the point electrode in the presence of a Plexiglas, (PMMA) dielectric plate The dielectric plate was charged positively, irrespective of the polarity of the applied impulse voltage The breakdown probability was measured for different crest voltages The spatial distribution of the accumulated charge was measured for positive and negative impulse voltages With DC applied voltages, the charge accumulated on the dielectric plate had the same polarity as the corona ions migrating from the point electrode The repetition rate and charge content of Trichel pulse corona was measured for points stressed by negative DC voltage The repetition rate and charge content were measured for positive onset streamer pulses All measurements are discussed in the light of gas-discharge physics >

Discharges in air from point electrodes in the presence of dielectric plates-experimental results

The effect of dielectric surfaces on the breakdown voltage of gas spark gaps were investigated both theoretically and experimentally Experiments have been carried out using N2 and other gases at pressures ranging from one to five atmospheres Several different dielectric materials were used as substrates, having plain and cylindrical surface geometries The observed results were interpreted by a physical model which was developed for this purpose In the model, the major physical processes, ie, electron production and electron loss due to diffusion, were treated in the presence of the dielectric surface In addition, electron loss due to electron traps on the dielectric surface has also been taken into consideration The model yields an analytical expression for the breakdown voltage of a given gas spark gap For the cases studied, the model predictions fit the observed results reasonably well

S. T. Pai

Papers

Filamentary breakdown of gases in the presence of dielectric surfaces