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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.

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 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.

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.

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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.

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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

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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.

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.

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.

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 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.

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 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.

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.

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 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.

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.

With the emergence of renewable energy, new technologies for energy distribution across long distances, such as High Voltage Direct Current in Gas Insulated Substations (HVDC GIS) are developed. Under HVDC, the electric field distribution is not anymore determined by permittivity of materials but by resistivities and charge accumulation. In presence of gas/solid interfaces, the role of the surrounding gas on charge accumulation must be considered. In this work, we first present characterization methods which allow measuring the current passing through gas and solid insulators for different environmental conditions. Using these methods, we investigate the influence of several parameters (temperature, electric field, relative humidity) on the measured current. The variations of volume and surface resistivities are obtained. Finally, the results obtained for both solid and gaseous insulations are used to develop a numerical model with a shape close to the industrial application, and observe the modification of electric field distribution in presence of water concentration and temperature gradient in an HVDC GIS., Avec l’émergence des énergies renouvelables, de nouvelles technologies pour la distribution de ces énergies sur de longues distances, telle que les Postes Sous Enveloppes Métalliques (PSEM) soumis à une haute tension continue (HVDC), sont développées. Sous HVDC, la distribution du champ électrique n’est plus déterminée par la permittivité relative des matériaux mais par leurs résistivités et l’accumulation de charge en surface des isolants. En présence d’interfaces gaz/solide, le rôle du gaz environnant sur l’accumulation de charge doit aussi être pris en compte. Dans ce travail de thèse, nous présentons tout d’abord des méthodes de caractérisations permettant de mesurer le courant au travers d’un gaz et dans les isolants solides, pour différentes conditions d’utilisations. Grâce à ces méthodes, nous avons pu étudier l’influence de plusieurs paramètres (la température, le champ électrique, l’humidité relative) sur le courant mesuré. Les variations des résistivités de volume et de surface sont ainsi obtenues. Enfin, les résultats obtenus pour les isolations solides et gazeuses ont été utilisés pour développer un modèle numérique présentant une forme similaire à celle utilisée dans l’application industrielle. Nous avons donc pu observer les modifications de la distribution du champ électrique induites par la présence d’eau et/ou des gradients de températures dans un PSEM sous haute tension continue.

With the emergence of renewable energy, new technologies for energy distribution across long distances, such as High Voltage Direct Current in Gas Insulated Substations (HVDC GIS) are developed. Under HVDC, the electric field distribution is not anymore determined by permittivity of materials but by resistivities and charge accumulation. In presence of gas/solid interfaces, the role of the surrounding gas on charge accumulation must be considered. In this work, we first present characterization methods which allow measuring the current passing through gas and solid insulators for different environmental conditions. Using these methods, we investigate the influence of several parameters (temperature, electric field, relative humidity) on the measured current. The variations of volume and surface resistivities are obtained. Finally, the results obtained for both solid and gaseous insulations are used to develop a numerical model with a shape close to the industrial application, and observe the modification of electric field distribution in presence of water concentration and temperature gradient in an HVDC GIS.; Avec l’émergence des énergies renouvelables, de nouvelles technologies pour la distribution de ces énergies sur de longues distances, telle que les Postes Sous Enveloppes Métalliques (PSEM) soumis à une haute tension continue (HVDC), sont développées. Sous HVDC, la distribution du champ électrique n’est plus déterminée par la permittivité relative des matériaux mais par leurs résistivités et l’accumulation de charge en surface des isolants. En présence d’interfaces gaz/solide, le rôle du gaz environnant sur l’accumulation de charge doit aussi être pris en compte. Dans ce travail de thèse, nous présentons tout d’abord des méthodes de caractérisations permettant de mesurer le courant au travers d’un gaz et dans les isolants solides, pour différentes conditions d’utilisations. Grâce à ces méthodes, nous avons pu étudier l’influence de plusieurs paramètres (la température, le champ électrique, l’humidité relative) sur le courant mesuré. Les variations des résistivités de volume et de surface sont ainsi obtenues. Enfin, les résultats obtenus pour les isolations solides et gazeuses ont été utilisés pour développer un modèle numérique présentant une forme similaire à celle utilisée dans l’application industrielle. Nous avons donc pu observer les modifications de la distribution du champ électrique induites par la présence d’eau et/ou des gradients de températures dans un PSEM sous haute tension continue.

The emergence of renewable energy leads to a development of new technologies for energy distribution across long distances. The latter will be based on High Voltage Direct Current (HVDC) to avoid capacitance losses. This network is interconnected using Gas Insulated Substation (GIS), which insulation is composed of pressurized gas (SF6) and solid insulators (epoxy resin), which have to withstand HVDC. The electric field is not anymore determined by permittivity of materials, but by resistivities and charge accumulation. In the case of an insulator with an interface with gas, electrons or ions will move across electric field lines and will charge the surface of the solid insulator. The behavior of insulator's properties (gas and solid) constitutes a major challenge for the development of HVDC GIS, to understand the charge relaxation/accumulation mechanisms.In this work, the characterization of solid insulator has first been investigated, based on a low-noise current measurement method. It is thus possible to measure the leakage current through samples and onto their surface, in a pressurized gas, at high electric field and for different temperatures. Those measurements permit to evidence that both volume and surface resistivities are strongly impacted by the increase of temperature and water concentration. It has also been shown that surface resistivity has a non-linear behavior with electric field. A numerical model was developed, to simulate experimental results, showing that the surface properties of the insulator can be implemented.Furthermore, the insulating properties of the gas were also investigated through different electric field geometry (coaxial and uniform), in order to estimate the contribution of current through gas on the charge accumulated on solid insulators. It has been found that a non-negligible current passes through the gas (~pA to nA). To determine the mechanisms responsible for such currents, the latter has been characterized depending on several parameters (electrode surface roughness, material nature, electric field, temperature and relative humidity). It revealed that the variations of currents are strongly impacted by the conditioning of the device and thus by the relative humidity adsorbed on electrodes and enclosure surfaces. In presence of a dry system (dry gas and device) low current were measured (~pA), which increases with temperature. On the contrary, in case of a “wet” system (humid gas and device) the current decreases with increasing temperature. Those results combined with the influence of the electrode roughness, strongly suggest a mechanism of charge injection at the electrode surface, enhanced by water adsorption.Finally, the results obtained for both solid and gaseous insulations are used to develop a numerical model with a shape close to the industrial application, and observe the modification of electric field distribution in presence of water concentration and temperature gradient. An estimation of current flowing through the insulator and gas is thus possible in case of uniform and gradient temperature.In conclusion, this work gives the variations of both volume and surface resistivities in an epoxy resin with temperature and electric field. It also evidences the major influence of relative humidity and temperature on charge injection mechanisms which contribute to the current measured through gas. The extensive characterization performed, enables to develop a simulation which predicts the variations of electric field distribution within an HVDC GIS., L'émergence des énergies renouvelables a entraîné le développement de nouvelles technologies pour la distribution de l'énergie sur de longues distances. Ces dernières sont basées sur le transport via de hautes tensions continues (HVDC) pour éviter les pertes capacitives. Ce réseau de distribution est interconnecté via des Postes Sous Enveloppes Métalliques (PSEM), dont l'isolation est composée de gaz sous pression (SF6) et d'isolants solides (résine époxy), qui doivent résister sous HVDC. Dans ces dispositifs, le champ électrique n'est plus déterminé par la permittivité relative des matériaux, mais par leurs résistivités et les phénomènes d'accumulation de charges. Dans le cas d'un isolant solide présentant une interface avec un gaz, des électrons ou des ions vont être susceptibles de se déplacer suivant les lignes de champ électrique et charger la surface de l'isolant solide. Le comportement des propriétés des isolants (solides et gazeux) constitue un enjeu majeur dans le développement de PSEM HVDC, notamment dans la compréhension des mécanismes d'accumulation et relaxation des charges.Dans ce travail de thèse, la caractérisation de l'isolant solide a d'abord été étudiée, basée sur des mesures de courants faibles bruits. Il est ainsi possible de mesurer le courant de fuite dans le volume et sur la surface des échantillons, dans une enceinte sous pression, à haut champ électrique et pour différentes températures. Ces mesures ont mis en évidence que les résistivités de volume et de surface sont fortement impactées par l'augmentation de la température et la teneur en eau des échantillons. Il a également été montré que la résistivité de surface a un comportement non-linéaire en fonction du champ électrique. Un modèle numérique a été développé pour simuler les résultats obtenus, et implémenter les propriétés de surface de l'isolant solide.Les propriétés isolantes du gaz ont également été étudiées pour différentes géométries de champ électrique, dans le but d'estimer la contribution du courant passant à travers le gaz, sur l'accumulation de charge en surface de l'isolant solide. Des courants non négligeables sont mesurés dans le gaz (~pA-nA). Pour déterminer les mécanismes responsables de la présence de tels courants, il a été caractérisé selon plusieurs paramètres (la rugosité de la surface de l'électrode, la nature du matériau, le champ électrique, la température et l'humidité relative). Cela a mis en évidence que les variations de courants dépendent du conditionnement du dispositif, et sont donc fortement influencés par l'humidité relative adsorbée sur les surfaces du dispositif (électrodes et cuves). En présence d'un système sec, de faibles courants sont mesurés (~pA), et augmentent en fonction de la température. A l'inverse, dans le cas d'un système humide, le courant diminue avec l'augmentation de la température. Ces résultats, combinés à l'influence de la rugosité de l'électrode, suggèrent fortement un mécanisme d'injection de charge à la surface de l'électrode, favorisé en présence d'eau adsorbée.Enfin, les résultats obtenus pour les deux isolants solides et gazeux sont utilisés pour élaborer un modèle numérique ayant une forme proche de celle de l'application industrielle, et permettent d'observer la modification de la distribution du champ électrique en présence de la concentration en eau et du gradient de température. Une estimation du courant circulant au travers des isolants est donc possible.En conclusion, ce travail donne les variations des résistivités de volume et de surface dans une résine époxy en fonction de la température et du champ électrique. Il met également en évidence la forte influence de l'humidité relative et de la température sur les mécanismes d'injection de charges qui contribuent au courant mesuré à travers le gaz. Cette caractérisation approfondie permet de développer une simulation qui prédit les variations de la distribution du champ électrique au sein d'un PSEM sous tension continue.