No abstract

Deviations to Paschen's law for SF6 at high pressures are commonly explained by field enhancement due to protrusions supposed to exist at the surface of the electrodes. New experiments for positive polarity allow the authors to draw a different conclusion, i.e. to ascribe these deviations to the impurities of the gas, especially to the combined effect of O2 and H2O. For negative polarity, impurities play a minor role and the threshold is very sensitive to the surface state of the stressed electrode.

The breakdown mechanisms in dry and moist compressed SF6 for inhomogeneous and quasi-homogeneous field configurations for positive polarity are dealt with. Deviations from similarity law are ascribed to the impurities present in SF6, mainly water vapour. It is experimentally shown that these impurities induce a change in the discharge modes. Adequate breakdown criteria are formulated depending on the degree of purity of the gas and/or the non-uniformity of the field distribution. In particular, in pure SF6, a direct streamer breakdown takes place when the mean electric field equals the critical field. The leader breakdown regime corresponds to the build-up of the critical avalanche. The study confirms the rejection of the effect of the micro-protrusions present on the electrode surfaces.

No abstract

The calculation of the striking distance can estimate the probability of lightning strike on a structure and thereby evaluate the effectiveness of a lightning protection system (LPS). The dimensioning and the positioning of air-termination on structures is often performed with the Rolling Sphere Method (RSM). RSM originated from the electric power transmission industry and is based on the well-known Electrogeometric Model (EGM). The EGM relates striking distance to the prospective peak stroke current. To apply this technique, an imaginary sphere is rolled over the structure. All surface contact points are deemed to require protection, whilst the unaffected volumes are deemed to be protected. The main drawback of this method is that it disregards the upward leaders' development and assumes the same probability for attachment to the ground, to a structure, and to a LPS. The proposed model is based on physical phenomena leading to the formation and the development of positive upward leader in the field produced by the negative downward leader charge distribution and by some other competing upward leaders. Its purpose is to develop a 3-D numerical model in order to improve the interception efficiency of the Lightning Protection System.