Measurements of Swarm Parameters in C_4F_7N:O_2:CO_2, C_5F_(10)O:O_2:CO_2 and C_5F_(10)O:O_2:N_2 Mixtures

摘要

In the quest for an electrical insulation gas with low global warming potential for medium and high voltage equipment, perfluoroni-trile (C_4F_7N) and perfluoroketone (C_5F_(10)O) based gas mixtures have shown a high electric strength, low toxicity and lower GWP than that of SF_6, making them promising alternatives. Detailed swarm parameter investigations have already been made in binary mixtures with N_2 and CO_2. Manufacturers started to use mixtures of C_4F_7N or C_5F_(10)O with CO_2 and O_2, or with air, as insulating and switching media in medium and high voltage gas insulated substations. The addition of oxygen has shown to reduce the creation of soot and CO from discharges or sparks. The influence of the addition of oxygen on the electrical properties, in turn, has not yet been studied in detail and is the topic of the present publication. The electrical insulation properties of these gas mixtures are studied using a pulsed Townsend experiment. The effective ionization rate coefficient, the electron drift velocity and the longitudinal diffusion coefficient are obtained, and the density-reduced critical electric field is deduced for different mixtures containing up to 19% O_2. For C_4F_7N:CO_2:O_2 and C_5F_(10)O:CO_2:O_2 mixtures in which the fluorinated gas is kept at 5%, the addition up to 10% O_2 does not change the effective ionization rate coefficient. On the other hand, a clear increase in the latter is observed for mixtures of C_5F_(10)O:N_2:O_2 containing up to 19% O_2 therefore having a negative impact on the insulation strength.