Dielectric behaviour of gaseous insulation with synthetic air at AC stress

Abstract

The paper deals with basic investigation of AC breakdown behaviour of gaseous insulation with synthetic air at different level of field homogeneity and gas pressure. The measurement was performed at different gas pressure ranging from 0.1 to 0.25 MPa and gap distances from 1 to 9 cm, using an electrode test-set “tip-bowl”. The test parameters were chosen in such a way that different field types could be realized. At small distances the electrical field is weakly inhomogeneous and the breakdown values correspond to the discharge inception level. The measured values show a reasonable agreement to those values based on the calculation of the streamer criterion by using a relevant approximation formula for the effective ionisation coefficient. For larger distances the electrical field becomes more inhomogeneous and the calculated streamer inception voltage corresponds to the partial discharge (PD) inception voltage only. The calculated inception voltage values fit the measured values over the applied gap distance and gas pressure quite sufficiently. The best fit to the measured results was reached by slightly changing the parameters of the approximation formula depending on the gas pressure value. Additional PD measurement shows different PD activity dependent on gas pressure and gap distance.