An electrical breakdown model for liquids in response to a submicrosecond (∼100ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing microbubbles. Impact ionization within the microbubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the prebreakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission.
In a coaxial test apparatus enabling the measurement of voltage and current at the test gap, dc conduction and breakdown in transformer oil caused by the application of dc voltages are investigated. Current measurements cover the range from 10." A to 1 kA, with temporal resolutions of milliseconds at the lowest current amplitudes to sub-nanoseconds for currents larger than IO4 A. The dc currentlvoltage characteristic for sub-breakdown voltage amplitudes point to the injection of charge carriers, allow us to characterize the transport mechanisms, and the influence of space charges. For voltages approaching breakdown thresholds, quasi dc-currents rising from nanoamperes to microamperes are superimposed by current pulses with amplitudes of milliamperes and above, and durations of nanoseconds. The onset of these current pulses occurs up to 10 ps before breakdown. One of these current pulses reaches a critical amplitude causing voltage breakdown and current rise to the impedancelimited value within 2 ns. Additional optical diagnostics using photomultipliers and high-speed photography with gated microchannel plates yield information on hydrodynamic ' processes and charge carrier amplification mechanisms associated with the current pulses and fmal breakdown, such as bubble formation, as well as on the development of the spark plasma finally bridging the gap.
The breakdown physics of transformer oil is investigated using high speed electrical and optical diagnostics. Experiments are done in self-breakdown mode utilizing a needle/plane geometry. Shadowgraphy combined with high-speed electrical diagnostics are aimed at measuring streamer expansion as a function of external pressure. Assuming a breakdown mechanism for negative needle based on bubble formation with subsequent carrier amplification in the gas phase implies a pressure dependence, which is observed in the experiments, i.e. the expansion velocity decreases with increasing pressure.
pact pulsed power and switching systems. An insulating fluid such as transformer oil is a critical component in With a fast coaxial setup using a needle/plane geometry high voltage and pulsed power systems. The desire for and a high sensitivity electrometer, conduction mecha-compact pulsed power systems drives research to develop nisms in transformer oil at varying temperature and hy-a model that is capable of describing the basic processes drostatic pressure are quantified. There are 3 stages in the of liquid breakdown, similar to the promotion of the conduction process prior to breakdown for highly non-model for gaseous breakdown almost a century ago. A uniform field geometries. Stage I is characterized by a descriptive model of conduction currents leading to diresistive current at low fields. Stage II consists of a rapid electric breakdown will aid the development of more effirise in the injection current associated with increasing cient switches and improved insulating systems. field due to a "tunneling" mechanism through the This paper discusses experiments on the basic conducmetal/dielectric interface. The transition from the resistive tion mechanisms associated with liquid dielectrics for to tunneling stage occurs when the applied field reduces non-uniform and uniform applied fields. Past studies the barrier at the metal/insulator interface to a point where have done similar tests but at much smaller gaps or only tunneling of charge carriers through the barrier begins. for a single geometry [1,2]. The variation of electrode This transition point is polarity dependent. In stage III, at material tests the conduction current as a result of varying high fields the current reaches space charge saturation at metal/liquid interfaces. Also, changes to the pressure clarelectron mobilities >100 cm2/V*s prior to breakdown. ify these mechanisms. The data leads to the explanation of The processes of final breakdown show distinct polarity the conduction current and how it evolves with applied dependence. Data for the negative needle exhibits strong voltage. Also, experiments on the investigation of the pressure dependence of the breakdown voltage, which is basic breakdown mechanisms as a function of reduced reduced by 5000 if the hydrostatic pressure is lowered hydrostatic pressure are performed. Optical diagnostics from atmospheric pressure to hundreds of mtorr. Such a combined with fast electrical measurements strongly supstrong pressure dependence, at reduced hydrostatic pres-port a bubble or vapor model for cathode initiated breaksure, indicates breakdown is gaseous in nature. This is down. Data presented reveals that cathode initiated breaksupported by images of bubble/low density regions form-down is strongly pressure dependent, while anode initiing at the current injection point. Positive needle dis-ated breakdown is less dependent upon the pressure. charges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. A weak pres-II. EXPERIMENTAL SETUP sure dependence indicates the break...
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