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.
Investigation of streamer propagation along insulating surfaces
Solid insulation materials are used in high voltage gas-insulated devices for mechanical stability reasons or for the separation of different compartments of the device. While solids have generally higher breakdown strength than gases, the weakest parts are often the solid-gas interfaces of the insulation. This contribution presents first measurements of characteristics of surface flashover in a specially constructed setup. The studied samples are epoxy rods with embedded electrodes, especially designed for localized electrodeless streamer inception and discharge propagation along the surface. The capacitances of the overall system are tuned such that current measurements of the discharge can be performed. In this research we focus on the initial phase of discharge propagation – the formation and development of streamers. The main results are the streamer inception voltage and the evolution of the current shape in nitrogen at different pressures.
Resonant Auger decay study of C1s -> pi* core-excited OCS
The present work aims at characterizing short-lived C1s((-1)) pi*((1)) core-excited states of the OCS molecule based on the analysis of the vibrational fine structure and lineshape profiles of the high-resolution resonant Auger decay spectra recorded at the excitation energies along the C1s -> pi* resonance in the binding energy region 15-19eV. Very different behavior in terms of lineshape and resonant enhancement is observed for the (A) over tilde, (B) over tilde and (C) over tilde final states. This is explained by (1) the variation in the C-O bond lengths for the states involved in the electronic relaxation and (2) different contributions in terms of Mulliken population to the molecular orbitals determining the electronic character of the corresponding states. Since the final-state geometries are known from a number of previous experiments and ab initio calculations, the geometry of the C1s((-1)) pi*((1)) intermediate states can be predicted in analogy with e.g. the N-2, CO2 and N2O molecules. (C) 2009 Elsevier B.V. All rights reserved. (Less)