Electrical breakdown of solid dielectrics and rocks on the trailing edge of a voltage pulse

Abstract

622.243An analysis of modern notions of breakdown in condensed media opens up new possibilities for a decrease in the microsecond voltages for solid dielectric breakdown. Investigations of solid dielectric and rock breakdowns on the trailing edge of a voltage pulse in a sharply nonuniform field demonstrate that the pulse amplitude and the slope of the working voltage pulse edge can be decreased significantly.Keywords: breakdown initiation, breakdown on the trailing pulse edge, dielectrics.When a microsecond voltage pulse is applied to the electrodes located on one solid dielectric surface embedded into a liquid, the discharge channel develops inside the solid dielectric, that is, the discharge channel penetrates into the dielectric [1,2]. This phenomenon provides the basis for a number of technologies used to destruct rocks and artificial materials, including drilling, cutting, crushing, utilization, and destruction of concrete and ferroconcrete parts of machines, mechanisms, and constructions. Electropulse (EP) drilling of rocks is most widely used.Among the main factors hindering the application of the electropulse technology are strict requirements on the voltage pulse parameters, including very high pulse amplitude and small pulse edge duration (τ ≤ 1 μs), that is, high slope of the pulse edge A = U m /τ, where U m is the voltage amplitude.Attempts to decrease the working voltage were repeatedly undertaken, because this allowed the overall dimensions and mass of the pulse voltage generator to be decreased and the reliability of the generator and high-voltage EP devices, for example, a drill to be increased.Proceeding from the behavior of the volt-second characteristics of solid dielectrics and liquid media, the voltage U can be decreased by increasing the pre-breakdown time. In this case, however, the probability of discharge channel penetration into the solid body also decreases together with the destruction efficiency. It is well known that the increased contact area of the electrodes also decreases the breakdown voltage [3,4]. However, this method can be used to increase the contact area only to a certain critical value, because undestroyed regions of the solid body retained under the electrode hinder its further drilling [5].Let us briefly consider the modern notion of the breakdown in condensed media, in particular, in solid dielectrics upon exposure to pulsed voltage. The physical and chemical processes proceeding in the gap from the moment of application of the potential difference to the electrodes to sparking of the discharge channel over the gap are conventionally subdivided into two stages: discharge ignition, that is, formation of the initial part of the channel at the potential difference U 0 and then its sparking over the gap when the potential increases from U 0 to the breakdown value U [6].The initial voltage U 0 is taken to be the voltage at which a light flash arises in a local region of the strong field. The flash is due to an electrodeless dielectric breakdown for a proper field strength w...

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Electric strength of dielectrics under influence of bipolar voltage pulses of submicrosecond duration

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