SF6-N2 gas mixtures-electrical insulation and discharge development under fast oscillating impulse conditions in a non-uniform gap

The insulation performance of SF₆-gas insulated switchgear GIS is strongly reduced in the case of non-uniform fields. From this the discharge development in a coaxial electrode configuration with a needle protrusion, fixed on the inner conductor, was investigated for different SF₆/N₂ gas mixtures (5/95, 30/70) and pressures (0.1 to 0.3 MPa). The configuration was stressed by applying low damped monofrequent oscillations up to 10 MHz with amplitudes of about 300 kV. The different gas mixtures are compared with pure SF₆ and nitrogen (N₂). The local extension of the discharge light emission was recorded by a high speed image converter camera. Different figures of streak records are presented and illustrate the influence of the SF₆ percentage, the VFT frequency and the entire gas pressure on the discharge development. The results of the experimental investigation show that in general the discharge development in gas mixtures progresses similar, but not in the same patterns known for pure SF₆