Prebreakdown phenomena in mineral oil under step and ac voltage inlarge-gap divergent fields

This paper presents an experimental study of the propagation of prebreakdown phenomena in transformer oil, in large point-plane gaps (5 to 20 cm), in positive polarity under step and ac voltages. The prebreakdown phenomena are characterized via the simultaneous recordings of transient currents, charges, photocurrents, and high-speed photographs of the emitted light. In these experimental conditions, prebreakdown phenomena in oil are basically composed of weakly luminous branched filaments, most of the time not connected to the point electrode, and propagating continuously. Bright luminous fast discharges reilluminate periodically the main branch of the propagating discharge from the point electrode to the extremity of these weakly luminous filaments. Below the breakdown voltage, streamers stop at an average length which increases linearly with the voltage. Above a critical length, they cannot stop any more and lead to breakdown. The main propagation velocity remains constant throughout the propagation. These observations, which present a number of similarities with leader phenomena in gases, are then discussed     

UHF detection of leader discharges in SF6

The ultra-high frequency (UHF) technique of partial discharge detection has been used to study the partial discharges produced by electrode protrusions and surface contamination in a gas insulated substation (GIS). The paper describes the stages of discharge development from inception to breakdown. The work has shown that the UHF technique may be used to identify distinct phases of discharge activity corresponding to discharge inception and a prebreakdown condition. The prebreakdown activity is detected as an increase in the UHF signal magnitude which has been attributed to the formation of leader-type discharges. These discharges form an essential part of the breakdown process, and their detection can be used to identify when the system is close to breakdown. The paper describes a filtering technique that may be employed to optimize the sensitivity of UHF leader detection     

Laser triggering of electric breakdown in liquids

Electric breakdown in semihomogeneous geometries has been studied in n-hexane and transformer oil using a laser triggering method for fields far below the spontaneous breakdown field. The method uses a laser-produced plasma to initiate the breakdown. Strong self focusing in the liquids was limited by increasing the numerical aperture of the laser beam to >0.20 prior to focusing. Electrical breakdown could be triggered at fields as low as 50 kV/cm in transformer oil and 25 kV/cm for n-hexane. The time lags are between 30 and 200 μs, depending on voltage, liquid, and gap size. Current and light emission measurements of the prebreakdown phenomena are presented and a model for the triggering process is proposed     

Dynamics of prebreakdown phenomena in a uniform field in water

For a breakdown time lag of 10^-7 to 10^-3, both slow subsonic and thin supersonic streamers were found to emanate from a point electrode in deionized water (σ=5×10^-5 S/m). The slow streamer-formed dendrite probably consists of gaseous microbubbles. Relaxation of the slow streamers was studied. The development of fast streamers resulted from kinetic phase transition in the liquid, with subsequent ionization of thin discharge channels. It was shown that heating of water and electrohydrodynamic flow do not affect the prebreakdown phenomena under these conditions. A model has been proposed of an anodic supersonic streamer which is generated by the plasma in the condensed medium     

Visualization of high‐speed phenomena using an acousto‐optic laser deflector

An acousto‐optic laser deflector was used for visualization of high‐speed phenomena, such as shock waves and density perturbations accompanying an impulse discharge, or shock waves generated by laser‐induced breakdown in air. Using a continuous wave laser as the light source, shadowgraphs of shock waves and density perturbations were obtained at shutter speeds down to 1µs. Results showed that shock waves propagated at a speed of 417 m/s in the case of an impulse discharge, and 485 m/s in the case of laser‐induced breakdown. Prebreakdown phenomena such as leaders progressing from the high‐voltage electrode were also visualized. Compared to conventional high‐speed imaging techniques, this method is useful when using a laser light source, since the acousto‐optic crystal can accommodate high‐intensity laser light. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(3): 9–15, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20279     

Prebreakdown phenomena in dielectric liquids

The aim of this paper is to make a synthesis of a number of characteristic features of prebreakdown phenomena in liquids, e.g. those events which precede arc development. The subject is restricted to short term processes in the bulk, more specifically of hydrocarbons. First, the basic physical properties involved in the prebreakdown mechanisms are considered, with special emphasis on the influence of pressure and temperature on the physical and electronic properties of liquids and their vapor. Following this, the advantages of using a point-plane electrode geometry to study separately cathodic and anodic prebreakdown stages are demonstrated. To begin with, there is injection of carriers which consequently produces avalanches in the liquid followed by bubble generation. These mechanisms are now well characterized. Eventually, discharge channels are developed, streamers or leaders. The initiation and the propagation of these channels depend on many experimental conditions which are discussed. Characterization and understanding of these phenomena are in progress. Finally, prebreakdown phenomena in uniform fields are considered, either at short (μm) gaps or at large gaps, in the presence of large protrusions or free moving particles     

Streamer propagation and breakdown under ac voltage in very largeoil gaps

This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps (⩽80 cm). The investigations presented concern the measurement of breakdown voltages together with the recording of streamers in rod-plane and sphere-plane electrode systems, at different gap sizes. A breakdown mode specific to ac voltage is found, in which `bursts' composed of streamers initiated at each half cycle appear. This mode leads to the lowest breakdown fields recorded under ac. Values of the average stress required for burst or direct breakdown modes are deduced from the experiments. Positive streamers are responsible for breakdown in large gaps: their propagation is easy and does not depend on the gap geometry and on the presence of particles. On the other hand, the propagation of negative streamers is quenched when not very divergent fields are used     

Effect of electrode ice layer on prebreakdown current in liquidnitrogen

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 H₂O ice has marked effects on prebreakdown current, corona onset voltage and breakdown voltage. The breakdown voltage at atmospheric pressure depends on microscopic cracks in H₂ O ice     

The impulse breakdown trajectory in air between rod and planeelectrodes

Image processing was used to analyze video recordings of electrical breakdown paths in a rod/plane electrode air gap. The number of segments in the breakdown path was found to vary as the applied over-voltage was changed. The results are discussed in terms of a prebreakdown space charge model     

Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen and comparison with mineral oil

This paper presents a study of breakdown and prebreakdown phenomena (streamers) in liquid nitrogen and mineral oil under quasi uniform electric field, under ac and impulse voltage. Streamers preceding breakdown are studied up to 0.5 MPa by high-speed visualization and recording of emitted light. In these conditions, breakdown in LN/sub 2/ is mainly due to negative streamers, initiated at lower voltage than the corresponding positive voltage. Hydrostatic pressure has a limited effect on breakdown voltage, such as in mineral oil. It is shown that the ratio of impulse to ac breakdown voltage in LN/sub 2/ is surprisingly low (close to 1), whereas in the same conditions ac breakdown voltage in mineral oil is lower than impulse breakdown voltage. Practical consequences for the design of HV insulation in superconducting systems are discussed.     

Initial images of complete breakdown of liquid nitrogen/polymerfilm

This paper describes an experimental study,on the prebreakdown phenomena in a liquid-nitrogen/polymer-film composite subjected to dc ramp and step voltages under point-plane electrode system. In order to clarify the formative process of the complete breakdown of the composite, photographic observation was carried out using a long image guide scope and an image intensifier with high speed gate in the nanosecond region. When a negative-plane electrode was covered by a polymer film (N-cover arrangement), a luminous region on the initial image of the complete breakdown was located near the negative-plane under both the ramp and the step voltages. These results suggested that the complete breakdown of the composite in the N-cover arrangement was caused by the film breakdown due to positive charges deposition and subsequent electron emission from the film breakdown spot. When a positive-plane electrode was covered by the film (P-cover arrangement), film breakdown could not trigger the complete breakdown under the ramp voltage because the film breakdown occurred at a low applied voltage. However, intense light near the positive-plane was observed at the initial stage of the complete breakdown under the relatively high step voltage. This was because the film breakdown under the high applied voltage induced the positive streamer propagation from the film breakdown spot     

High field conduction and prebreakdown phenomena in dielectric liquids

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (/spl sim/ MV/cm), and of propagation (requiring low electric field, /spl sim/ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data.     

Dynamic breakdown characteristics of liquid helium induced by aquench of superconducting wire and coil

For practical insulation design of superconducting power apparatus, it is necessary to take into account an inherent phenomenon known as quench, the transition from the superconducting to the normal state. We investigated quench-induced dynamic breakdown and prebreakdown characteristics of liquid helium (LHe). Experimental results revealed that the quench of the superconductor drastically reduced the breakdown voltage of LHe in the thermal bubble disturbance. Moreover, dynamic breakdown characteristics in a quasi-uniform field using a superconducting coil were investigated. The results revealed that the breakdown was induced in the quench-initiated region where the largest thermal energy was generated     

Pre-breakdown photoelectric monitoring of AC gaps

Prebreakdown photoelectric monitoring of gaps is introduced. The photo and electric channels of measurements are discussed in detail. Photo and electric traces obtained for a point-to-plane air gap at natural temperature and pressure and under AC voltage are presented as examples. The gap geometry or gap length does not change the general character of the traces. Distinct relations between the photo and electric traces are presented and discussed relating to the breakdown mechanism. The presence of streamers at successively decreasing intervals leads to a plateauing in the photo trace which is a prebreakdown indicator. It is concluded that the proposed photoelectric monitoring provides a good tool for investigating the prebreakdown and breakdown processes of a gas gap     

Breakdown and prebreakdown phenomena in liquids under positiveimpulse voltages

We examine the breakdown and prebreakdown behavior of two insulating liquids: benzyl-toluene and mineral oil in a nonuniform electric field at large distances (up to 20 cm) under a positive impulse voltage. We evaluate and compare the breakdown strengths of both liquids in these conditions as a function of interelectrode distance via Weibull type plots, a method commonly used when dealing with insulating gases and solids. The comparative behaviors of the liquids change greatly when the cumulative probability of breakdown is decreased from 100%. The second part of this study concerns streamer stopping lengths and the corresponding current and light emission signals     

Activity of moving metallic particles in prebreakdown state in GIS

The paper presents experimental results of measurements of prebreakdown activity, caused by free metallic particles in GIS, obtained using a phase resolved partial discharge (PRPD) analysis and acoustical measurements. The numerous parameters characterizing prebreakdown activity of particles were identified and analyzed. These included phase resolved distribution of charge, maximum charge, average charge per period, or per pulse, a balance of charge in positive and negative half-wave of voltage, pulse rate (number of pulses per period) for electrical and acoustic methods. The measurements were conducted in a compartment of real GIS, and Al as well as Cu particles of different shapes were investigated. The pressure varied from 150 to 550 kPa, however most measurements were taken at 450 kPa. A discharge model which takes into account the effect of a free metallic particle in GIS is presented     

Partial discharge and breakdown mechanisms in ultra-dilute SF6/N2 gas mixtures

With the goal of reducing SF₆ gas usage, we investigated partial discharge (PD) and breakdown (BD) mechanisms in ultra-dilute (0 to 1%) SF₆/N₂ gas mixtures. The experimental results and discussions are given in this paper. In particular, we focused our research on the transition characteristics of the electrical insulation performance with an extremely small amount of SF₆ content, the PD behavior on the applied voltage, and the relation among prebreakdown streamer, return stroke and BD. Moreover, we discussed the PD and so mechanisms with reference of space charge behavior. From these results, the transition threshold at which the influence of SF₆ gas on the discharge characteristics began to appear, was clarified to be SF₆ content k=10 ppm for a total pressure of 0.1 MPa     

Dielectric strength of a vacuum interrupter contact gap after making current operations

Experimental results concerning the breakdown voltage and prebreakdown current of a vacuum interrupter contact gap, after making current operations, are presented. The dielectric strength of the contact gap is lowest for a switching sequence during which a making of high current is followed by a no-load contact opening. The breakdown voltage is much lower than after making operation, followed by interruption of a sufficiently high current. The basic process of breakdown initiation appears to be field electron emission from a specific structure on the contact surface, created by weld rupture.     

Prebreakdown phenomena at high voltage in liquid nitrogen and comparison with mineral oil

This paper presents a study of prebreakdown phenomena (usually called streamers) occurring at high voltage in liquid nitrogen. A special cryostat has been constructed to allow dielectric measurements in liquid nitrogen up to 200 kV, and 0.5 MPa hydrostatic pressure. Streamers are detected and studied with various techniques: high speed visualization, streak camera, and recording of transient currents and emitted light. The propagation of positive and negative streamers is investigated at gap distances up to 80 mm. Streamers are then compared to similar phenomena observed in mineral oil. In liquid nitrogen, streamers are different compared to mineral oil. A transition from slow to fast positive streamers (velocity about 10 km/s) is observed at a threshold voltage below the breakdown voltage. The hydrostatic pressure has almost no influence on this phenomenon. Some experiments carried out with a solid barrier perpendicular to the gap axis are also presented. The practical consequences for the design of HV insulation in superconducting systems are then discussed.     

Experiments on paper insulation in a corona environment

Insertion of an insulating paper between point-plane electrodes normally increases the total breakdown potential threshold, except when its position is next to the plane. A series of DC [both (+) and (-)] experiments was designed and conducted to examine this latter distinction. It was found that visual ionic spots on the paper surface are a dominant prebreakdown phenomenon. Local failure at one of such discrete spots ultimately brings about a streamer sparkover in air at a sufficiently high potential. This paper describes the variability and other characteristics of the pattern in a two-dimensional spot distribution. Related experiments reveal that, with double paper layers already in the barrier, the pattern of spots becomes less dense visually but the breakdown potential would increase for the case of (+) point but would decrease further for the case of (-) point if a third sheet is introduced. A sharply defined “dark” band on the double-layer side in which glow spots are absent was always observed to clearly bridge the separate domains of, respectively, single-layer and double-layer spot distributions. Also related were the mechanical agitations of the paper accompanying all phases of the different discharge activities