1 st Annual International Interdisciplinary Conference, AIIC 2013, 24-26 April, Azores, Portugal - Proceedings- 72 POLARITY EFFECT ON CORONA DISCHARGE VOLTAGES IN SF 6 - N 2 GAS MIXTURES M. Assia Guerroui Dr. Ahcene Lemzadmi LGEG, Université 08 Mai 1945 Guelma, Algeria Abstract: This work is to investigate the behaviour of SF 6 -N 2 mixtures that can be considered as a potential substitute of pure SF 6 . The inception voltages have been determined from the measurements of the current-voltage characteristics for both polarities under high pressures and with highly inhomogeneous fields. The results show that the onset voltages increase with the increase of the gas pressure and the values of positive polarity are higher to those obtained with negative polarity. Key Words: Inception voltages, corona discharge, sulphur-hexafluoride Introduction Sulphur-hexafluoride SF 6 has found a wide range of applications due to its superior insulating properties and chemical stability. As the size of the high voltage equipments increases the cost of insulating gas becomes appreciably high. The quantity of SF 6 released in atmosphere is therefore rising. Due to its ability to absorb and reemit IR make it a potent greenhouse gas [1-2]. With its high lifetime (more than 2000 years), it accumulates in the atmosphere and contribute to the global warming of the atmosphere, therefore SF 6 will be severely controlled in the next years. The by- products issued from decomposition of SF 6 exposed to electric discharges may be dangerous to the equipments (corrosion) and to the personnel (poisoning) [3]. These factors have stimulated the research of a replacement gas with little environmental impact. The most promising alternative is the use of gas mixtures of SF 6 with inexpensive common gases such as nitrogen (N 2 ). SF 6 -N 2 gas mixtures have good dielectric strength, are non-toxic, non-flammable and they have a high arc quenching capacity with a good self healing ability. The purpose of this paper is to provide and discuss the measurements of the onset corona discharge voltages in SF 6 -N 2 gas mixtures at higher pressure ranging from 3 to 15 bars and with different percentage of SF 6 . The onset voltages were determined from the measurements of the current-voltage curves in both negative and positive polarities. A tip-plane configuration was used with the tip radius of few micro-meters and the gap between the electrodes is lower than 10 mm. Most of the data available deals with the breakdown voltages rather than the threshold voltage of corona discharge. Experimental set-up Experiments were made in a stainless-steel cell of 50 cc equipped with two quartz windows as shown in figure 1. Electrodes in a tip-to-plane configuration were mounted inside the cell. The tip electrode of few micrometers is made of tungsten and, is prepared by electrolyse technique, steel tips are also used. The gap between the electrodes varies from 5 to nearly 10 mm. The tip electrode is connected to the high D.C. voltage up to 60 kV. The stainless steel plane electrode with a radius of 12 mm is connected to a galvanometer which measures currents down to some microamperes. Before the cell was filled with the gas, pumping was undertaken pushing the vacuum down to nearly 3.10 -2 Pa. The gas was introduced in the cell without prior purification. The SF 6 used in these experiments is delivered with a purity of 99.97%. The measurements were made for pressures ranging from 3 bars to 15 bars. A partial pressure method is used to mix the gases in the cell. SF 6 was first introduced followed by nitrogen. It is supposed that SF 6 and N 2 do mix quite well. The current was measured when the voltage varies upwards and downwards and after each set of measurements a resting time is observed to allow the mixture to settle down. The tip electrode is regularly changed in order to limit