Gaseous Dielectrics IX

The dynamical behaviour of slow electrons traversing ases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances)1. For quite a few atoms and molecules, special behaviour of the s - wave (L = 0) phase shift leads to a deep Ramsauer-Townsend minimum in the scattering cross section between 0 and 1 eV which strongly affects the electron mobility in these gases. Even more importantly, resonances (compound states of the electron-molecule system with lifetimes ranging typically from 10-15 to 10-11 s) are often found to dominate the dynamics of electron-molecule collisions over the energy range 0 to 10 eV. The extended time interval (compared with the direct transit time which is below 1 fs), spent by the incoming electron close to the target while in the resonance state (lifetime τ = ħ/Γ, Γ= resonance width), has profound effects especially on collision channels which involve a reaction of the nuclear framework, i.e. on vibrational excitation (VE) and on dissociative attachment (DA). Apart from well-known shape resonances such as H₂-(2∑_u), N₂-(2∏_g), CO-(2∏), O₂-(2∏_g, v ≥ 4) which are located below the lowest limit for DA and owe their lifetime to the centrifugal barrier of the electron, repulsive anion states above the DA limit are important for VE as well as DA. The importance of resonances for vibrational excitation (VE) as well as negative ion formation via dissociative attachment (DA) is illustrated in Fig. 1.

Electron initiated processes play a key role in any kind of laboratory plasma. It is primarily the electron-molecule interaction from which the feed gas molecules receive energy and which maintains the plasma. These primary interactions generate molecules in various excited states, in ionized forms (cations and anions) and finally as fragmentation products, also in excited and ionized forms1. All these particles mutually interact, incuding photons from emission processes. It is hence a vast variety of different interactions between primary and secondary particles which characterize a plasma. In principle, knowledge about the relative density of the components in their different states and the respective cross sections would be necessary to model and eventually control the plasma. In actual pactice, however, it is often sufficient to restrict on two body interactions between the most abundant components which, in the case of laboratory plasmas, are usually electrons and neutral gas molecules. The plasmas used in materials processing are often so called cold or anisothermic plasmas.2 Although they contain a variety of high energy species (neutrals, radicals and ions in excited states) the plasma does not considerably heat its container, i. e., the excited species are far from equilibrium. In particular, the electron energy distribution in such a cold plasma peaks at a few eV and is hence much higher than the average energy of the heavy particles (kT (300K) = 0.026 eV). The weak coupling between the electrons and heavy particles is a consequence of the large difference in masses. From energy and momentum conservation it follows that in a collision, an electron can only transfer an energy amount of the order m/M (m: electron mass, M: mass of the heavy particle) onto the heavy target. Exceptions are low energy electron collisions with polar molecules, collisions when resonances are involved (see below) but also collisions at higher energies when electronic excitation becomes accessible.

Dielectric gases are used in a wide variety of important applications such as gas insulation and semiconductor fabrication (Christophorou 2000). As important as their use is their abatement (Kiehlbauch 2001), since gases like SF₆ or some perfluorocarbons are potent greenhouse gases. The above processes require that at some stage the gas be subjected to discharge conditions, which are nowadays optimized by previous modeling and process simulation, which in turn demand the knowledge of cross sections and/or swarm and transport data for the numerous physico-chemical processes occuring in the discharge. It is well known that the extent of the present quantitative knowledge on electron-molecule (atom) interactions with dielectric gases, like electron scattering, electron impact ionization and dissociation, and electron transport, by far overwhelms that on ion-molecule (atom) interactions, in the form of elastic and inelastic momentum transfer and reaction cross sections, coefficients of mobility, diffusion and reaction rates, to name only the most relevant quantities. There exist many cases in the literature where a researcher faces the problem of having only meagre, or even lacking, ion swarm or cross section data to provide a quantitative explanation to the phenomenon under study or simulation.

Because of the potentially serious greenhouse effects that SF₆ may cause on the atmosphere, efforts are being made to find gaseous substitutes that would retain many of the outstanding properties of this gas as a high voltage insulator and, at the same time being more environmentally friendly. Thus, intensive research has been carried out on SF₆ mixed with other gases1-3 such as SF₆-Ar, SF₆-N₂, SF₆-He, SF₆-CH₄, SF₆-CHF₃, and SF₆-CO₂. In particular, the latter mixture has a higher minimum impulse breakdown voltage than pure SF₆4, and therefore it could be a good substitute for some insulation applications.

In the use of perfluorocyclobutane (c-C₄F₈) as a plasma processing gas for silicon dioxide etching, perfluoroethylene (C₂F₄) is produced as a by-product of electron and photon impact on c-C₄F₈, and also as a product of thermal decomposition of c-C₄F₈. Consequently, dissociative processes can make C₂F₄ a significant gas constituent. Therefore, in order to model c-C₄F₈ plasmas, it is important to know the electron transport, ionization, and attachment coefficients of C₂F₄ in order to understand fully the chemical and physical processes occurring in the discharge. This knowledge may also be of relevance to gaseous dielectrics since C₂F₄ may be produced in SF₆ circuit breakers by vaporization of polytetrafluoroethylene insulators.1 Measurements of electron transport, ionization, and attachment coefficients are reported in this paper for pure C₂F₄ gas. In addition, measurements of the electron drift velocity and the effective ionization coefficient as functions of the density-reduced electric field E/N are reported for mixtures of C₂F₄ with Ar, which may be useful in efforts to obtain electron collision cross sections for C₂F₄ using Boltzmann codes. The effect of Penning ionization on the measured effective ionization coefficients in C₂F₄/Ar mixtures is also investigated.

Sulfur hexafluoride (SF₆) is widely used as a gaseous dielectric in high-voltage applications due to its extremely large cross section for electron attachment [1-3] and the stability of SF₆ with respect to decomposition in subsequent collisions with SF₆ [4]. It is also recognized as a potent greenhouse gas and it has been suggested that a mixture of SF₆ and N₂ might serve as a substitute for pure SF₆ in certain applications which require gaseous dielectrics [5,6]. Even with a very low SF₆ content, a SF₆/N₂ mixture exhibits many of the desirable properties of SF₆ as a gaseous dielectric. It has been suggested that this mixture may constitute a synergistic combination: the buffer gas (N₂) serves to cool energetic electrons into the low-energy region where the electronegative gas (SF₆) captures them with a remarkably high cross section, thereby inhibiting the buildup of free electrons that could cause ionization leading to electrical breakdown. The dielectric properties of this mixture have been the subject of numerous recent investigations [5,6].

The motion of electrons in gases under the influence of an external electric field has been the subject of research for almost a century. The results of these investigations give a base to such applications as radiation detectors, gaseous dielectrics and many others.

Besides its use in plasma etching, perfluorocyclobutane (c-C₄F₈) has potential applications as a gaseous dielectric, especially in gas mixtures (e. g., see Refs.1-3). Perfluorocyclobutane is also of environmental interest because it is a global warming gas.4 For these reasons, we have recently synthesized and assessed electron collision cross-section and electron transport-coefficient data for this gas.5 The results of this critical assessment are summarized and briefly discussed in this paper.

Halocarbons play important and very disgraceful role in the atmosphere not only destroying the ozone layer but also acting as the greenhouse agents. To diminish their negative influence on the environment it is necessary to find the methods which allow to destroy existing in the atmosphere halocarbons. There are some attempts for develop such procedure. The proposed techniques include several plasma methods: by an electron beam or by using a free localized microwave discharges1. The key processes in these techniques are electron attachment reactions. So, for modeling the system it is necessary to know the rate of these processes, their mechanism and products.

The study of the transport properties of excess electrons in dense non polar gases is aimed at understanding the nature of the electron states in a disordered medium and at elucidating how the electron-atom interaction is modified by the interplay between the quantum nature of the electron and the short average interatomic distance in a dense gas.

The processes of deexcitation of rare gas atoms excited by several means, including discharges, ionizing particles, lasers and so on, are extensively studied to investigate the potentiality of rare gases as sensitive media in radiation detectors1 and high energy electronic transition lasers2. In fact, rare gases have the ability to convert efficiently electron kinetic energy to electronic excitations and to convey the excitation energy to lower-lying atomic and excimer levels which emit a considerable fraction of the released energy in a narrow band vacuum ultraviolet (VUV) range3.

Data presented by F. Moore1 regarding recent measurements of the concentration of sulfur hexafluoride (SF₆) in air suggest a revision to the atmospheric lifetime for this compound. Concentrations of SF₆ and other trace atmospheric gases were measured at altitudes ranging from 3 to 30 kilometers (well into the stratosphere). Mesospheric air (representing air from altitudes above 50 km) was sampled from air masses moving through the polar vortices. Preliminary comparison of the SF₆ concentration profile to that for CO₂ indicates that the previous calculation of its atmospheric lifetime (i.e., the time to reach 1/e of its original concentration) at 3200 years is overestimated by a factor of approximately 5, assuming the dominant atmospheric sink for SF₆ is in the mesosphere.

Power electronics and high temperature operation capable micro electro mechanical (MEM) device fabrication, using wide bandgap semiconductors, require development of plasma processing tools where both ion energy and radical fluxes can be controlled to obtain reasonable etch rates with minimal surface damage. Recent work indicates that reactive ion etching (RIE) process optimization can be achieved primarily through a consideration of plasma electrical properties [1,2], which may lead to improved material removal performance. The concept involves modifying the electronegative character of strongly attaching etchant gas discharges through dilution with an electropositive gas species such as Ar, N₂, or even a weakly electronegative gas such as O₂. This gas mixture changes the radio frequency (RF) current-voltage (I-V) phase shift from a capacitive maximum of -90° to a value in the range of -45°, corresponding to the transition from sheath to bulk dominated discharge regimes. The changes in I-V phase shifts increase power deposition efficiency in the plasma, which leads to a greater production of the ions and radicals required for material removal. The optimal discharge condition is a function of pressure, fractional dilution, and electronegativity of the total gas mixture. Several authors have demonstrated various aspects of this phenomenon [3-6]. However, Sobolowski, Langan, and Felker [1], and Langan et. al.[2] have conclusively correlated RF electrical measurements with optical emission data and dielectric (SiN, SiO₂) etch rates for NF₃, CF₄, and C₂F₆ diluted with Ar, He, O₂, N2, and N2O. Based upon their observations, and simple models of the bulk and sheath regions of a parallel plate RF discharge, they proposed this RIE discharge optimization scheme to be generic in nature and applicable to any RF discharge utilizing electronegative gases.

Plasma assisted etching is a vital technology for microelectronics manufacturing. The quest for higher speed, denser circuitry and enhanced functionality in integrated circuits is pushing all microelectronics manufacturing processes including plasma etching to their limits. Transition towards new materials (e.g. Cu, low-k dielectrics), control of plasma damage mechanisms (e.g. charging) and new environmentally friendly etching chemistries (e.g. C₄F₆) are introducing additional challenges. It has therefore become imperative to thoroughly understand the plasma etching processes and the behavior of plasma equipment. Computational modeling is one tool that, in conjunction with experiments, can be invaluable in this quest. We have recently developed integrated plasma equipment - feature evolution models for investigating the physics and technology of plasma assisted etching. These models along with their application to c-C₄F₈ based dielectric etching are the focus of this paper.

Plasma assisted chemical vapor deposition is a proven method for the formation of thin films. The application of non-thermal low pressure plasmas containing organic compounds for thin film deposition by plasma polymerization is well known1. These films are successfully applied for corrosion protection and as diffusion barriers2. Operating non thermal discharges under atmospheric pressure conditions requires no vacuum devices, therefore the integration of the plasma process into production lines is greatly simplified. Batch processing can be avoided thus reducing production costs significantly. The plasma of the atmospheric pressure dielectric barrier discharge (DBD) has been used for technical applications for many years starting with ozone generation (Siemens, 1852). Other fields of application are flue gas cleaning, surface treatment of polymeric foils and films and thin film deposition3. The dielectric barrier discharge is usually a filamentary one and therefore strongly inhomogeneous. A homogeneous DBD without filaments was described by Okazaki et al. in a planar electrode configuration in He, later also in other gases and gas mixtures4. According to some similarities with the dc glow discharge this discharge is called atmospheric pressure glow discharge5. Its homogeneity favours this discharge for thin film deposition techniques.

In plasma etching of silicon substrates, electron-impact on a halogen-containing feed gas creates chemically-active ions and radicals that react with silicon to form volatile SiX_y (X = halogen) species. Unfortunately, the most widely-used feed gas, CF₄, has a high global warming potential.1 As a consequence, other halocarbons are being considered as replacements for CF₄. Trifluoroiodomethane, CF₃I, is a promising candidate: It has a low global-warming potential2 and plasma-etching of silicon dioxide with CF₃I has recently been demonstrated3-5.

Nitrogen trifluoride is used extensively in several aspects of semiconductor processing and manufacture and was also employed as an atomic fluorine source in pulsed electrical-chemical lasers. The electron collision database is of interest for modeling and simulation of plasma enhanced etching of materials. We have recently made comprehensive measurements of the absolute dissociative ionization cross-sections of nitrogen trifluoride and also of its dissociative charge transfer from argon ions. These results are reviewed and compared with previous data in the literature. We also compile, where available, the results for electron attachment, momentum transfer, vibrational excitation, and dissociative excitation. This data set is compared with the results from swarm experiments for mixtures of NF₃-argon and NF₃-nitrogen. The needs and opportunities for additional experimental studies are outlined.

This paper proposes a hybrid PIC-MCC/fluid model for streamer discharges in a needle-plane electrode system at a SF₆ gas pressure of 0.1 MPa. The Particle-in-cell and Monte Carlo (PIC-MCC) scheme is used to follow electron kinetics, and calculate electron drift velocity and the rate coefficients α ( ionisation ) and η ( attachment) in the region of highly non-uniform field. The obtained data are used in fluid model to find the charge density distribution of ions and electrons. The simulation results show the streamer formation and development and prove the effectiveness of the hybrid model.

The applications of gaseous dielectrics are lying on their physical, chemical and electrical properties and on these properties couplings. This justifies the search for appropriate and accurate diagnostics on these properties. A good knowledge of the currents displayed across gaseous dielectrics is of prime interest to understand and master the behavior of the equipment using them: this will be true for the control of gasinsulated (SF₆, SF₆-N₂) high voltage equipment, as well as for the optimization of plasma reactors devoted to plasma chemistry applications. In fact, in all cases, when such systems are energized with continuously applied voltages (dc, ac or others), they do not only deliver impulsive currents (partial discharges pulses only taken into account by many people managing with gas-insulated equipment or streamers pulses, only taken into account by various people concerned with plasma chemistry applications), but also nonimpulsive currents (glow discharge currents) which in all cases (i) consume energy, (ii) have chemical effects, harmful in the case of gas-insulated equipment1 and useful in the case of plasma reactors2, (iii) have heating effects to be avoided in the case of gasinsulated equipment3,4 and useful or not in the case of plasma chemistry applications.

Non-stationary streamer-free corona at slow (for seconds or dozens of seconds) voltage change is theoretically studied and numerically simulated. The obtained results are used to investigate the effect of injected space charge on initiation and development of a leader in long air gaps. The focus is on the initiation of upward lightning from a stationary grounded object in a thundercloud electric field. The main results are also applicable to leader process in long laboratory air gaps at constant voltage.

The plasma opening switch (POS)1 is a vacuum switching device used in modern pulsed power apparatus for physical and industrial applications. In the POS, a plasma is first injected into the A-K gap to conduct large current of hundred to thousand kiloamperes driven by Marx-generator-type or other pulsed power sources for hundred to thousand nanoseconds. Then the POS opens by means of a kinetic process that is not very clear at present, and converts the current to a load with voltage or power multiplication.

A two-dimensional model for simulation of gas discharges along a resin insulator in SF₆/N₂ gas mixtures is developed by means of the Particle-in-cell and Monte Carlo (PIC-MCC) approach. The electron swarm and the following streamer developments are fully simulated, in which the elastic, exciting, ionising and attaching collisions between electrons and neutral gas molecules are considered, and the gas photoionisation and the photoelectric emission from cathode surface are realised by using a probabilistic scheme. Considering the effects of the space charges, Poisson fields are solved for each time step. The attached charges at the insulator surface are calculated according to the electric field distribution and taken into account for the solution of Poisson fields. The dynamic behaviour of electron swarms and streamer formation during the discharge are presented and analysed.

The first study on luminous layers phenomena was reported by Hoist and Oosterhuis1 in 1923. They observed the phenomena between a pair of plane parallel electrodes at several V·cm−1Torr−1 and 0.1 µA in a Ne-filled tube. Druyvesteyn2reported on luminous layers phenomena in He, Ne, and Ar. We can recognize the layers which appear at equal intervals between the anode and the cathode by some photographs in the paper. Hayashi3 observed circular luminous layers in a Ne-filled discharge tube with a concentric electrode. Hölscher4 investigated the characteristics of the transition from luminous layers to striations in Ne. Emeleus5 discussed the space charge effect in the transitional region from Townsend discharge to glow discharge.

Observations and numerical simulation showed a step-wise development of a long streamer in highly non-uniform gaps at rising applied voltage.1,2This was obtained in electronegative gases such as air, 0₂ and SF₆. It is known that the plasma in the streamer channel is characterized by a high density of charged particles (electron density n _e ~ 1013 −1015 cm−3) and a considerable difference between electron temperature and gas temperature. Electron-ion recombination and electron attachment to a molecule in the streamer channel of radius r reduce fast the value of n _e and consequently the conductivity γ = πr2en _e μ _e per unit channel length where µ _e is the electron mobility. The plasma decay does not need to lead to a decrease of the streamer current which is written as (1)$$ {i_s} \approx {C_s}{\varphi _t}{v_s} + {C_s}{L_s}\frac{{dU}}{{dt}} $$ The first term on the right-hand side is determined by delivery of charge to the new streamer sections that must be charged up to the potential φ ₁ of the streamer tip. This component of the current decreases with decreasing channel conductivity.The second term in Eq. (1) describes recharging the formed channel of the length L _s and of the capacitance Cs per unit length when the applied voltage U₀(t) rises in time. Here, U is the time-varying average potential of the streamer. If the streamer develops at a sharp front of the voltage impulse (at high positive values of dU/dt), the total current i _s can remain constant or even increase in spite of the plasma decay. This will result in a growth of the electric field in the decaying streamer channel: (2)$$ {E_s} = \frac{{{i_s}}}{{\pi {r^2}e{n_e}{\mu _e}}} $$

As part of an overall effort to investigate partial discharge initiated deterioration of polymer dielectrics, partial discharge measurement and simulation have been explored. Fast time resolved measurements of corona in air have been made using electrodes and a passive component geometry designed for high resolution time measurements. The cathode employs a split configuration for use as a measuring electrode and coupling capacitor. A simple model has also been developed for the experimental system. The simulation uses swarm parameters and continuity equations coupled with electric field calculations to simulate the corona pulse. Both experimental results and simulations are reported and compared.

My assigned topic, the gaseous dielectric properties of matter resident in near-Earth space, is unusual. It is necessary first to identify the diverse plasma populations, then explain the sense in which they may be regarded as gaseous dielectrics. We focus specifically on regions called the ionosphere and the magnetosphere. The ionosphere is a layer of cold plasma that extends from < 100 to >IOOO km above the Earth. This plasma is created by solar ultraviolet light and by energetic particles from the magnetosphere. It is usually divided into D, E, and F layers. The D and E layers range in altitude from 60–95 km and 100–150 km, respectively. Outside the auroral oval they are mostly dayside features that disappear due to ion-electron recombination after sunset. Maximum plasma densities of ~106 cm−3 occur at F layer altitudes between 300 and 400 km on the dayside.

Partial discharge (PD) has been recognized as an important signal of pre-breakdown phenomenon and the PD measurement has been utilized for electrical insulation diagnosis of SF₆ gas insulated equipment such as switchgears (GIS) and circuit breakers (GCB). However, it can be said that the PD measurements, so far, have not always been based on their physical mechanisms, because PD phenomena are very complicated and hard to be measured precisely. Thus, it is generally very difficult to predict breakdown (BD) and life time of electrical insulation.

SF6 gas insulated electric power apparatus such as GIS and GIL have been operated under high gas pressure condition at around 0.5 MPa-abs[1,2]. Here, metallic particle contaminants in GIS would generate negative partial discharges (PD) firstly, because negative PD inception voltage is lower than positive one. In such high gas pressure condition, there is a possibility that breakdown occurs without replete positive PD precursor to it. In this case, only negative PD measurement could be a method for the insulation diagnosis. Therefore, in the commissioning process and under operation, it is important for the insulation diagnosis at the early stage to clarify the negative PD mechanism under high gas pressure condition.

Since SF₆ gas has excellent properties like a high dielectric strength, and is chemically inert, nontoxic, and so on, it has been widely used as an insulation material for gas insulated switchgears (GIS) and insulated transmission lines (GIL)(1,2). However, SF₆ gas is a potent green house gas and its global warming potential (GWP) is estimated as a very large at 23,900. Accordingly, SF₆ gas was designated as one of the emission gases to regulate at COP3. It is necessary to decrease the use of SF₆ gas, design techniques to retrieve SF₆ gas and reduce its emission into the atmosphere and develop alternative gases having much lower GWP values. SF₆/N₂ gas mixtures have been proposed as a promising alternative to SF₆. To date, the authors have shown that adding a small amount of CO₂ to an SF₆/N₂ gas mixture results in a prominent increase in the insulation performance under both uniform and nonuniform electric fields(2,3).

The UHF method is very well suited for PD measurements even in N₂-SF₆ gas mixtures with very low SF₆ contents [1]. An increase of the sensitivity of this method would be very advantageous in GIL in order to reduce the number of couplers needed for sensitive PD measurements. The UHF energy which is fed into the ducts by the PD processes is damped during the propagation. This is described mathematically by [2]: (1)$$ P\left( z \right) = P\left( 0 \right) \cdot {e^{ - 2 \cdot \alpha \cdot z}}. $$

Loss of insulation integrity from a gas-insulated system is at first a subtle phenomenon. It is contended that this early stage of degradation contains privileged information, to be gleaned in order to unravel precursor signs preceding the eventual outbreak of minor faults, viz. partial discharges and associated patterns.

Since SF₆ gas was specified to be a greenhouse gas at COP3 in 1997 because of its high global warming potential (GWP=23900), the development of new insulating gases or gas mixtures alternative to SF₆ gas is greatly needed. Especially, gas mixtures without the use of SF₆ gas will be expected as the ultimate substitutes to SF₆ gas.

Environmental problem of SF6 gas has been regarded as an up-to-date issue in the field of high voltage engineering. Binary gas mixtures such as SF6/N2 are considered to be one of the most promising candidates[1-3], because N2 gas has no greenhouse effect and toxicity.

Partial discharge behavior of epoxy resin with air void (open type and sealed type) was discussed as a function of accelerated time duration. The q — ϕ discharge patterns were modified according to the accelerated duration, and giving a well-known “swarming pulsive micro discharge” character. The void surface of the epoxy was damaged by the partial discharge energy due to the decomposition of the epoxy materials, which shows an orange skin structure with silica particles.

PD detection sensitivity is quite an important factor for the PD detection system. In this paper, to compare with the PD detection sensitivity, experiments are carried out using three kinds of insulation defects. Magnitude of detected PD charges is different case by case, however, estimated net charge of PD for the same background electric field is nearly the same.Equivalent circuit for PD sensitivity is also discussed. The ratio of net charge divided by apparent charge is about 15 for free particle in actual 300kV GIB.

High pressure glow discharges are used in plasma processing, gas lasers, chemical and bacterial decontamination of gases, and as mirrors and absorbers of microwave radiation. Transient high pressure glow discharges, such as barrier discharges1 and ac discharges2 are already well established, but recently high pressure dc discharges in noble gases3 and in air,4,5 with dimensions of up to centimeters have been generated by using novel plasma cathodes. One of them is a microhollow cathode discharge sustained plasma, where the microhollow cathode discharge provides the electrons for the main discharge. The elimination of the cathode fall, the cradle for glow-to-arc transition has allowed us to generate dc glow discharges with electron densities as high as 1013 cm-3, at gas temperature below 2000 K.4,6

Chemical and semiconductor industries are using volatile organic compounds (VOCs) such as toluene, xylene, trichloro-ethylene (TCE), trichloroethane (TCA), benzene, and acetone as solvents and for substrate cleaning [1]. However, the use of VOCs poses considerable health hazards. For example, inhalation of toluene with concentrations of 600 ppm for more than eight hours causes headache and dizziness [2]. Benzene is carcinogenic at long term exposure [2].

When sulphur hexafluoride (SF₆) was first considered as a replacement for compressed air in gas-blast switchgear, it was attractive both for its high intrinsic strength and for its excellent dielectric recovery characteristics. These factors, coupled with the very rapid rates of voltage collapse, and hence the high values of dl/dt which are achievable at breakdown, also make SF₆ the dielectric of choice in high-power plasma closing switches. For single-shot applications, SF₆ spark-gap switches can be designed for voltages of 10kV-5MV, high current-handling capability and switching speeds up to 1015V/s.

Fluorocaibon (FC) liquids are now interested in the field of electrical insulation technologies, since their vapors, under some conditions, show higher dielectric strength than that of SF ₆ gas and their polymerized films show high breakdown strength.1–5 They show, also, zero ozone depletion and fewer greenhouse potentials, and are nonflammable and highly resistant against thermal breakdown. C _x F _y polymer films composed of fluorine and carbon made by FC are known for their chemical inertness, gpod electrical stability, and low dielectric constants.3,4,6,7 The main disadvantages of FC for application of electrical insulator were of their low saturation vapor pressure, and their poor adhesion on substrates as a film. However, plasma enhanced chemical vapor deposition (PECVD) for film formation has provided many advantages that overcome these problems. In this case C _x F _y films contain strong C-F bonds that decrease the dielectric constant and cross-linked C-C structure that sustains high thermal stability, accordingly they show high hydrophobicity, good adhesion, and high stability in high temperature environment._7,8

SF₆ gas is widely used as insulation media for gas-insulated switchgears (GIS) because of its excellent insulation properties such as high dielectric strength and arc quenching ability(1). However, since SF₆ is a potent greenhouse gas, its use and emission must be decreased. From this point of view, a lot of studies on searching for an SF₆ substitute have been conducted_(1–4) and SF₆/N₂ gas mixtures seem to be the most promising and most thoroughly characterized gaseous dielectric media besides pure SF₆(5).

The very high global warming potential (GWP) of SF₆ gas has stimulated worldwide research on finding a new gas or gas mixtures applicable for gas insulation. As it is now considered very difficult to find a suitable pure gas substituting for SF₆, the research principally focuses on applying SF₆/ N₂ mixtures. In a previous paper, we have proposed to use mixtures including low content PFC (perfluorocarbon) mainly for the purpose of reducing latent GWP drastically, among which we have selected mixtures with c-C₄F₈ (perfiiluorocyclobutan)1) This gas is completely nontoxic and has no ozone-depleting ability. Its GWP is about 40% as low as that of SF₆. Furthermore, liquefaction recovery is much easier in mixtures with c-C₄F₈ than in SF₆ / N₂ due to the higher liquefaction temperature of c-C₄F₈ (about -6°C). Sparkover characteristics of c-C₄F₈ / N₂ mixtures were already reported for limited experimental conditions at a relatively lower gas pressure of 0.66 atm or 1 atm2). In this study, we measured sparkover voltage of gas mixtures including 5–20 vol. % c-C₄F₈ in N₂, CO₂, or air under a quasi-uniform electric field condition. The applied voltage was DC, 60 Hz AC, or a lightning impulse voltage. The pressure ranged from 0.1 to 0.4 MPa. The sparkover voltage so obtained increases more or less nonlinearly with the increase in mixing ratio of c-C₄F₈, irrespective of an applied voltage waveform or a mixing component gas. From the experimental results, we have evaluated the degree of nonlinearity, or synergism, by using an empirical formula that was proposed by one of the authors in 1972. Although the degree of nonlinearity depends on a mixed component, it becomes larger with increasing Pd (P is gas pressure and d electrode separation). In the case of SF₆/ N₂, on the other hand, the degree of nonlinearity is almost independent of Pd. We have also compared the degree of nonlinearity between applied voltage waveforms.

Partial discharge (PD) measurement is expected to be an effective method for electrical insulation diagnosis and breakdown prediction of the power apparatus. In order to establish the reliable insulation diagnostic techniques, physical mechanism of PD phenomena are required to be clarified. Especially, electronegative gases and gas mixtures are of interest from the viewpoint of high insulation performance due to corona stabilization effect. Although many investigations_1,2 have been so far carried out, PD phenomena have not yet been fully understood.

One of the remaining and long standing dilemma facing the Vacuum Circuit Breaker, (VCB) designer is the occurrence of the infamous, ”Late Breakdown”.

In the interval of pressure and gap distances, characteristic for SF6 equipment, the reasons of the deviation from the Pashen’s law are connected with changing of the breakdown mechanism from streamer to leader1–4. The analysis of the papers on the leader breakdown shows, that the study of the phenomenon is still at the first stage. For the creation of the sufficient complete picture of the phenomenon and development on its basis of the engineering recommendations the further more detailed study of the leader process is necessary.

It is well known that in a horizontal section of a GIS/GITL bus-duct a dielectric coating on the inside of the outer enclosure inhibits the movement of metallic contaminating particles. Several researchers have developed computational models for particle movement in such a co-axial bus-duct. The models, however, make assumptions about the particle charging processes and charge exchange mechanism when a moving particle returns to the dielectric coated enclosure. The paper describes an experimental procedure for recording the movement of particles in a co-axial bus-duct system, and compares the observations with the computational model. Some improvements in the computational model are also suggested.

In gases the breakdown voltage was defined by Paschen (1889) to be proportional to the product of pressure p (or to be more exact density ρ) and electrode spacing d. This has been widely accepted to be true independent of the actual values of pressure and electrode spacing. Paschen himself on the other hand only validated this ”law” for gap sizes of .01 cm to .15 cm with atmospheric pressure and .1 cm to 2 cm with pressures of 2 cmHg to 75 cmHg, i.e. 26.67 mbar to 1 bar. It was never confirmed, that the breakdown dependence from the product p · d really applies to electrode distances below 100 µm. All measurements done in the past to obtain the p · d values in the range below .1 bar-mm as presented by Dakin et al. (1971) were performed at small pressures but rather large gaps. Nevertheless it is widely accepted to use these p · d values for atmospheric pressure and gap widths below 100 µm. As Germer (1959), Boyle and Kisiluk (1955) and Hartherz et al. (2000) have shown, the general assumption of the so called ”Paschen Minimum” for small gaps below 10 µm is not applicable in general. Especially the increase of the breakdown voltage for p · d values below 8 bar-urn does not apply to air pressures in the range of 1 bar.

SF6 gas has been applied to the gas insulated power apparatus like gas insulated switchgears (GIS) and gas insulated transformers because of its excellent insulation performance and high reliability. However, SF6 gas has a high global warming potential and its consumption and emission are required to be reduced from the viewpoint of mitigating global warming by SF6 gas. Many studies have been carried out to develop novel insulating gases or gas mixtures alternative to SF6 gas1–3, while few studies have so far focused on creepage discharge characteristics on solid dielectrics in the alternative gases, which is important for insulation design of gas insulated power equipment.

With excellent insulation and current interruption characteristics, SF₆ gas has been applied since the 1960’s in electric power equipment, and is finding a wide range of applications today in GIS, GCB and GIL of classes from several kV to 1OOOkV. In COP3 of 1997, however, it was designated a greenhouse gas and since then there has been demand for alternative insulating gases to pure SF₆ gas.(1)

The fact that the specific heat and insulation capability of SF6 is very high compared to other gas species makes SF6 an excellent arc quenching media in gas circuit breakers. The global warming effect of SF6, however, causes strong environmental concern and the search for alternative gas insulator is in progress all over the world. The goal of our research is to find alternative gas insulator to be used as the arc quenching media in a gas circuit breaker. For that purpose, we investigate the insulation capability of various candidates of the alternative gas at high temperature of the order of 1000K or more. The study is intended to provide basic data to prevent the reignition of arc in the high temperature gas which still remains even after the arc is extinguished at the zero current point. In order to model the high temperature gas in a laboratory, we use a laser-produced plasma. The advantages of the laser-produced plasma are that we can produce the high temperature gas at any time and at any point in space in a very controlled manner. The gas properties produced by focusing a laser beam can be reproduced with a good precision. As we produce the plasma without any help of electrode, we can also distinguish the effects solely due to gas property from the effects of electrodes. The purpose of the present paper is to build and test the new experimental system to study the insulation capability of gas at the high temperature. As a test, we investigate how the flashover voltage changes at different gas temperature taking pure SF6, N2, CO2 as examples.

Recently, SF₆ has been identified as a greenhouse gas with a long atmospheric lifetime1. Therefore, recycling guidelines for SF₆ in electrical power equipment have been discussed, and the reduction of the release of SF₆ to the atmosphere has progressed2. In addition, many researchers have investigated alternative insulation gases to SF₆3-5. High-pressure N₂ gas is one of the proposed alternative gases that have no need of SF₆. Pure N₂ has a lower withstand voltage level than SF₆, but has the advantage of being not only environmentally friendly, but also low in cost and safe. The breakdown characteristics of compressed N₂ have already been discussed as part of the basic study on gas discharge in the 1930s6. However, most data on the electrical breakdown of compressed N₂ were obtained under limited experimental conditions (small electrode scale or kinds of applied voltage waveform), and are not sufficient to estimate the insulation performance.

SF₆ gas is widely used in electric power apparatus such as gas insulated switchgears (GIS) and gas circuit breakers (GCB) due to its excellent dielectric and arc-quenching performance. However, SF₆ gas has recently been recognized as a greenhouse gas and it was decided to reduce its emissions at COP3 at Kyoto in 1997. Thus, surveys on the amount of emissions and efforts to reduce emissions have been started all over the world. Simultaneously, studies on alternative gases to SF₆ gas have increased. In fact, studies on alternative gases, the purpose of which was to find a gas that had better performance than SF₆ gas, had been carried out previously. However, the purpose of recent studies is to reduce environmental impact.

SF₆ as insulating gas has a very good dielectric strength. Therefore, SF₆ is used in many applications. Nowadays, it is investigated again whether SF₆-N₂ mixtures can replace pure SF₆ in for example gas-insulated substations and lines. For pure SF₆ the optimal gas pressure varies between 4 and 6 bar. In order to find a suitable SF₆-N₂ gas mixture to substitute SF₆, the gas pressure has been varied in such a way that the similar lightning impulse breakdown voltage levels were obtained. In further experiments, we compared gases of corresponding dielectric strengths.

This paper presents the results of an experiment designed to investigate the effect of a conducting particle on a spacer surface upon the dielectric strength in SF₆/N₂ gas mixtures. The experiment, which involves a comparison with pure SF₆ gas, investigates the dielectric strength using a spacer model with an adhering particle on the surface under homogenous field conditions. The flashover field strength for a clean and particle contaminated spacer under AC and LI stress is discussed.The results of the investigations show the sensitivity of SF₆/N₂ gas mixtures to conducting particle on spacer surfaces in a gas pressure range of 300 to 1100 kPa. Moreover, the correspondence between pure SF₆ and SF₆/N₂ mixtures for a AC and LI flashover field strength range from 103 to 178 kV/cm are determined.

The influence of space-charge on the breakdown behaviour of SF₆ is investigated. The insulation properties in case of impulse voltage stress, AC stress and VFTO (very fast transient overvoltage) stress are essential for the design and testing of gas insulated switchgear. The manifold shapes of these overvoltages have great influence on the spacecharge and discharge development. In case of slowly rising impulse voltage, the ion drift during streamer development causes a high charge displacement at the streamer tip, leading to high breakdown voltages. A criterion for a simulation model of the phenomena is proposed. It is proven by theoretical considerations and experimental results.

The interruption of high alternative currents in high-voltage SF₆ circuit breakers leads in general to rather strong erosion of the contacts and of the nozzle surrounding the electric arc. The plasma created by the arc is thus established in a mixture composed of the filling gas, the copper vapor, the vapor of the nozzle material and several impurities such as water and oxygen coming from wall desorption. In modern circuit breakers, the nozzle ablation erosion is enhanced in order to increase the pressure in the upstream part of the apparatus, which leads to a strong blowing of the arc at current zero without using an energetic piston effect.

Sulfur hexaflouride (SF₆) is a man-made gas, offering excellent electrical, thermal, and acoustical insulation properties, and as such, it has been extensively employed for a variety of applications, i.e. power distribution (mainly), semiconductor foundries, metallurgy, tracer gas, sound sealing (double glazing, torpedo noise silencer), thermal sealing, diving equipment, aeronautical, wind tunnels, telecommunications, relays, antennas, medical (ultrasounds, ophthalmology) etc1 . However, SF₆ has been identified to be one of the strongest greenhouse gases, having a global warming potential 25000 greater, compared to that of CO₂, and an exceptionally long lifetime (estimated between 1800 and 3200 years). SF₆ is primarily employed worldwide (approximately 80% of its total production) as a gaseous insulator for high-voltage systems (GIS) including switchgear, gas insulated transmission lines, and power transformers. The current trend is to reduce SF₆ quantities within electrical equipment by mixing it with natural gases. As a result, the interest for possible SF₆/gas mixtures for power applications has been reignited. Most of the research work is now focused on SF₆/N₂ gas mixtures2, as it appears to be suitable for industrial non uniform electric field applications, combining environmental compatibility with comparable dielectric performance of pressurized SF₆ and overall cost effectiveness. Other possible mixtures of SF₆ that have been recently proposed are the SF₆/CO₂/N₂ instead of the SF₆/CO₂ mixtures for transformer applications, and also, SF₆ with dielectric mixtures of CHF₃ and CF₄ diluted at 50% to 75% with N₂ for possible applications at lower temperature environments, thus combining environmental compatibility with reduced gas decomposition rates3.

For the last three decades, sulfurhexafluoride has accumulated into the atmosphere to a concentration of ≈4 pptv, due mostly to leaks and releases from electrical equipment. Recognized as a very persistent gas, with lifetime exceeding three millennium, and as the molecule having the largest global warming potential, it was included as the sixth gas in the Kyoto Protocol. Since 95% of all the SF₆ mass is contained in the troposphere, a test program was conducted to study the chemical stability and reactivity of the inert SF₆ molecule in the typical conditions of the lower atmosphere (< I5 km). It was focused on the almost exclusive decomposition path resulting from the dissociative capture of an electron. Chemical kinetics of SF₆ in SF₆/air mixtures (1-1000 ppmv) were studied using reaction cells and setups favoring the interaction of SF₆ with low energy electrons (≈0,2 eV). These were generated in-situ by photo-emission from the UV-irradiated inner Al cell walls. The following experimental conditions were chosen to simulate those of the lower atmosphere: pressure (25-100 kPa), temperature (-40 to +25°C), humidity (0.05-3.5%). The O₂ concentration was also varied between 2-21%. In all cases, the decomposition of SF₆ was observed to proceed at very low rates to form exclusively SO₂F₂, the only byproduct observed. A simple reaction mechanism is derived from the ion-molecule reaction of SF₆ with O₂, the typical negative ion formed by electron capture in air at atmospheric pressure. In our experimental conditions as in the atmosphere, this reaction must compete with many other neutralization paths of the O₂, but the SF₆ destruction certainly occurs, due to its large electron affinity. Indeed, the dissociation of SF₆ into SF₅ is certainly irreversible in those situations. The decomposition kinetics of SF₆ measured in tropospheric air in this study can certainly improve the chemical reaction model used to estimate its persistence lifetime.

For environmental reasons the quantities of sulfur hexafluoride (SF₆) used in high voltage equipment must be reduced by mixing it with some lower-risk gases: N₂ for instance 1_5. The stability of such mixtures under electrical constraints during operation must then be checked and compared to that of undiluted SF₆. Among the various types of stress to which SF₆ is subjected, sparking may occurs, especially in substations. The discharges degrades the gaseous insulator as well as other struck materials (as solid insulators); so the sparking-induced decomposition of SF₆ or SF₆-N₂ mixtures was to be studied in presence of additives (vaporized insulator, O₂, H₂O).

Future applications of gaseous dielectrics such as SF₆ will be mostly conditioned either by a stronger control of leakage in the atmosphere, and/or by the choice of a more appropriate dielectric gas. The latter should satisfy the conditions of a low global warming potential and zero ozone depletion potential. The importance of both parameters strongly depends on the lifetime of the candidate molecule in the atmosphere and, hence, on its reactivity. For this reason, theoretical studies of the reactivity of the candidate gas remain a major challenge: correct modelling of atmosphere reactions and related computational efforts can indeed help to decrease the huge amount of experimental work that such a choice of a new gas would require.

SF₆ gas insulated high voltage switchgear (GIS) has been installed in an increasing number over the last three decades. GIS covering the voltage range from 72,5kV to 8OOkV are now in service world-wide.The driving forces for installing GIS instead of traditional air insulated switchgear (AIS) were, among other the needs to cope with space limitation in densely populated urban areas (installations in buildings and underground)to increase availability in areas exposed to high atmospheric stress (pollution, extreme climatic conditions)to improve safety of operation in earthquake areasto meet special requirements of hydroelectric plants.Economic considerations and the progress in GIS technology have reduced the gap in the overall cost relationship between AIS and GIS and thus increased the application range of this technology.Besides the economic aspects such issues as the space saving potential, improved safety and reliability, favourable maintenance, increased service availability, and insensitivity to external disturbances have played a key role.This paper comparatively presents the major layout and design features of GIS and AIS and the related performance characteristics. These allow to determine functional advantages, to carry out life cycle cost (LCC) comparisons, and to assess environmental issues (LCA - life cycle assessment). For the value based comparison exemplified LCC has been given the main impact followed by performance issues and environmental impact.As a result GIS and AIS technology achieve similar ranking values, depending on the specific assessment. The optimisation of GIS technology with respect to lowest life cycle cost, highest availability and zero-maintenance has made GIS competitive with AIS not only at special application fields.

In Japan, demand for electric power has been concentrating in large cities recently, but it has been becoming increasingly difficult to build substations in urban areas because of the difficulty in obtaining land and fireproofing related to them. This has been inviting rapidly growing demand for large-capacity underground substations. Gas-insulated transformers have been finding growing applications primarily in power distribution, and a large number have been installed in densely populated buildings and underground. Recently, large-capacity gas-insulated transformers have been developed and today gasinsulated transformers are coming to be used not only for power distribution but also in large-capacity substations.

This paper describes the effect that a lower SF₆ fill pressure and SF₆/N₂ gas mixtures have on the short-line fault (SLF) interruption capability of SF₆ power switchgear. Detailed measurements of current zero hereby allow us to differentiate between the effect on the thermal and that on the dielectric breaking capacity during a current interruption.

Recently, the study for replacing SF₆ gas that is one of the potent greenhouse gases by N₂/SF₆ gas mixture has been advanced in the gas insulated switchgear (GIS).1-6 The sets of mixture ratio and pressure which dielectric performance is equivalent to that of pure SF₆ are being clear. However some component devices of GIS such as the gas circuit breaker (GCB), the disconnecting switch (DS) and the earthing switch (ES) are required of the capability of the current interruption besides the dielectric performance.

Evaluation of lightning surge waveform, which actually enter into substations, is important when investigating the test voltage of equipment. The waveform of the standard lightning impulse waveform (1.2/50 µs) is used for testing; however, the lightning surge waveforms in actual fields are complex waveforms in which various different oscillations are superimposed1,2). Investigation of the insulation characteristics of the equipment against the complex waveforms and standard one has significant importance. The insulation characteristics of Gas Insulated Switchgear (GIS) were investigated for these waveforms.

The high reliability and compactness achieved by compressed gas insulation in HVAC systems, has led to the development of HVDC Gas Insulated Substations (GIS). As in case of HVAC GIS, the basic insulation components of HVDC GIS are SF₆ gas and solid spacers. In this respect, it has been recognized that the breakdown strength of GIS is mainly influenced by spacers. For HVAC GIS, the problems related to spacers can be eliminated to a great extent by a careful electric stress control design1-3. However, for HVDC GIS stress control designs based on dielectric interfaces with no trapped charges are not valid, since charges get accumulated on spacer surfaces which distort the designed Laplacian field distribution. In this context, it is also kept in mind that SF6 insulation is very sensitive to local enhancement of electric field. According to this surface charge accumulation and its effects in HVDC GIS have been studied in various aspects by several researchers4-11. In order to improve the dielectric performance of epoxy spacers by properly shaping the gas-dielectric interfaces, studies on electric field optimization along the profile of the gas-dielectric interface in gas insulated systems have been reported in literature12,13. For HVDC GIS, investigations on the effects of spacer shapes on flashover characteristics have also been carried out14. Since, in contrary to HVAC GIS, the electric field distribution at steady-state in HVDC GIS is mainly controlled by the conductivity k, of the dielectric media, studies on the influence of volume and surface resistivities on the electric field distribution around GIS spacers have been done15.

SF₆ gas has widely been used as the insulating and arc-quenching gas for power apparatus such as gas insulated switchgear (GIS) and gas insulated bus (GIB), which contribute to realize compactness and high reliability of the apparatus. However, SF₆ substitutes such as N₂/SF₆ gas mixtures are now in great concern to reduce the consumption and emission of SF₆to the atmosphere,1 since SF₆ is identified as one of the potent greenhouse gases.

Since 1950 years’, in order to minimize the substation area, SF₆ gas insulated switchgear has been energetically developed. Even at this moment, a new principle to improve interruption ability or insulation is considered and further reduction of switchgear size is being made to innovate new GIS. At moment, 550kV-63kA one-break circuit breaker as well as 1100kV UHV breaker is realized and the former is practically used.

Conducting particles, such as small aluminium splinters which can freely move in GIS, often result in dangerous situations for the operation of the GIS [1]. Such free particle can start to move under influence of the applied electrical field. In particular when a particle approaches the high-voltage conductor, breakdown can occur [2]. In this contribution, we analyse UHF measurements obtained from free particles to assess their risk for the insulation condition.

Gas insulated transmission lines (GILs) are a good alternative high power transmission system1,2, when overhead lines could not be installed. As insulating medium N₂ - SF₆ gas mixtures with a SF₆ content about 20 % are used. Such mixtures with the same intrinsic dielectric strength turned out to be more sensitive to defects like particles or fixed protrutions3.

Measurement of the impulse voltage breakdown, U_b50 characteristics with switching surge of wave shapes ±250/2500 and ±190/1900 μs and with the lightning surge of ±1.2/50 μs in atmospheric air are reported with increasing gap distance for electrode configurations in extremely nonuniform field. The propagation velocity of the breakdown channels and the time required for the breakdown to accomplish could also be measured. The results show that breakdown strength strongly depends upon the electrode configurations, the waveform and the polarity of the applied impulse voltage. The average leader propagation velocity is found to depend upon the rate of rise of the applied impulse voltage.

SF₆ gas has excellent dielectric insulating properties, and the application to the electric power apparatuses such as a GIS has contributed to the improvement of the reliability and miniaturization.1 However, a significant decrease of dielectric strength is induced under a nonuniform electric field by the existence of a protrusion on the conductor or a metal particle, or by invasion of an steep front surge.2,3 Moreover, recently the leakage of SF₆ is regulated like CO₂, because the ability of contribution to green-house effect is very high compared with that of CO₂.4

Electrical breakdown in air has been extensively studied by a large number of investigators and the already published literature on this subject has contributed toward an understanding of the mechanism of breakdown. The differences between the sparkover voltages and breakdown mechanisms of rod - rod and rod - plane gaps suggest that the examination of intermediate gap configurations’ would be of interest.

In order to investigate the breakdown mechanism of a conductor — tower gap under transient overvoltages in the laboratory, the simulation of overvoltages is done by the use of standard lightning or switching impulses. If these impulses are superimposed or preceded by a steady direct pre — stressing voltage it has been shown1,5 that this affects the dielectric strength of the configuration used. Previous study1 showed that the influence of the superimposition of positive impulses on a negative DC pre — stressing conductor-rod gap with a spacing of 50 cm is significant in that under certain conditions the overall sparkover voltage tends to increase. Several parameters affect this behaviour, i.e. the value of the pre — stressing DC voltage, the diameter of the energised conductor, the position of the rod with regard to the conductor, the waveshape of the applied impulse and humidity. All this was explained in terms of the various DC and impulse coronas that occur at the conductor and the rod which tend to create a more «uniform» field distribution in the gap. In order to have a better simulation of a tower — line clearance, a construction similar to a tower and a line has been constructed. Since a conductor — rod gap shows a gap factor of about l.610, rods were inserted in the gap and the influence of their length on the breakdown mechanism has been evaluated.

Sulfur hexafluoride (SF₆) is the insulator gas used in most of equipments handling high and very high voltages.Owing to its high global warming potential, SF₆ has been classified during the Kyoto conference on climate change among the greenhouse gases. Its emission in the atmosphere must therefore be reduced1. In order to comply with this aim, one of the solutions adopted by the manufacturers and the users of some of these equipments, consists in reducing the quantity of SF₆ used by mixing it with zero (or very much lower) risk gases.For instance, as SF₆/N₂ mixtures with SF₆ concentrations lying between 5 and 15% realise a satisfactory compromise between dielectric performances and cost, Electricité de France plans to use these mixtures to fill its future gas-insulated lines2. Under the effect of the corona discharges which can occur in these equipments, part of the SF₆ and of N₂ will dissociate and lead, in the presence of even low concentrations of H₂0 and/or 0₂, to the formation of a lot of other gaseous compounds: (SF₄+SOF₂), SOF₄, S0₂F₂, S₂F₁₀, NF₃....Previous studies on the decomposition of SF₆/N₂ (10:90) mixtures under coronas3 showed that, with no impurity added, the total quantity of by-products formed is not very different from that measured in pure SF₆ for the same charge transported value. This indicates that the percentage of SF₆ consumed increases as its concentration in the gas phase decreases 3,4.This result led us to extend our experiments to SF₆/N₂ mixtures only containing 5% of SF₆.

All SF₆-insulated equipment leaks to some degree. These losses of SF₆ gas are usually low and usually less than 2% of the whole gas content of typical GIS. They occur slowly and almost undetected for many years, until a low gas pressure alarm sounds. Over the last fifteen years, manufacturers have continuously improve the gas tightness of their metal-clad equipment which now exhibit leak rates roughly four time lower. Nevertheless, environmental concerns have been expressed in the last five years, peaking at the Kyoto meeting, about the cumulative effects of SF₆ in the atmosphere. Remembering the PCB problems, electric utilities start evaluating their SF₆ losses, since 80% of the SF₆ gas lost into the air annually come from electrical equipment. At Hydro-Quebec, we realized an evaluation program aimed at the direct measurement of the SF₆ leak rate of indoor GIS. Two substations were fitted with a sensitive SF₆ trace analyzer and monitored during 9 and 13 months respectively, in two consecutive periods.SF₆ gas was always present in the GIS surrounding air, at concentrations varying typically between 20 and ≈100 ppbv, but sometimes higher. These fluctuations were not due to the GIS leak rate changes but resulted mostly from the variations of the air exchange rate of the building due to external (climatic) and internal (heating, ventilation, human activities) factors. Interestingly, these measurements have shown that the two 20-years-old GIS studied exhibits average SF₆ lost rates of only 0.55% or less.

SF₆ gas recovery is considered one of the most important issues when we utilize a gas mixture at a low SF₆ content in gas insulation1). One of the plausible methods is to recover SF₆ as liquid using the difference in boiling point between gas species in the mixture. However, it has been pointed out theoretically that the recovery loss by this method is very large when SF₆ content in the mixture is lower than ~50 vol.%1). When gas mixtures with much lower SF₆ contents are considered, the loss increases so much to make the liquefaction practically impossible.

GIL’s (gas insulated transmission lines) provide an alternative technology for overhead lines e.g. in city centres. Mixtures of SF₆ and N₂ in varying percentage ratios are used in GILs, depending on the manufacturer involved.

In CIGRE there are several Working Groups, which are active in the field of gas insulated equipment for electric power supply. Two of them are dealing with the whole technology of gas insulated switchgear (GIS) (WG 23.02) and gas insulated transmission lines (GIL) (JWG 23/21/33.15) whereas WG 15.03 (Insulating Gases) is specialised on problems of gas insulation systems applied in these and further technologies. Its work bases on experience of manufacturers and utilities and results of scientific research published in relevant conferences like the International Symposium on Gaseous Dielectrics. Missing knowledge for the technologies is ascertained and necessary investigations are promoted. Of high importance is the development of commonly approved methods for testing, diagnosis and assessment of insulation condition and the expected life time. At the moment there are three running Task Forces in action - TF 15.03.07 Long Term Performance of Gas Insulated SystemsThis topic is of increasing interest due to the growing age of the equipment.- TF 15.03.09 Risk Assessment of defects in GIS by PD diagnosticsBoth topics are important for efficient maintenance procedures to ensure a sufficient reliability- TF 15.03.10 Insulation properties of N2/SF6-mixturesN₂/SF₆-mixtures are applied in GIL. Further work is necessary to get the same knowledge as for pure SF₆. Actual problems in the past were handled in similar Task Forces. This paper presents by three examples how and which work has been done in CIGRE WG 15.03.

In Japan, gas insulated switchgears (GIS) have been applied widely since the field test and the commercial operation of 66 and 77kV switchgears in 1968 and 1969 respectively. The application has been expanded up to 500 kV and furthermore 1000 kV class equipment. This is due to the fact that the gas insulated equipment can meet the requirements in the country that huge electric power has to be supplied in the metropolitan and industrial areas and also the severe environmental conditions in these areas. The applications of the gas-insulated equipment enable downsizing the substation and increasing the reliability and safety of the substation by applying metal enclosure construction. Also, the excellent characteristics of maintenance-free enable laborsaving in maintenance works. Therefore, gas insulated equipment such as, gas circuit breaker (GCB), gas insulated switchgear (GIS) and gas insulated transformer (GIT) have been developed and used widely in almost all classes of the electric power systems in Japan.

This paper summarizes EPRI’s present and future research in the area of SF₆. EPRI is a nonprofit research institute funded by global energy customers of all types. EPRT’s focus was once the US alone, but today EPRI serves over 1000 energy related organizations in over 40 countries. The focus of EPRI’s activities is the management of Research and Development for collaborative use. EPRI’s Headquarters are in Palo Alto, with 30 specialized facilities and technical centers throughout the US.

Sulfur hexafluoride (SF₆) is a potent and persistent greenhouse gas. Over a 100-year time horizon, SF₆ has a per-molecule global warming potential (GWP) 23,900 times greater than carbon dioxide (CO₂), the greenhouse gas of highest concentration in the atmosphere. With an atmospheric lifetime of 3,200 years, SF₆ is also a very stable chemical.1

In addressing specific problems such as the fixture of gas dielectrics for power industry use, we must think about the general needs of the industry in order to position these problems correctly in the whole socioeconomic structure of society.

H. OKUBO: In the case of SF₆ GIS, particle movement is not of interest under AC voltage application because the breakdown occurs just under the impulse voltage application condition. However, for a pure N₂ or for small content SF₆ in N₂ GIS, AC breakdown would occur just under the condition of particle standing on the tank. In this case, I think that we need to change the testing method of the GIS. What do you think about it?

Electron collisions are the most fundamental processes in a plasma for gaseous dielectrics or plasma processing.1,2 The plasma is sustained mainly by the process of electron-impact ionization. Electron collision produces various active particles (i.e., excited species, ions, and radicals), which, in turn, induce many chemical processes of practical importance. Those inelastic collisions are also important in slowing down of the electrons accelerated by the electric field. Elastic scattering should be taken into account when we determine the distribution of electron velocity, which controls the transport processes of electrons in the plasma. Finally, electron-attaching process plays a key role in the gaseous dielectrics. Thus, in the study of gaseous dielectrics or plasma processing, it is essential to have the cross section data for such processes of electron collisions with atoms and molecules.