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Partial discharges (PDs) are believed to be one of the key factors accelerating high voltage (HV) equipment ageing. The chemical by-products of PDs actively contribute to the modification of insulations surface quality, especially in air medium due to the diversity of its constituents. Although nonthermal plasmas in air have been widely studied, their adaptation to HV systems is still highly complex and prone to controversies. In this work, a typical simulation of a fault leading to PD in air, through a Dielectric Barrier Discharge (DBD) point-to-plane configuration, is performed in order to measure ozone (O 3 ), nitrogen oxide (NO) and dioxide (NO 2 ) by-products gases. The evolution of these species regarding PD duration, PD power and humidity is analyzed. Ozone production rate and dynamics are found to predominate over NO 2 and NO ones. The effect of humidity on gases evolution is found to be indirect, as it mainly affects PD power.

Partial discharges characterization on ceramic substrates and dielectric liquids under high temperature up to $350^{\circ}\mathrm{C}$ is investigated. Aluminum nitride (AlN) and alumina (Al 2 O 3 substrates, and a high temperature non-polar dibenzyltoluene (DBT) liquid are used. Several complementary measurements are also done in a synthetic ester liquid. PD measurements on AlN substrates embedded in DBT show a good behavior up to $300^{\circ}\mathrm{C}$, and up to $350^{\circ}\mathrm{C}$ with Alumina. This shows that high temperature insulating liquid may constitute a good alternative to silicone gels for the encapsulation of power electronics circuits at high temperature and high voltage. The comparison between PD characteristics in the liquid alone and with substrates show that PDs mostly occur within the ceramic material at temperatures up to $300^{\circ}\mathrm{C}$.

An experimental study of discharges in air at atmospheric pressure under impulse voltage, creeping on insulating solid surface is presented. The objective is to get a better understanding of creeping discharge by studying streamer initiation and propagation, and by looking at the influence of the nature of the insulating solid. Several solid materials, of different chemical natures and permittivity are investigated. Pre-breakdown streamers are investigated by high speed visualization and transient current measurements. In the presence of insulating solids such as epoxy resin, PPA/glass fibers and polycarbonate, surface streamers appear during the voltage rise, and the time delay to appearance is correspondingly rather short. With two materials of polyolefin and fluorine modified polyolefin families, characterized by lower permittivity, the first creeping streamer appears at the maximum voltage after a much longer statistical time delay, very close to that obtained in air without solid.

Breakdown, at least in point-plane geometry, is the result of inception and propagation of pre-breakdown phenomena called "streamers". The purpose of the present investigation is a spectroscopic measurement of the essential plasma parameters, i.e. the gas temperature and the neutral particle and electron densities. In this paper we focus our spectroscopic investigation on fast positive filamentary streamers obtained at large gap distance under high impulse voltage in LN 2 .

Dielectric properties of synthetic ester oil are investigated in wide frequency range (10 mHz–1 MHz) and temperature (−60°C −200°C) ranges. The aim is to determine the ability of liquid insulation, to be used as encapsulating materials for high temperature power electronics modules. As far as the ester remains in liquid state (above approximately −16°C), a typical frequency spectrum of the dissipation factor tan δ with two parts is observed. At low frequency, dielectric losses (tan δ > 0.10) are due to ionic conduction, and at high frequency losses are mainly due to dipolar relaxation. When temperature is increased, losses increase at low frequency, and decrease at high frequency. Results show that the liquid conductivity increases by more than 3 orders of magnitude between room temperature and 200°C. A slight decrease of permittivity is also observed, correlated to the lowering of density at high temperature. This behavior totally changes when the ester becomes a gel at low temperature. Dielectric relaxations reminiscent of solid materials then appear.

Gas Insulated Substation (GIS) under High Voltage Direct Current (HVDC) are becoming widely investigated with the development of DC networks. Under DC voltage, mechanism of charge accumulation on the insulation surface is of major importance since it may distort the electric field distribution. The latter directly depends on material's conduction and so, on its volume and surface conductivities. Previous work has evidenced that those properties are influenced by the temperature, the electric field and the surrounding relative humidity [1-3]. The insulation characteristics must be clarified to allow a better estimation of the electric field distribution, as well as a better insulation design.

In this paper, we present the results of electrical breakdown experiments performed in a contact gap ranging from 100nm to 10µm under air and nitrogen environment. The electrodes arrangement corresponds to a point-plane system and consists in a probe tip with a radius of curvature of 20µm and a perfectly flat electrode. A high precision piezoelectric actuator provides to the system a static resolution of ±30nm allowing to investigate submicrometer contact gaps. Numerous experiments have been performed at those distances (

Dark current measurements in pressurized gases at high electric field are performed in a coaxial electrode geometry. Three main parameters are investigated: the influence of electrode surface roughness, the nature and pressure of gas, the presence of water. Currents are measured in both negative and positive polarities. A large influence of electrode surface roughness and of the gas water content is observed. Most of results suggest that charge injection from the electrode constitute the basic mechanism of high field conduction through pressurized gases, and this mechanism is considerably enhanced when water is present.

The dielectric properties of two grades of Bi-oriented isotactic polypropylene (BOiPP) are studied, using a variety of techniques: breakdown field measurements, dielectric spectroscopy, thermally stimulated depolarization currents, dc conduction currents. Standard (STPP) and high-crystallinity (HCPP) polypropylene films are investigated. Measurements are carried out over a wide temperature range, up to (-150°C/ +125°C). Breakdown field in both materials show a very small difference. On the other hand, dielectric losses and dc conduction currents are significantly lower in HCPP. Both materials show a decrease of dielectric losses versus temperature in the range (20-90°C), favorable for the application to ac power capacitors. The analysis of dc currents allows to evidence two main conduction mechanisms: (i) below 80°C in both materials, a hopping mechanism due to motion of electrons occurring in the amorphous phase; (ii) above 80°C, an ionic conduction in HCPP and a hopping conduction in STPP.

Metalized polypropylene films are used to build power capacitors for ac voltage. The objective of this paper is to study the de-metallization processes that may occur under high electric field. A laboratory model of capacitor is first described. It allows us to visualize the de-metallization and record the corresponding capacitance loss, under various conditions of applied voltage, frequency, mechanical pressure, and gas nature. Four different de-metallization processes can be observed. The residual gas layer existing between films has a primary importance. Replacing air by nitrogen shows that the de-metallization is basically due to the oxidation of the metal.

Aluminium nitrite (AlN) or alumina (Al 2 O 3 ) substrates are widely used in power electronics modules, due to their ability to provide both electrical insulation, and heat conduction properties. A silicon gel usually covers the substrate, semiconductor chips, and bondings to prevent partial discharges (PD) from occurring within the module. However, at high voltage PDs can be observed in high electric field regions, mainly at the sharp edges of copper tracks on the substrate. In this study, we try to determine the origin of these PDs. Phase resolved PD recordings (PRPD) and optical visualization at very high sensitivity are carried out on substrates embedded in gel or insulating liquids. PD features are very different from those obtained in needle-plane geometry in gel or liquid, and do not depend on the nature of the liquid or gel. The main conclusion is that PDs recorded with substrates originate from the ceramic material itself, due to the presence of numerous μm-sized pores in sintered materials.

Ester-based insulating liquids are considered today for electrical insulation in medium and high voltage transformers. The main advantages expected are their non-toxicity, excellent biodegradability, and fire resistance. Esters can be either of natural origin or synthetic. In this work, we present a study of prebreakdown and breakdown properties at high voltage and large gaps in several natural and synthetic esters, and a comparison with the well-know mineral transformer oil. This study is focused on the study of streamer propagation in large gaps up to 10cm. Compared to mineral oil, the transition to fast streamers in esters occurs at much lower voltage. Measurements also show no difference between esters, whatever their origin, viscosity, etc. From these results, it appears that the properties of esters are less favorable for very high voltage insulation than in mineral oil: once triggered, streamers are able to propagate very quickly to breakdown at lower voltage. In consequence, esters show a lower resistance to fast transients such as the standard lightning impulse.

Pulsed high-voltage discharges induce simultaneously several physical processes (UV light, shock waves) and chemical processes (active species and radicals such as °OH, °H, °O, °HO 2 , H 2 O 2 , O 3 , etc.), able to degrade harmful organic compounds in wastewater. This paper aims to compare various discharge types (either in the solution volume, or in the gas phase in close proximity to the liquid surface), based on the measurement of their energetic efficiency. This is done by correlating electrical measurements (injected energy) with chemical measurements (pollutant degradation), with various pollutants and catalysers. Pulsed corona discharges in the gas above the liquid appears to be the most effective process in terms of mol pollutant degraded per joule of electrical energy, and also the most simple to implement.

Aluminium nitrite (AlN) or alumina (Al 2 O 3 ) substrates are widely used in power electronics modules, due to their ability to provide both electrical insulation, and heat conduction properties. A gel layer usually covers the substrate, semiconductor chips, and bondings to prevent partial discharges (PD) from occurring within the module. However, at high voltage PDs can be observed in high electric field regions, mainly at the sharp edges of copper tracks on the substrate. In this study, we try to determine the origin of these PDs. Phase resolved PD recordings (PRPD) and optical visualisation at very high sensitivity are carried out on substrates embedded in gel or insulating liquids. PD features are very different from those obtained in needle-plane geometry in gel or liquid. The main conclusion is that PDs of low amplitudes probably originate from the ceramic itself, due to the presence of numerous µm-sized pores in sintered materials.

Trifluoroiodomethane CF 3 I has been recently considered as a potential alternative to SF 6 , thanks to its high electronegativity and extremely low environmental effect. However, due to its high boiling point, CF 3 I should be mixed with other gases such as N 2 or CO 2 in order to avoid the liquefaction at low temperature. In this paper, we present breakdown measurements carried out in CF3I and CF 3 I-N 2 gas mixtures, under dc voltage, and with a pressure-distance product (p.d) up to 5 mmbar. Pure CF3I has a higher breakdown voltage (V b ) than SF 6 . However, the favourable synergistic effect observed in SF 6 gas mixtures with N 2 , at low SF 6 content, is not observed in CF 3 I gas mixtures. In CF 3 I-N 2 mixtures, the dc breakdown voltage increases proportionally to the CF 3 I content. As a consequence, breakdown voltages for mixtures containing up to 50% CF 3 I are lower than with SF 6 at the same ratio. These results are in good agreement with calculated values using breakdown model and reduced critical field strength (E/N) crit reported by others authors. In CF3I, the appearance of solid by-products (particles) generated by breakdown is also observed. The nature of these particles is tentatively attributed to the formation of iodine. In our sphere-sphere geometry, the accumulation of such solid particles leads to a measurable conductivity.

In gas insulated switchgears (GIS), high voltage electrodes are supported by solid insulators. A "triple junction" (TJ) exists at the interface between metal, insulating solid, and gas. This area constitutes a preferential zone were partial discharges (PDs) can appear. A further degradation of insulation performance can occur if solid particles accumulate at that place. This paper presents experiments carried out to obtain a quantitative account of the influence of a metallic particle located close to a triple junction area, both in dry air and SF6. PDs are detected and measured by conventional electrical measurements, and observed optically with a high-sensitivity intensified camera. Measurements under AC voltage show the influence of the particle on PD inception and flash-over voltage, in comparison with the same geometry without particle. In SF6, the particle induces a large decrease of PD inception and breakdown voltages, which become nearly independent of the gas pressure.

This paper discuss the relations between laboratory studies of breakdown precursors in liquids (usually called "streamers"), and practical applications such as high voltage insulation of power transformers and testing of liquids. The conditions existing within high voltage power transformers (ac voltage, moderately divergent electric field, presence of insulating solids, particles in the oil, etc.) are rather different from those frequently used in the laboratory to study streamers (very divergent field -point-plane geometry-, and impulse voltage). Parameters such as field geometry, voltage waveform, liquid conditioning, can influence either streamer inception or propagation. After a short presentation of streamer phenomena in liquids, the most important effects of these parameters are recalled in this paper. Many features previously observed empirically by the measurement of breakdown voltage can be explained by the behaviour of streamers in conditions close to those actually present in transformers. It remains rather difficult today to model and predict streamer properties (initiation fields, propagation length, velocity, ...) when the liquid nature is changed. The large variety of physical processes involved during initiation and propagation, and the non-linearity of many processes make the study of breakdown in liquids complex. However, knowledge about streamers is helpful to define and measure parameters of practical importance (initiation and propagation conditions), providing a more scientific basis to test and compare the properties of liquids.

This paper presents an experimental study of pre-breakdown discharges in water (usually called ldquostreamersrdquo) and transient breakdown arcs (called ldquosparksrdquo) generated by high voltage pulses, in relation with their potential application to degrade organic molecules for depollution purposes. An experimental set-up is described using semiconductor high voltage switches to produce high voltage pulses. In a point-plane gap, depending on the high voltage pulse duration, either streamers alone or streamers followed by a transient breakdown arc can be produced. Electrical measurements provide data correlated to streamer and spark processes: current, charge, power, and dissipated energy. Coupled to these electrical measurements, the degradation of 4-chlorophenol and 4-nitrophenol molecules is studied by HPLC measurements. It is reported that the energy yield for the degradation of these products is higher when sparks are used instead of streamers.

The purpose of this work is to evaluate the chemical stability of a bio-degradable insulating oil composed by a mixture of esters based on rape-seed oil. The measurements are done in comparison with a commercial mineral oil. Both oils are subjected to accelerated ageing conditions at 120degC under air (open beaker method). Dielectric properties (dissipation factor, permittivity, conductivity, AC breakdown voltage) and chemical parameters (acidity, water content) are monitored during the ageing process. In the rape-seed based oil, ageing has a negligible influence on the breakdown voltage, whereas conductivity and acid content show a large increase.

In this paper, we have investigated the dielectric properties (losses, conductivity) of a modified rape-seed oil (RS50). Dielectric spectroscopy has been done over a wide frequency range (10-2-106Hz) and conductivity measurements according to CEI 61620 standard. The parameters investigated are temperature (-20degC to 120degC) and water content. Frequency spectra of the dissipation factor tgzeta at different temperatures are typical of losses by conduction (in the low frequency range) and of losses by dipole relaxation (in the high frequency range). The influence of temperature is well explained by conduction and relaxation theories taking into account viscosity change. The increase of conductivity with temperature is rather large: a factor of 103 from -20degC to 120degC. Up to about 200% of relative humidity, no significant influence of moisture on the dielectric properties (permittivity, losses, conductivity) has been observed. These results are then discussed in correlation with potential applications of this oil in electrical systems.

Rape-seed oil is considered for electrical insulation at high voltage. In this work, breakdown and streamer propagation in rape seed oil are studied in point-plane gaps up to 20cm, under impulse voltage of both polarities. In both polarities, two main streamer types are observed: "slow" streamers with a velocity of about 2km/s, and much faster streamers up to 150km/s. Compared to mineral oil in the same conditions, fast streamers in rape-seed oil appear at much lower voltage. They are able to propagate over large distances within a short time at moderate voltage. From these results, it is concluded that the properties of streamers in rape-seed oil are less favorable for high voltage insulation than in mineral oil.

This paper describes experimental measurements carried out to study the dielectric properties of composite dielectric materials (glass cloth/mica/epoxy) used for the insulation of stator bars in high power generators. Due to the structure of the material, composed of a stack of layers, its dielectric properties are nonisotropic. If the material is polluted by water, water diffusion is much faster in the direction parallel to the layers. Dielectric measurements also show that the degradation of properties due to water (increase of DC conductivity, of dielectric losses) is more marked when the electric field direction is parallel to the layers.

This paper describes the effect of water on dielectric properties at high field of composite insulation, based on epoxy reinforced by fibre glass and mica flakes. Samples with controlled water content are prepared by immersion. Breakdown measurements under ac voltage show a marked decrease of breakdown field, when large amounts of water (up to 1.5%) are present in the material. High voltage dielectric spectroscopy measurements were carried out from 0.001 Hz to 1 kHz with various water content and applied fields (up to 5 kV/mm). At low frequency, a very large increase of dielectric losses versus water content, due to the increase of material conductivity is reported. At 50 Hz, tan /spl delta/ higher than 1% are recorded with water content above 0.5%.

This paper presents an experimental study of the influence of vapor bubbles on breakdown in liquid nitrogen. Vapor bubbles are initiated by creating a small hot spot at the surface of an electrode. The behavior of vapor bubbles is first investigated by visualization, versus power dissipated by the heater, pressure and electric field. When breakdown occurs in the presence of bubbles, it is due to formation of either a vapor bridge between the electrodes (at low pressure), or a vapor layer on the hot spot (at high pressure). The results are discussed in comparison with gas breakdown fields obtained from the Paschen's law.

This paper presents streamer charge measurements carried out in liquid cyclohexane, and calculations of the electric field acting on the streamer. The time resolved measurement of streamer charge is made with a differential system that allows to subtract the electrode charge in order to obtain the streamer charge under impulse voltage with a high sensitivity (below 1pC). Measurements are carried out in a point plane gap versus applied voltage and hydrostatic pressure, and concern slow subsonic streamers observed in positive polarity. With the charge data obtained, and with the measured streamer size, numerical calculations of the electric field within the gaseous streamer and outside the streamer are done. This allows to calculate the voltage drop existing within the streamer, and also to determine the intensity of the electrostatic pressure acting at the gas/liquid interface.

This paper presents a study of streamer inception and propagation in point-plane and semi uniform-geometry. The latter is constituted of parallel plane electrodes, with a thin triggering point of calibrated size attached at the centre of one. By reducing the length of the point, it is possible to move progressively from a point plane geometry to a quasi uniform geometry. The initiation and propagation of streamers is impeded due to the presence of a metallic plane behind the triggering point, that reduces the field on the metallic tip and on propagating streamers. The propagation of negative streamers is quenched, whereas filamentary positives are less affected.

A real-time optical detection method combined with high-speed current measurement and visualization was used to investigate bubble generation and growth in purified cyclohexane. The generation of gaseous bubbles in the high field region present on a sharp tip appears to be a consequence of the detected current pulse. An analysis of the influence of the energy input together with that of the hydrostatic pressure shows that vaporization of the liquid is the most probable mechanism of bubble generation. The energy input is achieved in a very short time (a few nanoseconds). As the bubble grows, the pressure and temperature decrease in the bubble down to steady-state boiling conditions; the bubble size is then maximal, and as the temperature decreases further to the ambient value, the vapour condenses and the bubble disappears. >

An experimental study of the initiation of positive streamers under step voltage in a variety of hydrocarbon liquids is presented. The experimental conditions (point tip radius, pulse risetime, and gap length) required for obtaining valuable measurements of the initiation fields and threshold propagation voltages of streamers are defined. The results obtained are discussed with reference to the known properties of the liquids (ionization potentials and breakdown strength).

The authors present a study of the dynamics of positive streamer filaments in pentane, for hydrostatic pressures above 1MPa. They describe an experimental setup allowing the simultaneous visualization of the channels, together with the electrooptical time-resolved measurement of their diameter. The dynamics of the expansion and collapse is fairly well described by Rayleigh theory, showing that the filaments are gaseous channels. It is shown that the stopping of filamentary streamers at high pressure is determined by the collapse of the channel. The behavior of the filaments may be described by considering their extremities as moving point sources of energy.

The authors present an investigation of the breakdown and prebreakdown behavior of two insulating liquids; a benzyl-toluene and a mineral oil, in point-plane electrode geometry under positive polarity. To properly evaluate the breakdown voltage as a function of interelectrode distance (and voltage), it was necessary to utilize more statistically based methods than are usually used. In this light, a better and more realistic comparison of dielectric strengths between liquids can be made. The authors also investigate the length a streamer will travel across the interelectrode distance before stopping, for voltages lower than the breakdown values. These lengths are plotted nondimensionally against voltage for both liquids.

This paper presents a study of prebreakdown phenomena in mineral oil under impulse voltage, in point-plane gaps up to 10 cm. In highly overvolted gaps, the measurements show that above a critical voltage, a large increase in propagation velocity is recorded (from 2 to 100 km/s), correlated to large modifications in the streamer shape and charge. This phenomenon is analysed by modelling streamers as conducting objects of various shapes, calculating the charges and fields by a charge simulation method, and comparing them to measurements. The good agreement obtained shows that streamers behave as good conductors, and that the transition between streamer modes strongly depends on the "electrostatic" properties of streamers.

This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps up to 80 cm. The investigations concern the measurement of breakdown voltages together with the recording of streamers. Experiments were conducted with rod-plane and sphere-plane electrode systems at different distances. The results describe breakdown mechanisms for moderately divergent fields and large gaps. A breakdown mode specific to the ac voltage is evidenced, in which "bursts" of streamers initiated at each halfwave can appear and lead to the lowest breakdown fields recorded under ac. Values of the average fields required for "burst" or "direct" breakdown modes are deduced from the experiments. The influence of particles is also considered.

This paper presents a summary of the introductory lecture to be presented at the 1996 ICDL on the particular problem of prebreakdown phenomena in long liquid gaps (centimeters to decimeters). The specific topics developed concern the characterisation of the propagation of positive discharges in mineral oil, including a description of the various propagation modes, together with the study and modelling of their gaseous nature and electrical properties.

This paper presents a study of the transition to fast streamers in mineral oil in overvolted point-plane gaps. Streamer propagation is studied in 5 cm and 10 cm gaps, either in the liquid alone or in the presence of solid surfaces parallel or perpendicular to the gap axis. The experiments presented concern the visualization of streamers, the measurement of breakdown and acceleration voltages. In the liquid alone, above some critical voltage, a large increase in propagation velocity is recorded from 2 km/s up to about 100 km/s. The presence of insulating solids parallel to the electric field is greatly in favor of the inception and propagation of fast streamers (velocities up to 300 km/s have been measured). Conversely it is observed that a solid perpendicular to the electric field stops more efficiently fast streamers than slow ones, which easily creep and get round such obstacles.

In this study we examine breakdown and prebreakdown phenomena in asymmetric geometry (point to plane) at large gap distances (up to 20 cm) under positive voltage steps (up to 500 kV) in liquids of distinct chemical families: saturated, aromatic and polyaromatic. Signal analysis shows that the currents are not continuous but are made up of bursts with different waiting times between them. The duration and number of these bursts vary according to the general chemical composition of the liquid. For example, there are numerous short bursts in mineral oil and a few long bursts in the aromatic liquids. The polyaromatic has a behaviour which retains features of both. >

This paper presents an experimental study of streamer inception in mineral oil in point-plane and semi uniform-geometry. The latter is constituted of parallel planes with a thin metallic point of calibrated size attached on a plane electrode. A systematic study of streamer initiation has been carried out versus geometric parameters: gap distance and point size (tip radius and length). As compared with the point-plane gap, the initiation of streamers is impeded due to the presence of the metallic plane behind the point, that shields the electric field on the metallic tip. The extrapolation to the general case of a particle-initiated breakdown in liquids and the effects of particle size are discussed.

This paper presents an experimental study of the influence of a polyaromatic additive (pyrene) on the propagation of positive streamers in cyclohexane. Experiments were carried out in gaps up to 5 cm over a wide voltage from the inception of streamers up large overvoltage. With pyrene, streamers are composed with a much larger number of branches than in pure cyclohexane. It is observed that the additive leads to slightly lower breakdown voltages, but also induces a large increase of the acceleration voltage (inception threshold of fast streamers). These results are explained by the macroscopic electrostatic properties of streamers, that depend on the streamer shape, and thus on their tendency to form branches. The transition to fast propagation modes occurs at higher voltage when streamers are more ramified.

This paper describes a study of streamer inception in mineral oil under ac voltage conditions with rod or point electrodes. Positive and negative streamer inception frequencies versus voltage have been investigated in gaps up to 40 cm with different concentrations of cellulose particles. Streamer inception is determined by the field at the rod extremity. Initiation fields decrease with increasing rod radii and when cellulose particles are added.

We study streamer initiation in liquid cyclohexane, in a point-plane geometry under impulse voltage. A new differential charge measurement system, allowing a high sensitivity to be obtained under impulse voltage (below 0.1 pC), is presented. The investigations presented concern the influence of applied voltage, impulse rise time and hydrostatic pressure. Under DC voltage, the negative streamer initiation is due to an avalanche in the liquid phase, and is not affected by pressure. In contrast, positive streamer initiation shows some variation when the pressure is increased.

This paper reports the results of experimental investigations about prebreakdown phenomena in liquid nitrogen, in point-plane electrode geometry (gap distance: 2 mm) under impulse voltage (up to 30 kV). As in hydrocarbon liquids, slow "bush like" streamers and fast filamentary streamers are observed in positive polarity. The spectral analysis of the emitted light during the propagation of filamentary streamers shows the second positive band emission system of nitrogen (C/sup 3/II/sub u//spl rarr/B/sup 3/II/sub g/) in the region 330 nm to 400 nm. The rotational and vibrational temperature T/sub r/ and T/sub v/ can be evaluated from a comparison of experimental and simulated spectra of vibrational bands for the transition /spl Delta/v=-2 of the second positive system. From these measurements, we can conclude that the temperature of gaseous filaments is about 540 K, i.e. a quite cold plasma.

This paper presents experimental investigations concerning the influence of space charge on the ac breakdown of moving mineral oil, between metallic electrodes. Space charges are injected in oil with sharp metallic points, and carried into the breakdown test cell through a perforated ground electrode. Breakdown voltages are measured under ac voltage in rod-sphere or sphere-sphere electrode at 5 and 10 cm gap distances. The results show that, whatever the flow rate of oil and the amount of charge injected, no significant influence on ac breakdown voltage is observed. As a consequence, it appears that the deleterious effect of static electrification of oil in devices like power transformers cannot be simply ascribed to a degradation of the intrinsic oil performance.