SF6-N2 gas mixtures: Optical investigation of the discharge development under fast oscillating impulse stress in a non-uniform gap

In the case of nonuniform fields, the insulation performance of SF ₆-gas insulated switchgear GIS is strongly reduced. The discharge development in a coaxial electrode configuration with a needle protrusion (nonuniform gap) was investigated for different SF₆/N₂ gas mixtures (5/95, 10/90, 30/70) and pressures (0.1 MPa to 0.3 MPa). The configuration was stressed by low damped monofrequent oscillations up to 20 MHz with amplitudes of about 400 kV. The main focus of this paper are stresses with 10 MHz oscillations. The performance of SF₆-N₂ gas mixtures is described by a factor of merit by means of the comparison with pure SF₆ and N₂. Predischarge current measurements were recorded by means of a Rogowski-coil, inside the inner grounded electrode. The high sensitivity and the fast response time of the measuring system enable a simultaneous detection of displacement current, superimposed partial- and predischarge impulses. The light emission caused by local extension of the discharge development, was recorded by a high speed image converter camera. Different figures of streak records are presented and illustrate the influence of the SF₆ percentage in the mixture and the entire gas pressure on the discharge development