This paper deals with the experimental characterization of discharges propagating over insulators of epoxy and glass, immersed in a gas or a gaseous mixture, under lightning impulse voltages (1.2/50 µs), using a point-plane electrode arrangement. The gases and mixtures we considered are SF 6 , N 2 , CO 2 , SF 6-N 2 and SF 6-CO 2. The morphology of creeping discharges and their final lengths are investigated versus the kind of insulator material, the amplitude and polarity of the voltage, the type of the gas (resp. mixture) and its pressure. It's shown that the shape of discharges and their final (stopping) lengths L f depend significantly on the solid insulator and the type of gas. For given solid and gas, L f increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges don't always present a radial structure as reported in literature. For given voltage and pressure, L f is longer when the point electrode is positive than when it's negative while the initiation voltage of discharges is higher with a negative point than with a positive one; and L f is longer with glass than with epoxy. L f is shorter in SF 6 than in CO 2 or N 2. On the other hand, the increase of SF 6 content in SF 6-CO 2 mixture leads to a significant decrease of L f. Therefore the addition of small concentration of SF 6 in a given gas mixture improve the dielectric strength of insulating structure.
This study is devoted to the optical and electrical characterisation of discharges propagating over insulators made of polytetrafluoroethylene (PTFE) filled with different kinds of micro-mineral fillers immersed in gas or gaseous mixture, under lightning impulse voltage (1.2/50 ms), using a point-plane electrode arrangement. The fillers the authors investigated are MoS 2 , Al 2 CoO 4 , SiO 2 and CaF 2 . The gases and mixture the authors considered are SF 6 , CO 2 and SF 6 -CO 2 . It is shown that the stopping length of discharges L f increases quasi-linearly with the voltage; L f is shorter in SF 6 than in CO 2 and it is higher when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one. The discharges do not always present a radial structure as reported in the literature. Also, the type of filler greatly influences the characteristics of creeping discharges. In a given gas or mixture, the shortest L f is obtained with PTFE filled with MoS 2 or Al 2 CoO 4 . Thus, insulators with these fillers appear as the best insulators.
The phenomenon of the commutation in a circuit breaker is the moment when the current switches from the nominal contact circuit to arcing contact circuit. Due to the difference of the impedance (resistance and inductance) between the two circuits, an arc appears between the contacts. So, this phenomenon can lead to two problems: the wear due to the commutation on the main contacts with bad issue if the main contacts could not withstand the nominal current and the particles of molten materials removed from the main contacts (pollution of the chamber with conductive particles). In this article we will analyze the influence of different parameters like materials nature, inductance and resistance of the circuits on the commutation. Calculations have been done and solution to improve the commutation has been proposed.
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