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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.

This paper presents an experimental study of Partial Discharges and light emission with ceramic substrates embedded in silicone oil, under ac and square high voltage. Two types of ceramic materials used in power electronics modules are considered: Aluminum Nitrite (AlN) and Alumina (Al2O3). With AlN, a large light emission due to the electroluminescence of the ceramics occurs during the negative dV/dt. This phenomenon is not observed with Al2O3. With Al2O3 and silicon oil, the light emitted is only due to PDs, and recording under square voltage wave shows that PDs are more numerous and/or more intense compared to ac. Square voltage is more relevant to actual working conditions within power electronics modules than ac.

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.

Surface resistance measurements on insulating solids are usually performed by measuring the current induced when an electric field tangential to the surface is applied. The measured current results from the addition of currents flowing along the surface, and also partly in the volume. In this paper, surface current measurements on an alumina-filled epoxy resin are performed under varying conditions of temperature (40-60°C), relative humidity (0-80 %), gas nature (SF6, N2, air), pressure (0.1-0.8 MPa). In dry conditions, measured currents do not depend on the gas nature and pressure. In air, when the relative humidity (RH) is varied, a large influence is measured, and the recorded variation of surface currents includes two phases: a rapid one attributed to the immediate change of surface resistance, followed by a much slower variation attributed to the progressive impregnation of the material by water. Numerical simulation shows the dominant influence of surface properties on measured currents, and evidences the existence of a surface layer with a higher conductivity compared to the material volume, even in dry conditions.

This work reports on the relationship between structure and dielectric properties of biaxially oriented polypropylene. The morphology of semi-crystalline bioriented isotactic polypropylene (BOiPP) films is investigated using Wide Angle X-ray Diffraction (WAXD) and Polarized Optical Microscopy (POM). A β-orthorhombic structure, with a crystallinity ratio of about 46%, and a “Crater” morphology of the β-form is identified. Dielectric properties are measured by Broadband Dielectric Spectroscopy (BDS) over a wide temperature range (-150°C to 125°C). 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 α-relaxation peak associated to the glass transition temperature (Tg) at low frequency and temperature about -7°C, and a broad β*-relaxation at high frequency at about -60°C, attributed to CH orientation. The variation of the dissipation factor versus sample thickness (from 3.8 μm to 11.8 μm) is correlated and explained by the increase of crystallinity ratio at larger thicknesses.

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.

In Insulated gate Bipolar Transistor (IGBT) modules, ceramic substrates are used as insulation between semiconductor chips and the ground potential. The substrate, semiconductor chips and connections are covered with silicone gel to prevent partial discharges (PDs) from occurring. In this work, partial discharges are investigated on laboratory samples made of metalized substrates embedded in silicone gel and various liquids. Substrates are made of aluminum nitride (AlN), alumina (Al2O3), or glass/epoxy composite. The objective is to determine the actual origin of PDs, by changing the nature of encapsulating material and substrate. The various liquids used are first characterized using a point-plane geometry. Experiments show that a rather large discrepancy on the PD behavior can be due to the ceramic substrate itself. "Good" and "bad" substrates can be identified on the basis of PD measurements. The influence of the sample preparation (degassing procedure) is also evidenced. The main conclusion obtained in this paper shows that partial discharges do not occur within the encapsulating material, contrary to most previous hypotheses, but within pores existing in sintered ceramic materials. Changing the substrate nature has a large influence on PD behavior, whereas a modification of the encapsulating material has almost no effect.

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.

Contact bounces are usually erratic and undesirable phenomena that greatly affect the lifetime and reliability of electrical contacts. This paper deals with bounce phenomena that are experimentally observed during DC contact make and break in a MEM switch. In such a device, the bouncing behavior cannot be explained by inertia forces that are far lower from those existing in macroscopic relays. Therefore this study aims at providing a better understanding of bounce phenomena at the nanometer scale. Experiments have been performed with a nano-indenter and an Atomic Force Microscope (AFM), both used to actuate a contact with a well controlled electrode velocity. Multiple voltage transients of several tens of ms are observed when make/break operations are performed at an extremely low velocity (few tens of nm/s). This occurs when the distance between contact electrodes is of the order of 10nm. A simple analysis of the static balance of forces shows that the electrostatic force becomes predominant at this scale, and that bounces result from a competition between the electrostatic force and the restoring force of the mobile contact. This paper highlights that in a MEMS switch a minimal actuation velocity (here 1µm/s) is required to prevent contacts from bouncing.

The reliability of electrical contacts is now the major challenge to improve the lifetime of Micro Electro Mechanical Switches. This paper investigates contact failure due to material transfer in real MEMS devices. The mechanisms of degradation observed on components have been reproduced with a good correlation on an AFM-based setup. Therefore, the importance of the closure phase and the voltage applied across the contact on the material transfer phenomenon has been highlighted with a fine quantitative analysis of the transferred volume.

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. >

This paper describes an experimental study of streamer inception in mineral oil under ac voltage, with rod and point electrodes. Positive and negative streamer inception frequencies versus voltage are investigated in gaps up to 40 cm with different electrode shapes and different conditioning of the oil (filtered oil, addition of cellulose particles, water). Streamer inception probability increases exponentially versus field, and it is not possible to simply define an “inception voltage”. A voltage (or field) value correlated to an inception probability must be used to properly compare different experiments (comparison between liquids, influence of pollution, etc.). Under ac, several effects are superposed to reduce dielectric strength: “scale effects”, influence of pollution, long time duration. With sharp points, injected space charges considerably influence experiments, and the results obtained cannot be extrapolated to practical applications in which the effect of space charge is mostly absent.

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.

International audience; An integrated solution for the galvanic isolation between power transistors and their control unit is presented in this paper. This solution is based on a monolithic integration of a photodetector within a power MOSFET without any modification of its fabrication process. This photoreceiver can be associated with a monolithic driver to drive high side switches. Exhaustive characteristics for several integrated photodetectors are presented and discussed: quantum efficiency, step response, small signal analysis and sensitivity to the High Voltage MOSFET's Drain. The results of this analysis are photoreceivers with a Full Width at Half Maximum above 300MHz and a responsivity above 0.15A/W at a wavelength of 500nm. This leads to an integrated low power and high bandwidth optical isolation.

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.

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 reports an experimental investigation about streamer propagation and breakdown in natural esters (rape-seed oils). Experiments are carried out over a large range of gap distances (from 2 to 20 cm) and voltage (up to 460 kV), in order to determine the properties of natural esters for high voltage insulation. A detailed description of positive and negative streamers is obtained by the measurement of propagation velocity, stopping length, transient currents, charge, streak photographs of the emitted light. As in previous studies carried out in mineral transformer oil, several propagation "modes" can be observed in rape-seed oil. They show the transition from "slow" streamers (velocity about 1 km/s), to fast streamers (velocity up to 200 km/s). Fast positive streamers propagate at much lower voltage in rape-seed oil compared to mineral oil. In turn, this induces lower breakdown voltages and shorter time to breakdown in this liquid. Measurements of currents and streak photographs bring the evidence of the stepped character of fast positive streamers and show the constancy of their velocity under very high voltage.

International audience; In this study, the efficiency of different types of pulsed electrical discharges for the removal of organic pollutants from wastewater has been determined. Three discharge types, either in the water volume or in close proximity to the water surface are studied. The production of hydrogen peroxide in pure water, and the degradation of two typical pollutants (4-chlorophenol and 4-nitrophenol) is measured together with the amount of electrical energy dissipated in discharges. It is shown that the energy yield for the degradation of organic pollutants, expressed in terms of degraded moles per Joule, strongly depends on the discharge type. The highest efficiency is obtained with pulsed corona discharges in humid air above the water surface. A lower efficiency is found with spark discharges in water, and the less efficient process is constituted by streamer discharges in water. The influence of ferrous ions added to solutions is also very different according to the discharge type. This helps to get a better understanding of the degradation processes involved with the different discharge types.

Water absorption in an epoxy composite used for insulation of stator bars in large generators is investigated in order to study both diffusion kinetics and effects on dielectric properties. Water absorption can be described by the superposition of two Fickian mechanisms, the first occurring within epoxy and the other at interfaces between epoxy and fillers. A large decrease of breakdown strength versus water uptake is observed. It points to the large deleterious effect of water on insulation reliability. Dielectric spectroscopy is performed under various conditions of water impregnation, electric field and temperature. A very large increase of permittivity and losses is recorded in the presence of water, especially at low frequency. At power frequency, dielectric properties do not show a non-linearity when the field is increased up to service values. At very low frequency, a decrease of losses when the field is increased is observed. It is due to the Garton effect. Most of results show that the wet material behaves as a low frequency dispersion system (LFD).

In this paper, the water concentration profile within the insulation of a stator bar in the presence of a water leak is calculated by resolving the Fick's equation. The temperature strongly activates diffusion processes, but the shapes of concentration profiles remain similar. From the measured dielectric properties versus water content, an electrical model is developed in order to calculate the field and potential distribution within the insulation. The field is reduced in wet zones due to the high local permittivity and conductivity compared to dry zones. This effect is favorable since the breakdown field is lowered in wet zones. On the other hand, the field in dry zones is reinforced, but the value attained remains lower than the short-term breakdown strength. This explains why stator bars in the presence of waters leaks may survive for long times, in spite of high local water concentrations in excess of 1%. The high sensitivity of dielectric properties at low frequency to the presence of water suggests non-destructive methods to detect wet bars in a real power generator.

This paper presents an experimental study of streamer inception in a silicone gel. Many parameters are able to influence the inception of discharges in a gel: voltage wave shape, electrode geometry, temperature, etc. In this work, streamer inception is studied in various conditions of electrode shapes (from sharp points up to quasi-uniform field), voltage (ac, dc, impulse voltage) and temperature. Streamer inception is detected by the measurement of breakdown voltages in selected conditions. The results obtained show that streamer inception field varies when the electrode shape is changed, in a similar way as in liquids: the inception field decreases when the electrode surface increases. On the other hand, temperature and voltage wave shape have almost no influence on the inception field. Results are discussed in comparison with data already obtained in liquids in similar conditions.

A study of transient currents associated with the propagation of filamentary positive streamers in liquids is presented. Experimental measurements of currents in several selected liquids were analyzed with a simple model in which the channel is compared to a cylindrical conducting filament. Good agreement was found between theory and experiment, which indicates a rather high conductivity of the streamer filaments. An estimation of the charge carrier mobility in the filament was obtained, and it is concluded that the high current densities recorded are likely carried by electrons moving in a gaseous phase. The propagation of positive filaments in liquids is found to be similar to that of leaders in electronegative gases. >

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.

Silicone gels are widely used to encapsulate power electronics modules. The objective of this paper is to study the partial discharge (PD) mechanism in a silicone gel, using electrical measurements and fast visualization. Experiments are carried out in a pointplane geometry, using either impulse voltage or ac. Under impulse voltage, the visualization shows that the first discharges recorded in a new sample are due to the initiation and propagation of "streamers", with features close to those previously reported in viscous liquids. Under ac, a stable PD regime rapidly establishes: a streamer creates a cavity with a long lifetime, and PDs occur in this cavity during the subsequent ac waves. Since slow streamers develop during long times (some ms), classical PD measurements do not provide an adequate evaluation of the actual discharge magnitude: very small PDs (1 pC) are recorded, whereas large cavities with a total charge in excess of 10 pC are actually present in the gel. When PDs are repeated permanent degradations of the gel rapidly occur, showing the limited self-healing capabilities of silicone gels.

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 presents an experimental study of pre-breakdown discharges in water (usually called 'corona' or 'streamer') used to degrade organic molecules for depollution purposes. Streamers in water are generated by high voltage pulses of short duration in order to minimize energy losses by Joule heating. Measurements of the electrical energy used to develop streamers can be taken only when losses due to the electrical conduction in the test cell becomes negligible, i.e. at high voltage. Coupled to electrical measurements, the formation of hydrogen peroxide and the degradation of organic compounds such as 4-chlorophenol and 4-nitrophenol is studied. Chemical effects are correlated with the amount of electrical energy released, which depends mainly on conductivity and applied voltage. On the other hand, the chemical yield of these processes, expressed in terms of mole/joule is independent of voltage and conductivity.

In order to increase the knowledge of the processes involved during the degradation of insulating silicone gels under ac stress, recordings of the activity of partial discharges (PD) are performed. Silicone gels are commonly used for the insulation and packaging of high voltage semiconductors such as IGBT modules. We worked on a commercially available dielectric silicone gel. Electrical and optical apparatus allows the measurement of PD's and their location, either on actual power module substrates, or in a point-plane gap. The temperature dependence of PD inception voltage is moderate, while the number of PD's and their charge steeply increases versus temperature. We show that the formation of cavities constitutes the main factor responsible for the degradation under ac conditions. The actual self-healing capabilities of silicone gels are also studied and discussed.

We studied the effects of water absorption (up to 1 wt%) on the dielectric properties of mica/glass/epoxy laminated composites used for high-voltage insulation. The DC resistivity, the dielectric constant and the dielectric loss were studied in the 10 −2 –10 6 Hz range, as a function of water uptake. The dielectric characteristics are strongly affected by water ingress. Upon water sorption the resistivity decreases by several orders of magnitude. Its variation is well described by a percolation law. The low-frequency permittivity rises to very large values. The large increase in the permittivity is related to an electrode polarisation mechanism.

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. >

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.

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.