Prebreakdown Events Research Articles

Space Charges as Pre-breakdown Phenomena in Solid Dielectrics: A Concise Approach and Some Critical Comments

This short review deals with some aspects of space charges in solid dielectrics, and in particular in polymeric materials. The relationship between space charges and pre-breakdown events is discussed and the importance of space charges for the breakdown of solid dielectrics is emphasized.

Optical high voltage breakdown prediction using thermal lensing effect in transformer oil

We describe an optical system to monitor microscopic pre-breakdown events in liquid insulation. The system has successfully demonstrated its ability to predict high voltage breakdown in transformer oil. A simple theory based on a thermal lens build up between electrodes as a result of applied voltage is presented to explain the system operation and obtained measured results.

Effect of electrode ice layer on prebreakdown current in liquid nitrogen

The effect of ice on dc prebreakdown events was investigated using a needle-to-plane electrode system in liquid nitrogen at 77.3 K. It was found that H/sub 2/O ice has marked effects on prebreakdown current, corona onset voltage and breakdown voltage. The breakdown voltage at atmospheric pressure depends on microscopic cracks in H/sub 2/O ice.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A study of the inception of positive streamers in cyclohexane and pentane

This paper presents an experimental study of the inception of positive streamers in point-to-plane geometry under pulsed voltage. The different pre-breakdown events (slow and filamentary streamers) are characterized via sensitive transient current recordings, obtained using a new differential measurement technique described in the text. The roles of the tip radius, voltage pulse rise time, gap distance and hydrostatic pressure are systematically investigated in two liquids, cyclohexane and pentane. We found two parameters to characterize inception of both streamer types in a liquid: a unique initiation field for slow and filamentary streamers (depending slightly on the point tip radius of curvature); and a threshold propagation voltage for filamentary streamers (depending on the gap distance). There is also a critical point-tip radius above which no slow streamers are observed. It is shown that the initiation field and the propagation threshold can be accurately determined only in very specific conditions, namely below the critical point-tip radius and with fast rise time voltage impulses (about 10 ns). The presence of a gaseous phase in the initiation process is identified via the influence of hydrostatic pressure. Finally, the significance of the critical point-tip radius and that of the propagation voltage of filamentary streamers are discussed by comparison between the streamer and a conducting filament.

An optical study of prebreakdown events in AC-stressed n-hexane

For a point-to-plane gap immersed in n-hexane it is shown that discrete prebreakdown streamers can be generated and recorded when 60 Hz AC is applied. The streamers are shown to be similar to those seen under impulse conditions. Simultaneous optical and electrical measurements indicate the existence of a correlation between conduction current pulse activity and streamer development. Two distinct types of streamer are observed. Bush-like events appear at the needle tip when a discharge is initiated on the negative half cycle of the applied voltage, whereas a thin filamentary streamer can be seen during the positive half cycle. Inception and breakdown resulting from the two different types of event appear to be independent. For the chosen geometry, negative events are dominant at low voltages, with the positive streamers appearing only at higher potentials. For most conditions it is the positive streamers that are responsible for breakdown. >

Impulse Breakdown Characteristics of 13.2 kV Covered Conductor Insulator/Tie Configurations

Utility companies which use covered conductors on 13.2 kV primary distribution systems have experienced a number of outages due to burndown conductors. Burndown is believed to result from a stationary arc, triggered by a lightning induced surge, in the insulator/tie region melting and separating an overhead conductor. An experimental investigation has been carried out to determine the lightning impulse characteristics of two types of insulator and four types of cable tie, used in conjunction with covered conductors on 13.2 kV distribution systems. The results obtained are discussed in terms of two types of prebreakdown events.

DC-Induced Prebreakdown Events in N-Hexane

A dc source was applied to a point/plane gap in n-hexane, at a level just below that required to cause breakdown. Detailed shadowgraph images of the gap were obtained by using a pulse laser for illumination. When the laser was synchronized with a conduction current pulse, a distinct prebreakdown event could be recorded. It is shown that disturbance inception occurs at the onset of a conduction current pulse, then the event develops as a streamer and grows into the gap and, finally, fragmentation occurs during the current tail. Provided that the applied voltage is maintained, repetitive events may be observed with a waiting period of as little as 100 μs.

Impulse Breakdown Characteristics of 13.2 kV Covered Conductor Insulator/Tie Configurations

Utility companies which use covered conductors on 13.2 kV primary distribution systems have experienced a number of outages due to burndown conductors. Burndown is believed to result from a stationary arc, triggered by a lightning induced surge, in the insulator/tie region melting and separating an overhead conductor. An experimental investigation has been carried out to determine the lightning impulse characteristics of two types of insulator and four types of cable tie, used in conjunction with covered conductors on 13.2 kV distribution systems. The results obtained are discussed in terms of two types of prebreakdown events.

Bubble Growth Following a Localized Electrical Discharge and its Relationship to the Breakdown of Triggered Spark Gaps in Liquids

In order to investigate the mechanism of breakdown of the liquid-filled, triggered spark gap, a flash-illuminated, shadowgraph optical system has been used to photograph the pre-breakdown events in a triggered gap. Photographs indicate that in all cases the trigger spark is followed by the growth of a hemispherical bubble, or vapor cavity, and this bubble appears to be the precursor of the main gap breakdown. A theoretical investigation shows that the expansion and collapse of the cavity in the low-viscosity limit, follows a simple hydrodynamic model. We find that only about 1% of the circuit energy is transferred to kinetic energy of the liquid surrounding the expanding cavity. The time required for an expanding bubble to fill a 1 mm gap is of the same order as the breakdown time lag for the triggered gap. It is concluded that the bubble generated by the trigger spark clears the gap of liquid, leaving a low-density gas or vapor between the electrodes, so that the actual process of electrical breakdown takes place through the low strength gas, not through the liquid. In the case of longer gaps, of 2 mm and above, the bubble may have time to expand across the entire gap and it is suggested that an electrohydrodynamic instability may cause the bubble surface to breakup into streamers, which cross the gap and cause breakdown.

Electrostatic and Hydrodynamic Effects in the Electrical Breakdown of Liquid Dielectrics

Optical studies of pre-breakdown events in insulating liquids enable one to distinguish three stages in the breakdown of negative point-plane gaps: (1) The creation of a rapidly expanding vapor cavity adjacent to the point electrode. (2) The instability of the surface of the cavity, characterized by the growth of wave-like disturbances, and (3) The runaway growth of the instability leading to vapor streamers that bridge the gap and cause the actual breakdown. This paper contains an analysis of the electrostatic and hydrodynamic forces acting on the cavity and on the resulting streamers. It is assumed that the vapor in the cavity is ionized due to the high Field, so that the potential at the bubble surface is close to that of the point electrode. This assunption enables one to calculate the electrostatic force expanding the cavity. The electrostatic force on the cavity wall also leads to an instability of the interface, and the runaway growth of the instability leads to the generation and propagation of streamers; by combining simple electrostatic and hydrodynamic concepts, we derive an equation for streamer velocity that is in approximate agreement with measured values.

Electric Discharge Pulses in Irradiated Solid Dielectrics in Space

Electric charging and discharging phenomena on spacecraft are reviewed. Laboratory tests of spacecraft materials, performed over the last six years by many workers, are summarized. A conceptual framework is proposed to explain the apparently diverse findings and to guide future studies: prebreakdown events (microdischarges) appear to produce small Lichtenberg trees with a plasma which conducts large current under certain conditions. An extensive set of measured pulse data in several polymers is compared to theoretical prediction of radiation induced internal electric fields which rise as high as 2×106 V/cm due to trapped irradiation spacecharge alone. Implications for spacecraft are discussed.

Observation of prebreakdown and breakdown phenomena in liquid hydrocarbons

Prebreakdown and breakdown events under quasi-uniform fields in n-hexane, iso-octane, cyclohexane and toluene have been photographed. For each liquid, two types of breakdown pattern are observed. One pattern is characteristic of growth from the cathode while the other is characteristic of growth from the anode. The measured postbreakdown phenomena were identical in all cases. In addition to the optical studies, microscopic observations showed that organic films were produced on the electrode surface and that lead accumulates preferentially in the area of the electrode surface where breakdown occurs.

Cavitation dynamics in stressed dielectric liquids

A simplistic dynamic mathematical model is proposed for the explanation of transient cavity behaviour in electrically stressed dielectric liquids. Experimental high speed photographic studies on the growth of bubbles in perfluoro-n-hexane under step voltage conditions provide some encouragement for the approach which does not suffer from some of the shortcomings of earlier equilibrium models. The approach offers some help in explaining qualitatively the nature of some observed prebreakdown events.

Comparative optical observations of prebreakdown events in n-Hexane

An optical study has been made of electrically induced disturbances in n-hexane. A high speed image converter camera was used that provided both sub-microsecond temporal resolution and a complete record of the events that terminate breakdown. In order to generate a benchmark for this comparative study, the high speed camera was first used with conventional shadowgraph optics. The time resolved records were seen to support earlier findings. Such findings, however, are subject to the uncertainties that are rooted in the sensitivity of the shadowgraph technique. Images are generated by an integration of all perturbations over the optical path. The net deflection that generates the recorded image may thus be produced by a range of events. Scattered light photography was used to augment and clarify the information gathered from the shadowgraph images, it being particularly sensitive to localized interfacial phenomena associated with phase changes in the test fluid. To furnish the experimental data, the electrode gap was illuminated off axis such that the high speed camera could record the light that was scattered by prebreakdown streamers. On comparison with the shadowgraph recordings, a strong image correlation was apparent for either positive or negative point polarity.

The electric field distribution associated with prebreakdown phenomena in nitrobenzene

The electric field distribution between a prebreakdown tree and the electrode toward which it is propagating is determined. The electric field is measured in nitrobenzene, using the electro-optic Kerr effect. High-speed photography is used to provide a 10-ns resolution of the transient, prebreakdown events. The measurements show that to predict the Kerr fringe pattern, and thus the electric field distribution, it is sufficient to model the prebreakdown tree as a spherical conductor about the same size as the tree and at the potential of the electrode from which the tree is propagating.

A divergent field study of prebreakdown events in n-hexane

High speed Schlieren studies of prebreakdown events under step voltage conditions have yielded an extensive series of records for divergent electric fields. The shuttered image tube technique has allowed event velocity profiles to be established for prebreakdown disturbances under a variety of thermal and prestress conditions. The results show clearly that the velocities for positive point conditions (3 km s −1) are about an order of magnitude greater than those for the negative case. Events leading to eventual failure exhibit a characteristic minimum in their velocity profiles which are discussed in terms of the prevailing field conditions. The dependence on the field is also demonstrated by similar studies in prestressed gaps which have been correlated with statistical time lag measurements taken from light emission and Schlieren records.

High-speed schlieren studies of electrical breakdown in liquid hydrocarbons

High-speed schlieren studies have been carried out on the electrical breakdown of liquid hydrocarbons using a pulsed ruby laser. The development and propagation of charge carrier trajectories were recorded photographically on prebreakdown, breakdown, and postbreakdown events occurring in very pure, oxygen-free samples of hexane, benzene, toluene, and isooctane using a parallel plate electrode geometry. The initiation of breakdown was characterized by the growth of narrow streamers the creation of which was attributed to field injected electrons at local asperities of the cathode surface. Once the streamers reached the anode, large currents were found to flow through the gap leading to formation of a plasma column. Subsequent emissions occurring at the anode were attributed to field reversal. Details of the schlieren pictures have been interpreted in terms of classical electromagnetic theory.

Optical studies of prebreakdown events in liquid dielectrics

A combination of time-resolved Schlieren and scattered-light optical techniques has been used to examine the prebreakdown events in dielectric liquids. When a negative high-voltage pulse is applied to the point of a nonuniform gap, a disturbance, associated with discrete shock waves, propagates outwards from the point. For the opposite polarity, the disturbance moves at supersonic velocity, and is characteristically different in appearance and self luminosity. In both cases, the disturbance is identified as a region of closely packed microbubbles which ultimately fragments into isolated bubbles if breakdown does not intervene. The events recorded are explained in terms of cavitation nucleated by transient cavity collapse, and the polarity differences are attributed to field emission. The mechanism is further supported by experimental findings using prestressing techniques.

New observations of pre-breakdown in n-hexane in a point-plane configuration

New observations of the optical and electrical disturbances associated with pre-breakdown events in n-hexane with a point-plane electrode configuration are reported. The sequence of events leading to the previously described black spots or trees at the tip is analysed, and a tentative interpretation accounting for the polarity effects is offered.

Prebreakdown events in liquid nitrogen

Prebreakdown events in liquid nitrogen