Influence of SF6 admixtures to nitrogen on critical parameters of surface streamer inception
Summary form only given. Sulfur hexafluoride (SF6) is commonly used as a gaseous dielectric in high-voltage equipment due to its excellent insulating properties and chemical stability. However it significantly contributes to the greenhouse effect, and can under certain circumstances (after a discharge event) be poisonous for persons. Therefore, SF6 - nitrogen mixtures are considered, in order to decrease the negative effects of the gas, while still keeping the required dielectric properties.The goal of the present work was to investigate prebreakdown parameters of discharges originating on the surface of a dielectric in the mixtures of SF6 with nitrogen. All experiments were performed in a specially designed vessel that allowed setting up and maintaining the parameters of gaseous environment (SF6/N2 ratio, gas pressure)1. Streamers were propagating along the surface of an epoxyresin dielectric, prepared in a shape, typical for various HV technologies1. The voltage of discharge inception was determined for different amounts of SF6 admixture. It was shown, that the inception voltage increases rapidly starting at very small admixtures of SF6 (~5%), and continues to increase at a lower rate with the further increase of SF6 percentage. The voltage of streamer inception also increases with the increase of gas pressure (in these experiments the concentration of SF6 was kept constant). The velocity of discharge propagation was estimated from the analysis of time-resolved discharge current measurements. The rise-time of the current increased significantly with the addition of SF6, moreover, for high concentrations of the above the majority of the discharges were originating as leaders, skipping the streamer phase. The fall time of the current behaves in a similar way, thus the total duration of the discharge decreases almost by a factor of 10 (from tens to several nanoseconds for pure nitrogen and 5% SF6 admixture respectively). Overall, it was shown, that major inception parameters (critical voltage, rise and fall times, current shapes) of streamer discharges propagating on a gas-dielectric interface change considerably starting already with a small (5%) admixture of sulfur hexafluoride to nitrogen.
Creeping Sparks: a study on surface discharge development
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Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms
SUMMARYThe Bushveld Complex has continued to serve as the basis for study into the fundamental nature of petrological processes for layered intrusion formation and for oxide and sulphide hosted Platinum Group Element (PGE)–Cu–Ni ore deposits. These studies have included discoveries in terms of the physical extent of Bushveld magmatism, both laterally and internally. Lateral variations in the mafic to ultramafic Rustenburg Layered Suite of the Northern Lobe of the complex have also revealed petrologically distinctive Upper Critical Zone equivalent rocks (the so-called Flatreef) with enhanced contamination and mineralization traits that reflect a transition between Eastern and Western Lobe equivalent stratigraphy and Platreef-style complexity. Traditional magma mixing models have been re-examined in light of radiogenic isotopic evidence for crustal involvement early in the chromite precipitation or formation process, combined with evidence for associated heterogeneous fluid contents, cryptic layering profiles, and textural evidence. A wide variety of alternative ore-genesis models have been proposed as a consequence. The fundamental mechanics of magma chamber processes and the existence of the magma chamber as an entity have been called into question through various lines of evidence which have promoted the concept of progressive emplacement of the complex as a stack of not-necessarily-quite-sequentially intruded sills (with or without significant quantities of transported phenocrysts), emplaced into variably crystallized and compacted crystal-liquid mush mixtures, modified by compaction-driven late magmatic fluid (silicate and aqueous) activity. Alternatively, petrological and geochemical observations have been used to discount these interpretations in favour of more conventional cooling and gravity-driven accumulation of silicate and ore minerals in a large, liquid-dominated system.RÉSUMÉLe complexe de Bushveld a demeuré à la base d’études sur la nature fondamentale des processus pétrologiques de formation d’intrusions litées et des gîtes des éléments du groupe platine (ÉGP)-Cu-Ni hébergés dans les oxydes et les sulfures. Ces études ont comporté des découvertes sur l’étendue physique, à la fois latérale et interne, du magmatisme de Bushveld. Les variations latérales de la suite stratifiée et mafique à ultramafique Rustenburg du lobe nord du complexe ont également révélé des roches équivalentes pétrologiquement distinctes de la zone critique supérieure (le communément désigné Flatreef) avec des traits de contamination et de minéralisation accrus qui reflètent une transition entre la stratigraphie équivalente des lobes est et ouest et la complexité de type Platreef. Les modèles traditionnels de mélanges magmatiques ont été réexaminés à la lumière de preuves isotopiques radiogéniques indiquant une implication de la croûte au début du processus de précipitation ou de formation de la chromite, combinées à des preuves de contenu fluide hétérogène associé, de profils de litage cryptique et de preuves texturales. Ainsi, une grande variété de modèles alternatifs de genèse de minerai a été proposée. La mécanique fondamentale des processus de la chambre magmatique et l'existence de la chambre magmatique en tant qu'entité ont été remises en question au moyen de divers éléments de preuve qui ont mis en avant le concept de mise en place progressive du complexe sous forme d'un empilement non-nécessairement séquentiel de sills injectés (avec ou sans quantités significatives de phénocristaux transportés) mis en place dans des mélanges de bouillie cristaux/liquide à cristallisation et compaction variable, modifiés par une activité tardive de fluide magmatique (silicaté et aqueux) induite par la compaction. Alternativement, des observations pétrologiques et géochimiques ont été utilisées pour écarter ces interprétations en faveur d'un processus plus conventionnel de refroidissement et d’accumulation de minérais silicatés et minéralisés induite par la gravité dans un vaste système à dominance liquide.