The DC electrical breakdown of SF6 gas at extra-high- voltages was studied experimentally in coaxial systems of moderate size. The results were compared to a similarity relationship of electric stress for breakdown based on ionization of the gas. For laboratory-clean systems, theory and experiment agree when the macroscopic gradient does not exceed 150 to 200 kV/cm. If the active area of the stressed electrode is small the law remains accurate for higher gradients. Departures from the law are qualitatively explained by including the electrode surface microstructure in its application.

Electrical Breakdown and the Similarity Law in SF6 at Extra-High-Voltages

Using a high speed image converter camera together with a shadowgraphic technique, positive streamer velocities have been measured as a function of instantaneous voltage on impulses of varying shape and duration in transformer oil and Marcol 70. They have been found to propagate at a constant velocity which is independent of the instantaneous value of the voltage. The velocity appears to correlate with the rate of rise of voltage rather than the initation voltage itself. This result has implications for the use of arresters as protection for electrical equipment and these implications have been examined using breakdown measurements.

Streamer Propagation in OIls and its Relationship to Arrester Application

In service, a transformer will be subjected throughout its life to three different types of voltages. These are: power frequency, switching surge, and impulse voltages. Recent computer studies and field investigations of EHV systems have provided data on the magnitudes and durations of these voltages. Laboratory tests and factory tests of transformer insulator structures have provided new data on performance. Impulse test levels have been defined for many years. Modern lightning arresters can be selected to provide adequate protection against switching surges.

The Effect of Test and Abnormal System Voltages on Transformer Insulation

Ce travail porte sur l'etude de la morphologie et de la longueur finale Lf des decharges se propageant sur divers types de dielectriques solides (PTFE avec differentes charges, resine Epoxy et verre) en presence des gaz SF6, N2 et CO2 ainsi que des melanges SF6 - N2 et SF6 - CO2, sous tension de foudre (1,2/50 µs), en geometrie pointe - plan Les materiaux consideres ont ete choisis pour leur grande utilisation dans les applications haute et moyenne tensions (disjoncteurs en particulier) Les caracteristiques de la decharge sont analysees en fonction de l'amplitude et de la polarite de la tension, de la nature et de l'epaisseur de l'isolant solide, du type et de la pression du gaz (ou melange) ainsi que de la concentration des gaz constituant le melange Il ressort des resultats obtenus que l'aspect morphologique des decharges varie selon les constituants de l'interface gaz/solide, l'amplitude et la polarite de la tension ainsi que la pression du gaz En presence d'un gaz ou melange donne, Lf augmente quasi-lineairement avec la tension et decroit lorsque la pression, l'epaisseur du solide dielectrique et/ou le taux de SF6 dans le melange augmentent Aussi, pour une tension et une pression donnees, Lf est plus eleve en polarite positive alors que la tension d'apparition des decharges est plus elevee en polarite negative Les resultats obtenus avec l'azote et le melange SF6 - N2 sont tres aleatoires Le calcul du champ par elements finis (Flux 2D/3D) montre que le renforcement du champ au voisinage de la pointe est d'autant plus important que la difference entre la constante dielectrique du materiau solide et celle du gaz est grande Les enregistrements de courants associes aux decharges ont revele l'existence d'une decharge secondaire de signe oppose a la tension appliquee Ce phenomene est du a l'accumulation de charges sur la surface de l'isolant solide ; certains materiaux accumulent moins de charges surfaciques et la longueur finale des decharges qui s'y developpent est plus courte Une relation entre le type de gaz (melange) et sa pression, la nature du dielectrique solide et son epaisseur, et la dimension fractale D des decharges a ete etablie D diminue lorsque l'epaisseur e du solide et/ou la pression du gaz augmente et/ou la permittivite dielectrique du solide diminue

Caractérisation des décharges électriques se propageant aux interfaces gaz/solide – Relation entre propriétés des matériaux et dimension fractale

Corona Studies - In Relation to Insulation [includes discussion]

A photographic technique was used to measure ionization under impulse voltage in transformer mineral oil and in transformer Askarel (a chlorohydrocarbon). Two electric-field conditions were investigated: 1. The liquid under test surrounded disk electrodes between which were placed impregnated pressboard and dried photographic paper. Above a threshold voltage, Lichtenberg figures occurred in the thin film of liquid under the rounded electrode edge. For mineral-oil immersant, the figures were independent of voltage polarity, while for Askarel immersant, the figures depended markedly on voltage polarity. The chlorine atoms in the Askarel and in carbon tetrachloride are shown to affect the liquid-ionization process and the puncture voltage of Askarel-impregnated pressboard. Chopped-wave tests demonstrate how the discharges develop with time. 2. The pressboard was replaced by a polystyrene sheet and the upper electrode was raised above the sheet to permit the liquid to fill the intervening space. Ionization records were obtained on the photographic paper at threshold stresses which, in most cases, were lower than either the breakdown stress with the polystyrene barrier present or the breakdown stress of the oil without the barrier. The threshold impulse-ionization stress was the same when the photographic paper was under the transparent barrier and not in contact with the stressed oil layer. Threshold-ionization stresses ranged from 400 to 800 kv per centimeter in virtually uniform fields.

E. L. C. Larson

Papers

Impulse ionization and breakdown in liquid dielectrics