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Dielectric liquids, in combination with solids, have been used for insulating power transformers for more than a century. While mineral oils have dominated the market, new liquids with different molecular structures are now in common use. This brochure addresses the dielectric performance of the liquids as used in a transformer - including the potential influence of solid insulation on the behaviour of prebreakdown streamers in the liquid. Topics discussed include how the chemical composition of liquids influences the streamer velocities and breakdown, and how the electric field configuration may govern breakdown modes. The aim of this brochure is to review the dielectric requirements and test standards of dielectric liquids as seen from a transformer designer, to review the behaviour of dielectric liquids.

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.

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.

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.

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

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.

International audience

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.

International audience; Surface resistivity of solid insulators is usuallydetermined by measuring the current induced when an electricfield tangential to the surface is applied. This classical methodcan become questionable when the electric field is raised. In fact,a current may also flow above the interface through the humidgas, in addition to currents flowing at the interface and below theinterface within the solid bulk. This paper presents anexperimental study of the variation of the surface resistivityversus electric field and relative humidity RH of the surroundinggas. The results show that in dry gas (RH < 1%) and for anelectric field up 8 kV/mm, the surface gets no specific propertiescompared to the volume. However, in humid gas (1% < RH

International audience

International audience

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.

International audience

Todays, power electronics cover wide range of applications in our daily life, starting from household appliances, communications, transportation systems up to harsh and extreme environment as in oil and gas exploration and the deep space missions. The main deliveries of power electronics are energy efficiency, compact size, reliability, long durability. Improving power electronics will surely mean to deal with materials, the packaging system, switching technologies, heat dissipation, dielectric properties, thermal stability etc. It was since the first arc-mercury rectifying in traction system, and then reshaped by the discovery of classical semiconductor (Si based) and ultimately the wide bandgap semiconductor materials, such as SiC, GaN and carbon based (diamond). They have superior thermal and dielectric properties compared to previous classical semiconductor technologies (Ge, Si and GaAs), and allow devices to operate at higher voltage, temperature and switching frequency in power modules. Unfortunately, these developments are not equally followed by other parts within, such as encapsulant.Despite their key roles to provide mechanical and electrical protection inside a power module, silicone gel as major encapsulant is limited to 200°C, which is far below devices (e.g. SiC at 500°C). Encapsulant came from polymerization and curing process of silicone liquids mixture and transforms into gel. They worked very well when assembly with classical SC devices, but not with WBG SC. Thus, it is necessary to solve this thermal related issue by improving silicone gel or start looking for other type of encapsulant with better thermal performance such as dielectric liquid or gas.Dielectric liquids have been used as insulating medium for high voltage (HV) applications for decades. Their excellent self-healing and arc quenching properties were used in the HV circuit breaker applications even though nowadays replaced by gas. Their low viscosity allow the fluid flow to exchange heat from internal source yielding effective cooling system as in power transformers. Other industries use dielectric liquids as heat transfer liquid at much higher temperature range compare to those in HV applications. Of course as heat transfer liquids, their dielectric properties are out of considerations. Nevertheless, having this wide range of applications spectrum, dielectric liquids seem rather promising and potential as alternative encapsulant. Some questions then aroused such as how are their electrical properties at high temperature (HT) approx. 400°C, are their dielectric properties stable at HT and can they contribute to cooling of devices inside power module.This work presents the initial study of dielectric liquids for HT power electronics module applications. We demonstrated the electrical characterization of several dielectric liquids under influence of temperature such as dielectric spectroscopy and ion mobility measurement, partial discharge, streamers and breakdown. Interesting physical phenomena such as liquid motions due to EHD and natural thermal convection were observed during experiments. Comparison among liquids are showed to indicate the most convenient. In term of application, conditions were adapted and simplified to replicate as those in power module when we performed characterizations to actual ceramic substrates under quasi-uniform to highly divergent electric field with AC, DC and impulse voltage. Many fundamental behaviours of liquids have been confirmed and evidenced at HT range. Governing parameters for electrical properties such as breakdown, charge injection etc. were affirmed.While not all aspects of encapsulant requirement in term of HT are covered, this work has established essential basis for electrical properties of dielectric liquids. Further works are required to fully assess their compatibility as alternative encapsulant, such as thermal ageing process, cooling contribution, complete modelling, etc., La recherche et le développement sur les matériaux semi-conducteurs ont permis de transformer la technologie des dispositifs électroniques de puissance, avec une densité de puissance, des performances thermiques et un dimensionnement plus compacts. Ils permettent aux appareils de fonctionner à des tensions, températures et fréquences de commutation plus élevées dans les modules de puissance. Pourtant, ces développements ne sont pas suivis de la même manière par d’autres éléments, tels que les encapsulants.Avec un matériau d'encapsulation récent, à savoir un gel de silicone, la température maximale de fonctionnement ne peut pas dépasser 200 ° C alors que les dispositifs à semi-conducteurs WBG sont très supérieurs (par exemple, du SiC à 500 ° C). Il s’agit là d’un obstacle majeur car il joue un rôle important dans la protection mécanique et électrique d’un module de puissance. Dans ce travail, nous proposons des liquides diélectriques comme agent d’encapsulation alternatif pouvant avoir une performance thermique supérieure au gel de silicone. Les caractérisations diélectriques de plusieurs candidats ont été effectuées dans le cadre d’une cellule d’essai spécialement conçue, capable de chauffer à haute température dans un environnement contenant de l’azote afin d’éviter les risques d’incendie et d’oxydation. Nous avons mesuré la conductivité de liquides soumis à une variation de température en appliquant une spectroscopie de champ alternatif dans une large plage de mobilité fréquentielle et ionique sous une variation de polarité inverse en courant continu. Nous étudions l'influence de la température et de la pression sur les pannes avec des champs quasi uniformes et divergents, ainsi que des décharges partielles, aussi bien dans les liquides que dans les substrats céramiques noyés dans des liquides, afin de démontrer leurs applications dans les modules de puissance. Des modèles numériques ont également été développés par simplification à partir des résultats de spectroscopie diélectrique afin d'estimer et d'observer la distribution de champ à un point triple critique.Enfin, nous présentons une comparaison de candidats liquides et de gel de silicone pour montrer leurs avantages et leurs inconvénients pour les applications d’électronique de puissance à haute température. Néanmoins, ces travaux ne couvrent pas tous les aspects fondamentaux et d’applications tels que le vieillissement thermique, la capacité de refroidissement des liquides, etc., ces résultats ont établi une bonne base pour les liquides diélectriques dans les applications à haute température.

Todays, power electronics cover wide range of applications in our daily life, starting from household appliances, communications, transportation systems up to harsh and extreme environment as in oil and gas exploration and the deep space missions. The main deliveries of power electronics are energy efficiency, compact size, reliability, long durability. Improving power electronics will surely mean to deal with materials, the packaging system, switching technologies, heat dissipation, dielectric properties, thermal stability etc. It was since the first arc-mercury rectifying in traction system, and then reshaped by the discovery of classical semiconductor (Si based) and ultimately the wide bandgap semiconductor materials, such as SiC, GaN and carbon based (diamond). They have superior thermal and dielectric properties compared to previous classical semiconductor technologies (Ge, Si and GaAs), and allow devices to operate at higher voltage, temperature and switching frequency in power modules. Unfortunately, these developments are not equally followed by other parts within, such as encapsulant.Despite their key roles to provide mechanical and electrical protection inside a power module, silicone gel as major encapsulant is limited to 200°C, which is far below devices (e.g. SiC at 500°C). Encapsulant came from polymerization and curing process of silicone liquids mixture and transforms into gel. They worked very well when assembly with classical SC devices, but not with WBG SC. Thus, it is necessary to solve this thermal related issue by improving silicone gel or start looking for other type of encapsulant with better thermal performance such as dielectric liquid or gas.Dielectric liquids have been used as insulating medium for high voltage (HV) applications for decades. Their excellent self-healing and arc quenching properties were used in the HV circuit breaker applications even though nowadays replaced by gas. Their low viscosity allow the fluid flow to exchange heat from internal source yielding effective cooling system as in power transformers. Other industries use dielectric liquids as heat transfer liquid at much higher temperature range compare to those in HV applications. Of course as heat transfer liquids, their dielectric properties are out of considerations. Nevertheless, having this wide range of applications spectrum, dielectric liquids seem rather promising and potential as alternative encapsulant. Some questions then aroused such as how are their electrical properties at high temperature (HT) approx. 400°C, are their dielectric properties stable at HT and can they contribute to cooling of devices inside power module.This work presents the initial study of dielectric liquids for HT power electronics module applications. We demonstrated the electrical characterization of several dielectric liquids under influence of temperature such as dielectric spectroscopy and ion mobility measurement, partial discharge, streamers and breakdown. Interesting physical phenomena such as liquid motions due to EHD and natural thermal convection were observed during experiments. Comparison among liquids are showed to indicate the most convenient. In term of application, conditions were adapted and simplified to replicate as those in power module when we performed characterizations to actual ceramic substrates under quasi-uniform to highly divergent electric field with AC, DC and impulse voltage. Many fundamental behaviours of liquids have been confirmed and evidenced at HT range. Governing parameters for electrical properties such as breakdown, charge injection etc. were affirmed.While not all aspects of encapsulant requirement in term of HT are covered, this work has established essential basis for electrical properties of dielectric liquids. Further works are required to fully assess their compatibility as alternative encapsulant, such as thermal ageing process, cooling contribution, complete modelling, etc.; La recherche et le développement sur les matériaux semi-conducteurs ont permis de transformer la technologie des dispositifs électroniques de puissance, avec une densité de puissance, des performances thermiques et un dimensionnement plus compacts. Ils permettent aux appareils de fonctionner à des tensions, températures et fréquences de commutation plus élevées dans les modules de puissance. Pourtant, ces développements ne sont pas suivis de la même manière par d’autres éléments, tels que les encapsulants.Avec un matériau d'encapsulation récent, à savoir un gel de silicone, la température maximale de fonctionnement ne peut pas dépasser 200 ° C alors que les dispositifs à semi-conducteurs WBG sont très supérieurs (par exemple, du SiC à 500 ° C). Il s’agit là d’un obstacle majeur car il joue un rôle important dans la protection mécanique et électrique d’un module de puissance. Dans ce travail, nous proposons des liquides diélectriques comme agent d’encapsulation alternatif pouvant avoir une performance thermique supérieure au gel de silicone. Les caractérisations diélectriques de plusieurs candidats ont été effectuées dans le cadre d’une cellule d’essai spécialement conçue, capable de chauffer à haute température dans un environnement contenant de l’azote afin d’éviter les risques d’incendie et d’oxydation. Nous avons mesuré la conductivité de liquides soumis à une variation de température en appliquant une spectroscopie de champ alternatif dans une large plage de mobilité fréquentielle et ionique sous une variation de polarité inverse en courant continu. Nous étudions l'influence de la température et de la pression sur les pannes avec des champs quasi uniformes et divergents, ainsi que des décharges partielles, aussi bien dans les liquides que dans les substrats céramiques noyés dans des liquides, afin de démontrer leurs applications dans les modules de puissance. Des modèles numériques ont également été développés par simplification à partir des résultats de spectroscopie diélectrique afin d'estimer et d'observer la distribution de champ à un point triple critique.Enfin, nous présentons une comparaison de candidats liquides et de gel de silicone pour montrer leurs avantages et leurs inconvénients pour les applications d’électronique de puissance à haute température. Néanmoins, ces travaux ne couvrent pas tous les aspects fondamentaux et d’applications tels que le vieillissement thermique, la capacité de refroidissement des liquides, etc., ces résultats ont établi une bonne base pour les liquides diélectriques dans les applications à haute température.

The increasing number of interconnections in electrical networks and the massive integration of renewable energies nowadays comes with an increase of short circuit currents, and more constraints on high voltage circuit breaker during the current clearance. To solve this problem, a solution consists in inserting a fault current limiting device on electrical lines. Among the available technologies, the superconducting fault current limiter is ideal from the perspective of transmission system operator. However the design of this device is non-trivial, especially the electrical insulation, which is very specific to this apparatus: electrical insulation in a cryogenic environment (liquid nitrogen at - 196°C), superconducting tapes inducing electric field reinforcement, and strong transient heating generating numerous vapor bubbles.In this context, the aim of this work is to gather data and knowledge supporting the design of future superconducting fault current limiters. First, the case study of a fault current limiter model inserted on an electrical line allowed to deduce the main stresses on this device during its working sequence. These constraints are then reproduced within a high voltage cryostat. Experimental results gathered include breakdown voltages measured in various conditions of voltage type (DC or impulse voltage), pressure (from 1 to 4 bars), temperature (from 65 to 77 K), inter electrode gap (from 5 to 10 mm), with and without transient heating of the tape. The observation of boiling conditions and pre breakdown phenomena evidenced several new phenomena, and allowed to perform a qualitative interpretation of the breakdown voltages. An important result is the observation of the complex and sometimes adverse effect of subcooling (temperature decrease or pressure increase) on breakdown voltages, due to the variation of nitrogen bubbles size.; L’interconnexion de plus en plus grande des réseaux électriques traditionnels AC et l’émergence de réseaux DC s’accompagne de courants de court circuit plus importants, et de contraintes plus fortes sur les disjoncteurs haute tension lors de la coupure du courant. L’une des solutions pour maitriser le courant de court circuit consiste à insérer un limiteur de courant en série sur les lignes haute tension. Parmi les technologies existantes, le limiteur de courant supraconducteur est un candidat idéal du point de vue du réseau, mais il fait face à de nombreuses contraintes de dimensionnement. Parmi elles, l’isolation électrique est particulièrement critique et très spécifique à cette application : milieu cryogénique (azote liquide à -196°C), rubans supraconducteurs générant des renforcements de champ électrique, et chauffage transitoire des rubans lors de la limitation de courant qui induit une forte ébullition de l’azote liquide.L’objectif de ce travail est de rassembler des données et des connaissances pour le dimensionnement de l’isolation des limiteurs de courant. Le cas d’étude d’un limiteur inséré sur une ligne électrique permet de mettre en évidence les principales contraintes que celui-ci subit durant son fonctionnement. Ces contraintes sont reproduites dans un dispositif expérimental associant cryogénie et haute tension. Les résultats expérimentaux incluent des mesures de tensions de claquage obtenues dans diverses conditions de tension (DC, choc impulsionnel) de pression (de 1 à 4 bar), et de température (de 65 à 77 K), de distance inter électrode (5 et 10 mm), avec et sans chauffage de l’électrode ruban. L’observation des phénomènes d’ébullition et de pré claquage met en évidence des phénomènes et paramètres nouveaux, et permet d’interpréter qualitativement les tensions de claquage observées. L’un des résultats importants de ces travaux est la mise en évidence de l’influence complexe et parfois délétère du sous refroidissement (diminution de la température ou augmentation de la pression) sur les tensions de claquage, liée à la variation de la taille des bulles d’azote gazeux.

The increasing number of interconnections in electrical networks and the massive integration of renewable energies nowadays comes with an increase of short circuit currents, and more constraints on high voltage circuit breaker during the current clearance. To solve this problem, a solution consists in inserting a fault current limiting device on electrical lines. Among the available technologies, the superconducting fault current limiter is ideal from the perspective of transmission system operator. However the design of this device is non-trivial, especially the electrical insulation, which is very specific to this apparatus: electrical insulation in a cryogenic environment (liquid nitrogen at - 196°C), superconducting tapes inducing electric field reinforcement, and strong transient heating generating numerous vapor bubbles.In this context, the aim of this work is to gather data and knowledge supporting the design of future superconducting fault current limiters. First, the case study of a fault current limiter model inserted on an electrical line allowed to deduce the main stresses on this device during its working sequence. These constraints are then reproduced within a high voltage cryostat. Experimental results gathered include breakdown voltages measured in various conditions of voltage type (DC or impulse voltage), pressure (from 1 to 4 bars), temperature (from 65 to 77 K), inter electrode gap (from 5 to 10 mm), with and without transient heating of the tape. The observation of boiling conditions and pre breakdown phenomena evidenced several new phenomena, and allowed to perform a qualitative interpretation of the breakdown voltages. An important result is the observation of the complex and sometimes adverse effect of subcooling (temperature decrease or pressure increase) on breakdown voltages, due to the variation of nitrogen bubbles size., L’interconnexion de plus en plus grande des réseaux électriques traditionnels AC et l’émergence de réseaux DC s’accompagne de courants de court circuit plus importants, et de contraintes plus fortes sur les disjoncteurs haute tension lors de la coupure du courant. L’une des solutions pour maitriser le courant de court circuit consiste à insérer un limiteur de courant en série sur les lignes haute tension. Parmi les technologies existantes, le limiteur de courant supraconducteur est un candidat idéal du point de vue du réseau, mais il fait face à de nombreuses contraintes de dimensionnement. Parmi elles, l’isolation électrique est particulièrement critique et très spécifique à cette application : milieu cryogénique (azote liquide à -196°C), rubans supraconducteurs générant des renforcements de champ électrique, et chauffage transitoire des rubans lors de la limitation de courant qui induit une forte ébullition de l’azote liquide.L’objectif de ce travail est de rassembler des données et des connaissances pour le dimensionnement de l’isolation des limiteurs de courant. Le cas d’étude d’un limiteur inséré sur une ligne électrique permet de mettre en évidence les principales contraintes que celui-ci subit durant son fonctionnement. Ces contraintes sont reproduites dans un dispositif expérimental associant cryogénie et haute tension. Les résultats expérimentaux incluent des mesures de tensions de claquage obtenues dans diverses conditions de tension (DC, choc impulsionnel) de pression (de 1 à 4 bar), et de température (de 65 à 77 K), de distance inter électrode (5 et 10 mm), avec et sans chauffage de l’électrode ruban. L’observation des phénomènes d’ébullition et de pré claquage met en évidence des phénomènes et paramètres nouveaux, et permet d’interpréter qualitativement les tensions de claquage observées. L’un des résultats importants de ces travaux est la mise en évidence de l’influence complexe et parfois délétère du sous refroidissement (diminution de la température ou augmentation de la pression) sur les tensions de claquage, liée à la variation de la taille des bulles d’azote gazeux.