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This paper presents partial discharges and streamers characterizations on 3 dielectric liquids under variation of wide temperature range, i.e.: aromatic hydrocarbon dibenzyltoluene, synthetic ester and halogenic perfluoropolyether liquids. Results indicate distinct behaviors among these liquids under highly divergent AC sinusoidal and impulse electric field. Dibenzyltoluene demonstrates excellent feature in suppressing discharges even at elevated temperature. In synthetic ester, positive discharges appeared to be less numerous but extremely high up to several nC in amplitude are strongly related to the further breakdown. Streamers inception voltage which denotes the initial discharges properties and streamer propagation velocity varies from several m/s up to order of km/s, show less dependency to temperature instead of the intrinsic characteristic of liquid itself. These prebreakdowns phenomena constitute interesting information in planning and selecting insulation liquids and analyzing fundamental mechanism during the operation and maintenance for HV utility applications.

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.

The characterization of pre-breakdown streamers in liquids is carried out under high temperature up to 200°C. Measurements are obtained either in a high temperature non-polar liquid (dibenzyltoluene, DBT), or in a synthetic ester (Midel®7131) for comparison. Streamer inception voltages are measured in both polarities, as well as propagation velocities. The results obtained show that only minor modifications are recorded when temperature is raised up to 200°C. Due to its stability at high temperature, DBT constitutes an interesting solution in the perspective of using liquids for the insulation of high temperature power electronics.

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.

Electrical breakdown properties of Dibenzyltoluene (DBT) are investigated over wide temperature range (from room temperature up to 350°C), and under pressure variation « 1 kPa to 200 kPa). The purpose is to investigate the suitability of DBT as potential insulating material for high temperature applications, such as encapsulant in high temperature power electronics modules. Measurements were carried out using a special high temperature test cell, under nitrogen atmosphere. Sphere-sphere electrodes and voltage ramp were used. Results show some reduction of breakdown voltage at high temperature, down to 58.2% at 350°C. A similar effect is observed when pressure is reduced. This reduction occurs when the liquid approaches boiling conditions, hence facilitating the appearance of vapor bubbles.

This paper deals with physical processes involved during inception of positive streamers in liquid cyclohexane. The inception of streamers is detected with a high temporal resolution by measurements of charge, visualization, and detection of phase change by scattered laser light. These techniques allow us to obtain measurements of inception voltages and inception delay time in various conditions of impulse voltage with different slew rates, and pressure. In positive polarity, a minimum "formative time" close to 40 ns is observed, as well as a large influence of both pressure and slew rate. The hypothesis of cavitation induced by the EHD motion, and liquid boiling due to heat dissipation are considered to explain positive streamer inception.

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.

This paper deals with physical processes involved during streamer inception in liquid cyclohexane under impulse voltage. In point-plane geometry, the inception of streamers is detected with a high temporal resolution (about 1 ns) by measurement of charge, visualization, and detection of phase change by scattered laser light. These techniques allow us to obtain precise measurements of the inception delay time, versus applied voltage and hydrostatic pressure. Measurements carried out in positive and negative polarity with a fast impulse voltage (20 ns risetime) show a large difference. In positive polarity, a minimum "formative time" close to 45 ns is observed, while negative streamers can appear almost immediately at high voltage. The hypothesis of cavitation induced by the EHD motion is considered to explain positive streamer inception.

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 .

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 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 (

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 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.

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.

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 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.

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.

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. >

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.

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.

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.

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.

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.

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 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 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 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.

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 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.

The two general specifications required for plasma-based ion implantation are low pressure, large size plasmas and high voltage high current pulse generators. In addition, pulses with rise and fall times of the order of the inverse ion plasma frequency and with much longer durations than those of the inverse ion plasma frequency are most often required. To fulfill these requirements, a new type of high voltage generator using a pulse transformer has been developed. A “mettglass”® magnetic core is used as step-up pulse transformer. Voltage at the primary is provided by transistor switches which can achieve rise and fall times of less than 1 μs and maximum pulse currents of 100 A. The primary of the transformer consists of 96 turns wired in parallel and the secondary of 96 turns wired in series. The performances reported with this pulse generator were obtained on a test resistor and then on a substrate immersed in a plasma.

Prebreakdown phenomena in transformer oil are studied under AC voltage in different field configurations, ranging from very divergent point-plane geometry to quasi-uniform fields produced by rods or spheres. A detailed study of phenomena in both polarities is presented, and the minimum conditions required for streamer breakdown voltage measurements under AC are established. In negative polarity, the generation of a microscopic gas bubble ( approximately 10 mu m in diameter) immediately precedes the development of streamers. In the case of bushlike streamers, it is concluded from energetic considerations that the vaporization of the liquid constitutes the main process of streamer growth. In positive polarity, similar conclusions are drawn for slow bushlike streamers, whereas in the case of fast filamentary streamers, it is observed that the conductivity of filaments determines their propagation. >

Power module reliability can be increased if problems due to high electric fields can be better understood. This paper proposes a global analysis using finite element simulations. Results are confirmed thanks to optical detection of partial discharge localization. Solutions to reduce field reinforcement are proposed and discussed. A discussion of maximum electric field handled by insulation gel is also provided.

This paper presents a study of slow positive streamers in cyclohexane under impulse voltage. A high sensitivity differential technique is used to record the streamer charge, together with high speed visualization. At voltages below the breakdown voltage, streamers stop while the voltage is still applied. It is shown that the first event preceding stopping is the hydrodynamic collapse of streamer filaments close to the point. This collapse "propagates" from the point to the streamer head, and the streamer then totally disappears. As soon as the streamer collapse starts, the streamer current is stopped, no more electrical energy is supplied, and liquid vaporization ceases. However, the streamer head still propagates at a reduced velocity. These observations constitute a direct evidence of the influence of space charges and electric field (i.e. electrostatic pressure) on propagation processes.