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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.

The conduction current recorded when a high voltage impulse is applied to a rod-plane gap containing water is modelled. The electro/thermal model considers the coupling between electric field, Joule dissipation, temperature rise, and variation of liquid resistivity with temperature. A fairly good agreement between current measurements and calculations are obtained. The model also shows the evolution of temperature and field distributions in the gap versus time. The calculated average temperatures appear insufficient to explain liquid boiling. Boiling can be explained by considering the influence of local surface defects. Values of the bubble pressure at discharge ignition are evaluated, with the hypothesis that discharges occur in vapor following Paschen's law.

The generation of vapor bubbles in water under high voltage impulses of long duration (500 μs) is investigated experimentally. In most cases, the appearance of bubbles is followed by the inception of discharges occurring within the vapor, eventually transforming the bubble into a propagating streamer. Time delays to bubble and discharge inception are measured versus conductivity and voltage, using fast visualization and optical detection of bubbles. Before bubble inception, the liquid heating and the onset of an electro-hydro-dynamic (EHD) motion can be observed by Schlieren visualization. Electrical measurements show a decrease of the gap resistance versus time, attributable to the heating of water. When conductivity and voltage are changed, the onset of bubble inception is correlated to a nearly constant resistance drop, suggesting that a constant temperature is present when bubbles appear. At short times, the energy dissipation required to observing bubbles is independent of conductivity and applied voltage. Experiments show that vapor bubble generation results from a competition between liquid heating by Joule dissipation, and forced cooling induced by the EHD motion.

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 breakdown and pre-breakdown phenomena in liquid nitrogen (LN 2 ) is performed in this work, aiming to provide useful knowledge for the design of high voltage superconducting systems. A metallic tape subjected to pulsed heating is used as ground electrode to properly simulate the operating conditions of a quenching superconducting apparatus. Measurements of breakdown voltages between the tape and a plane electrode tied to high voltage show the strong influence of tape polarity, LN 2 temperature, and delay between heating and voltage application. Various measurements obtained with either a highspeed camera or an intensified camera allow to characterize the sequence of pre-breakdown events (i.e. liquid boiling, vapor bubble development, and streamer propagation) leading to breakdown.

International audience; Breakdown measurements are carried out in liquids under point-sphere and pointplane electrode geometries, over a large range of gap distances up to 35 cm. Non-polar hydrocarbon liquids with different chemical structures are studied: saturated, aromatic, polyaromatic, mineral oils, and ester. Two high voltage impulse shapes are used: the standard lightning impulse (LI), and a specific “step” impulse (ST). Step impulses are favorable to interpret breakdown measurements since the applied voltage remains constant while prebreakdown streamers propagate. Conversely, with lightning impulse and long gaps the propagation of streamers with a low propagation velocity (a few km/s) is quenched due to the rapidly decaying voltage. In this case, breakdown can result only from the propagation of faster streamers, which appear in very different conditions according to the liquid nature. The results obtained allow us to interpret the large variations of breakdown voltage observed in large gaps, when either the high voltage impulse shape or the liquid nature is changed. These results also help to define proper conditions for testing and comparing liquids. Testing with lightning impulse in short gaps, such as in standard tests, does not provides data relevant for very high voltage applications.

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.

This paper presents an experimental study of the influence of pulsed electrical discharges of low energy (3 to 10 J) on the extraction of polyphenols from grape seeds. To obtain a better understanding, three basic phenomena involved in the whole discharge process are studied separately: pulsed electric field (PEF), pre-breakdown phase (streamer), and breakdown phase (arc). The polyphenol extraction is much more efficient with arcs, compared to streamers and PEF. Therefore, during the discharge process, the enhancement of polyphenol extraction can be mainly ascribed to the final arc phase. The total energy per unit mass CWp required to extract 5000 mg GAE/100 g DM with the arc (CWp = 16 kJ/kg) is 27 times lower compared to streamers alone, and 47 times lower compared to PEF. An optical study shows that the mechanical effects of arcs (shock waves, expanding cavity, and strong turbulence) are much more pronounced compared to streamers. These mechanical effects are responsible for the fragmentation of grape seeds, and strongly promote the release of polyphenols. Other parameters such as the test cell shape and the inter-electrode distance also show that the polyphenol extraction is further enhanced when grape seeds are in close proximity with the breakdown arc. In optimized conditions, the extraction efficiency with low energy discharges can be significantly higher than previous measurements with high energy discharges. Industrial relevance This paper presents relevant information for the design of generating electrical discharges treatment. The study also addresses a specific case of use of by-products and shows the effectiveness of such technology.

In gas-insulated systems, the presence of solid insulators used to hold conductors at high voltage induces the presence of triple interfaces where the metal, solid insulator and gas are in contact. These regions, usually named “triple junction”, constitute weak points for the insulation, mainly due to the mismatch between permittivities of solid and gas, which induces an enhancement of the electric field. If the design of these regions is not carried out with great care, partial discharges (PDs) may appear and accelerate the ageing of the insulator. This paper deals with an experimental study of PDs in a “triple junction” electrode configuration, carried out with sensitive electrical and optical PD measurements. Experiments are done in two pure gases, nitrogen and SF6. Two discharge regimes can be observed: very small discharges (

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 (

This work reports on the relationship between structure and dielectric properties of biaxially oriented polypropylene. The morphology of semicrystalline bioriented isotactic polypropylene films is investigated using wide angle X-ray diffraction and Polarized Optical Microscopy. A b-orthorhombic structure, with a crystallinity ratio of about 46%, and “Crater” morphology of the b-form is identified. Dielectric properties are measured by Broadband Dielectric Spectroscopy over a wide temperature range (2150 to 1258C). Since the dissipation factor of the PP is very low, special care was taken to obtain valid data. Two main relaxation processes are observed: a a-relaxation peak associated to the glass transition temperature (Tg) at temperature about 278C, and a broad b*-relaxation at about 2608C, partly attributed to CH orientation. The variation of the dissipation factor versus sample thickness (from 3.8 to 11.8 mm) is correlated and partly explained by the increase of crystallinity ratio and lamella size at larger thicknesses. It comes out that the thinnest film seems perfectly meet the application requesting, namely lowest dissipation factor and highest permittivity.

The dielectric properties of two grades of bi-oriented isotactic polypropylene were studied with a variety of techniques: breakdown field measurements, dielectric spectroscopy, thermally stimulated depolarization currents (Is), and direct-current (dc) conduction I values. Standard polypropylene (STPP) and high-crystallinity polypropylene (HCPP) films were investigated. Measurements were carried out over a wide temperature range (2150C/1125C). The breakdown fields in both materials showed a very small difference. On the other hand, the dielectric losses and dc conduction I values were significantly lower in HCPP. Both materials showed a decrease in the dielectric loss versus temperature in the range 20–90C; this is favorable for application in alternating-current power capacitors. The analysis of the dc I value allowed us to find evidence of two main conduction mechanisms: (1) below 80C in both materials, a hopping mechanism due to the motion of electrons occurred in the amorphous phase, and (2) above 80C, ionic conduction occurred in HCPP, and hopping conduction occurred in STPP.

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.

Streamers initiated in point-plane electrode geometry under impulse voltage are studied. Characterization up to 30 kV in both polarities shows two streamer types. In negative polarity, only slow `bush-like' streamers with a velocity of about 100 m s-1 are observed. In positive polarity, both `bush-like' and filamentary streamers appear. Positive `bush-like' streamers have similar velocity as in negative polarity, whereas filamentary streamers have a propagation velocity up to 30 km s-1. This high velocity is correlated to large transient current and light emission. Spectroscopic study of the light emitted by filaments shows mainly the second positive system of nitrogen. This system allows us to estimate the gas temperature inside a filament by making a comparison with a numerically simulated spectrum. A temperature close to 500 K is found. Dilution of hydrogen gas in liquid nitrogen allows us to determine also the electron density by Stark broadening of the Balmer Hα line. An electron density of about 1.1018 cm-3 is found.

Presents a study of streamer propagation in transformer oil, with point-plane and semi-uniform geometry. The latter is made of parallel plane electrodes, with a thin triggering point of calibrated size. By reducing the length of the point, it is possible to move progressively from a point-plane geometry to a quasi uniform geometry. The propagation of streamers is impeded by the presence of a metallic plane behind the triggering point, that reduces the field on propagating streamers. The effect varies widely according to the streamer type considered. The propagation of negative and fast positive streamers is nearly quenched, whereas slower filamentary positive streamers (usually responsible for breakdown in oil) are less affected. This shows that many results obtained in point-plane geometry can not be simply extrapolated to the more realistic case of uniform field.

Presents a study of streamer inception in mineral transformer oil, in point-plane and rod-plane geometry under impulse voltage. The measurements performed with points and rods over a wide range of tip radii show a marked decrease of initiation fields when the electrode radius is increased. The initiation field is divided by 30 over the investigated range (from 1 /spl mu/m points up to 2 cm rods). This effect compares fairly well with the "surface effect" known for breakdown voltages under uniform field with large electrodes. Plotting these results together shows the decrease of streamer initiation fields over a very wide electrode surface range (12 decades), proportional to S/sup -0.17/ (S: electrode surface area, cm/sup 2/). These results suggest the influence of electrode surface defects on streamer initiation under impulse voltage.

This paper presents an experimental study of streamer initiation in liquid cyclohexane. The aim is to compare specific features of streamer initiation in positive and negative polarity. Experiments are carried out in point-plane geometry under impulse voltage. A new differential charge measurement system is presented. It allows to obtain a high detection sensitivity under impulse voltage (below 0.1 pC). The investigations presented consider the influence of applied voltage, impulse rise time, hydrostatic pressure. As previously observed under DC voltage, negative streamer initiation is due to an avalanche in the liquid phase, and is not affected by pressure, up to 10 MPa. In contrast, the mechanisms leading to positive streamer initiation are not clearly established. In this polarity, an increase of streamer initiation voltage is observed when the pressure is raised.

This paper presents an experimental study on the influence of an aromatic additive (pyrene) on the propagation of positive filamentary streamers and breakdown in liquid cyclohexane. Experiments are carried out in point-plane gaps up to 5 cm over a wide voltage range. With pyrene, the propagation is facilitated, and thus the breakdown voltage is lowered in point-plane geometry. Streamers also become very branched, and a much larger number of filaments propagate than in pure cyclohexane. Correlated to this, a large increase of the inception voltage of fast streamers (acceleration voltage) is observed at high voltage. This fact can be explained by macroscopic electrostatic properties of streamers, which depend on their geometrical structure. When more and more filaments propagate at high voltage they shield each other, which in turn limits their tip field (and thus velocity) at a nearly constant value over a large voltage range. The propagation velocity is thus not only determined by microscopic processes at the filament heads. Consequently, it does not always constitute an adequate parameter to directly appreciate and compare microscopic propagation processes. A discussion about streamer mechanisms is also presented.

This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps (/spl les/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.

This paper presents an experimental study of positive streamer propagation in mineral oil, in large point-plane gaps under impulse voltage. A systematic investigation was done concerning the influence of gap distance (/spl les/35 cm), and voltage from streamer inception up to large overvoltages. The measurements presented concern breakdown voltage measurements, time to breakdown, visualization of streamers (streak and still photographs), electrical measurements (transient current, charge), light emission intensity. Streamers are characterized and then classified into different modes (2nd, 3rd and 4th modes) according to their propagation velocities up to more than 100 km/s. According to the applied voltage, transitions between modes are observed, and correlations are established between charge, shape, and streamer velocity. The physical mechanisms as well as practical consequences are then discussed.

This paper presents experimental data and calculations concerning the electrical properties of positive streamers in mineral oil at large gaps and HV. The experiments concern the measurement of charge, electric field, and the determination of the potential drop along streamers, either in the liquid alone, or for streamers guided within insulating tubes. Calculations of charge and field distribution around streamers are carried out by charge simulation. To do this, streamers are represented by objects with simple shapes (spheres or cylinders) equivalent to their macroscopic aspect. These models lead to a correct agreement with measured streamer charge and field on the plane electrode. Qualitative correlations are established between calculated field distributions and streamer behavior such as velocity, transitions between propagation modes. It is also concluded that the potential drop in streamers and branching both act as regulating mechanisms that help to keep the streamer tip field, and hence the velocity, constant over a wide voltage range.

For over a decade, our group has been working on the development of distributed electron cyclotron resonance (DECR) plasma sources. The uniform distributed plasma (UDP) is the latest outgrowth of DECR, which has proved to be a flexible concept, leading to task-adapted plasma sources. Our plasma-based ion implantation (PBII) reactor is a 60 cm diameter, 70 cm high cylinder. The inside of the cylinder wall is covered with an array of 24 tubular magnets, 2.45 GHz microwave power feeds, and wave propagators. This peripheral plasma source of a good square meter produces a uniform distributed plasma (UDP), suitable for the treatment of wafers, pipes or objects of arbitrary form. Initial PBII plans concern the nitridation of silicon wafers. At 1 mTorr pressure and 1.3 kW input power, the N 2 plasma has a density of 2 × 10 10 ions cm −3 and an electron temperature of 1.2 eV. The N + /N 2 + ratio of 7/3 in the N 2 UDP plasma, determined by quadrupole mass spectrometry, is favorable for PBII applications. A 45 kV pulsed power supply is available for initial tests, but should be supplanted by a more powerful source for meaningful experiments.

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.

This paper presents a study of the dynamics of positive streamer filaments in pentane using an optical method described in the text for pressures up to 9 MPa. It is observed that filaments expand and collapse, in a similar way to cavitation bubbles. Good agreement with the Rayleigh model shows that the dynamics of filaments is determined by liquid inertia. The influence of electric forces (electrostatic pressure) on the dynamics is found to be negligible. An evaluation of the energies involved shows that filaments are mainly composed of vapour, whose pressure varies with time and space and is on average higher than the hydrostatic pressure. Vaporization results mainly from energy dissipated at the filament extremity, which may be compared to a propagating point heat source of about 10 W power. At low pressure, a significant influence of energy dissipation within the filament, attributed to the transient current flowing during propagation, is also observed. At high pressure and/or high voltage, the stopping of the streamer is due to collapse of the gaseous filaments.

In this first part, the gaseous nature of filamentary positive streamers is demonstrated by the influence of hydrostatic pressure on their propagation in cyclohexane and pentane up to 7 MPa. At a fixed voltage, when the pressure is increased, the propagation velocity is found to be constant whereas the stopping length of streamers is greatly reduced. Correlated to this effect, the duration of transient currents and light emission signals is reduced. These effects are discussed in terms of the hypothesis that the conductivity of the streamer results from a gas discharge mechanism occurring within the filaments. These results constitute the experimental basis on which a study of the filament dynamics has been developed (part II).

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. >

This paper presents an experimental study of the initiation of positive streamers under step voltage in point-plane geometry, in a number of hydrocarbon liquids. The experimental conditions (point tip radius, pulse risetime, gap length) were strictly controlled in order to take valid measurements of the initiation fields and threshold propagation voltages of streamers. Transient currents associated with the initiation of positive streamers were recorded down to the /spl mu/A range with a sensitive differential measurement technique. The different current shapes obtained in the liquids studied are presented. The measured inception fields of positive streamers and propagation thresholds of filamentary streamers are discussed according to the properties of the liquids (ionization potentials and mobility of charge carriers). >

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. >

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.

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.

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.

Gassing phenomena under an AC voltage in various dielectrics liquids subjected to partial discharges produced in different gases are discussed. The roles of the numerous experimental parameters which affect the results have been studied using a uniform field test cell. It is shown first that from the classical PD equivalent circuit, a more refined analysis permits a good description of the influence of parameters such as gas and solid thickness as well as the amplitude and frequency of the applied voltage on gas volume variations, and discharge characteristics. This allows one to get suitable measurement of the charge and energies involved in the gaseous discharges, from which one can deduce the energy required to absorb (or to create) a gas molecule. This gassing energy, which is independent of the experimental parameters, is characteristic of each gas/liquid pair and constitutes a valuable quantitative evaluation of the gassing properties. >

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.