Investigation of corona discharges and electrical breakdown in sulphur-hexafluoride/nitrogen mixtures

Abstract

These days, corona was studied and investigated in different mixture ratio of SF6 and N2 by observing the pulse shape and frequency and the average current for a range of voltages from onset to breakdown. Models for the corona behaviour were developed and simulation programs written based on various proposed models for the ionisation and attachment parameters of gas mixtures. An important aspect was the use of the symmetrical point/cup electrode system introduced by MacAlpine, my supervisor whose symmetry allows the development of mathematical models not possible with the more traditional point/plane electrode system. A thorough understanding of the pre-breakdown (corona) phenomena is likely to lead to an explanation for the breakdown strength. The measurements were carried out over a range of pressures up to the commercially and industrially important 5-bar level. Both voltage polarities were studied. Experiments conducted in point-cup gaps can be considered to represent the geometric conditions in practical gas-insulated switchgear which are commonly used in power systems, where the breakdowns tend to originate with small metallic particles and the corona which develops at their pointed end. The mechanism of breakdown originating with a long metallic particle on the 'floor' of the earthed outer tube has some similarity with that of the point/plane system.