Non-uniform field breakdown study in three component gas mixtures containing SF6 under impulse voltages at elevated pressures

Gas mixtures comprising of SF6, CCl2 F2 and N2 have been studied under lightning impulse (1.2/50 μs) voltage conditions of both positive and negative polarity for gap spacings of 10 mm and 40 mm over the pressure range of 1 to 5 bar (0.1 to 0.5 MPa). The concentration of SF6 content in the gas mixture was in the range of 1 to 30%, while 10 to 39% was CC12F2 and the rest, the buffer gas (N2 fixed at 60%,). The electrode system consisted of a hemispherically capped steel rod of 5 mm dia and a 230 mm brass plane electrode. An analysis of the results show that the breakdown strength of N2 was enormously increased over all the gap spacings investigated, by the addition of 1% electronegative gases (SF6, CCl2 F2, SF6/CCl2 F2) under both the positive and negative voltage polarities. The influence of the addition of SF6 to the binary mixtures (CCl2F2/N2) was observed to depend on the gap spacing, the mixture pressure and their combination. In SF6/CCl2F2/N2 mixtures a rapid increase in positive breakdown voltages was observed up to a pressure of 0.4 MPa (4 bar); this increase being more rapid at large gaps (>40 mm).