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We develop a criterion for spark-breakdown in non-uniform field geometries with positive polarity and small electrode separations so that breakdown evolves without the formation of a leader. We arrive at the spark-breakdown criterion by framing it in terms of gain and instability conditions, whose relative importance is established from an analysis of the experimental breakdown characteristics and correlations with streamer behavior in short gaps. Results are presented in the context of two generic geometries having coaxial and point-plane electrodes. For nearly uniform field situations, we re-confirm that the breakdown criterion obtained by the usual extension of either the Townsend or Meek criteria satisfactorily predicts the experimental results. On the other hand, for increasing non-uniformity, the results for the corona and spark branches of the breakdown characteristics are shown inconsistent with a breakdown criterion solely based on either the Townsend or streamer mechanisms. In particular, the avalanche gain factor, the primary component of the Townsend and streamer criteria, does not determine the spark breakdown criterion. Streamers can cross the gap for a significantly wide range of applied voltages without triggering a spark. We find that it is the instability condition, derived from a relation between the minimum Laplacian field in the gap and the local streamer body field (which we relate to the streamer sustaining field), that is sufficient for determining the spark threshold thereby yielding a breakdown criterion. [ABSTRACT FROM AUTHOR]

For the safe design and operation of wet-mate (WM) DC power connector, a time-dependent full thermo-electrodynamic model comprised of Poisson's equation, three charge continuity equations-one each for the positive and negative ions and one for the electrons-, and a thermal diffusion equation is developed to study the streamer initiation and propagation in the oil portion of a WM DC chamber. The electric field dependent molecular ionization mechanism accounts for the source term for free charge carriers, and positive ion/electron recombination, positive/negative ion recombination and electron attachment represent sink terms in the oil section. The solid portion of the WM DC connector is modeled as a perfect insulator. Considering a needle-sphere electrode geometry with electrodes covered by a dielectric solid and oil enclosed by the dielectric solid, it is approached the complicated solid-liquid insulation system envisaged in a WM DC connector after mating. By using the model, the influence of three parameters including magnitude and rise time of applied voltage as well as the type of oil on streamer initiation and propagation is investigated. It is found that the shorter rise time the more prominent streamer growth in the oil portion. For oil comprising only aromatics, an electric field magnitude larger than about 2×108 V/m is needed to propagate streamers, while this value for the oil comprising naphthenics/paraffinics will be exceeding 4×108 V/m. [ABSTRACT FROM PUBLISHER]

Breakdown characteristics of air gaps under negative switching impulse are complicated. To better understand the nonlinear discharge characteristics of long air gaps, the 50% breakdown characteristics of rod-rod gaps, rod-plane gaps and rod-conductor gaps, with air gap distance ranged from 1 m to 10 m, are studied thoroughly, with two waveforms of negative switching surges. Saturating characteristics have been obtained for the above three typical gaps. For air gaps exceed 4 m, the 50% breakdown voltage appears to be lower for rod-plane gaps than that for rod-rod gaps, while the average breakdown electric field decreases with the length of air gaps. Gap factors under negative impulses tend to level off with gap spacing. Furthermore, new knowledge is obtained indicating that, the 50% breakdown voltage for rod-conductor gaps may be lower with conductor grounded than that with conductor ungrounded. Finally, it also indicates that the wave front of the applied voltage has great influence on the breakdown time of rod-rod air gaps. [ABSTRACT FROM AUTHOR]

An electrothermal hydrodynamic model is presented to evaluate effects of the applied lightning impulse voltage parameters such as polarity, magnitude, and rise time on the initiation and propagation of the streamers formed in an IEC defined needle-sphere electrode geometry filled with transformer oil. Instantaneous velocity, column diameter, head curvature, maximum electric field, and the volume charge density have been investigated as the main characteristics of the streamer. Modeling results indicate that greater applied voltage peak amplitudes form streamers with higher velocity, greater head curvatures, and thicker columns. The bushy negative streamers usually initiate at almost twice the applied voltage magnitude and propagate slower than filamentary positive streamers. Results also show that in transformer oil at the same impulse voltage peak amplitude, shorter rise times create thicker positive and negative streamers. [ABSTRACT FROM PUBLISHER]

Negative discharge induction experiments were performed with a rod-to-rod electrode configuration having a gap length of 0.8 m using a high-power XeCl excimer laser of maximum output energy of 2 J/pulse and an impulse generator of maximum voltage of 1.2 MV. Development of a leader in the laser-ionized plasma channel was observed by using an image converter camera. When a XeCl excimer laser is applied at a carefully controlled instant, the guiding effect for an impulse discharge is optimized. Three main results were obtained. First, the discharge started near the focal point and then developed bidirectionally toward the high-voltage electrode and the grounded electrode. Midgap leaders were frequently observed. Second, it was observed that the lifetime of the laser-ionized plasma channel was about 1 Ps. Third, the discharge processes were classified into six stages. The mechanism of the streamer and leader propagation is discussed. [ABSTRACT FROM AUTHOR]

The nanosecond single-pulse voltage was employed here to generate a large-volume discharge between the wire-plane electrodes in nitrogen. In order to find out the effect of pulse polarity on the propagation of a large-volume discharge, both the positive and negative pulses were applied to the wire-plane electrodes, respectively. Images presented here reveal that the discharge channels are more homogeneous under the negative pulse, and the propagation speed of the discharge channels is faster under the positive pulse. [ABSTRACT FROM AUTHOR]

The dynamics of an atmospheric pressure air discharge in a point-to-plane geometry with a dielectric layer obstacle on the discharge path is investigated numerically for different applied voltages. Whatever the polarity of the voltage applied, first, a streamer discharge of the same polarity ignites at the point and propagates towards the dielectric layer. After the impact on the dielectric surface, the streamer discharge spreads along the upper dielectric surface and charges it positively or negatively depending on its polarity. On the bottom surface of the dielectric layer, charges with an opposite polarity are deposited. Surface charges on both faces of the dielectric layer are shown to have a significant influence on the discharge reignition for a negative applied voltage, but not for a positive one. Furthermore, it is shown that the dynamics of the discharge reignition below the dielectric layer depends on the polarity of the applied voltage at the point electrode. For a positive applied voltage, the reignited discharge is a positive ionization wave propagating towards the grounded plane. For a negative applied voltage, a double headed discharge is observed with positive and negative fronts propagating in opposite directions. Finally, the minimal value of the ionization integral to have a discharge reignition below the dielectric obstacle is found to be less for a negative applied voltage than for a positive one. [ABSTRACT FROM AUTHOR]