Insulation Properties of CO₂/N₂/50%SF₆ Gas Mixture under Nonuniform Field

Abstract

This paper describes partial discharge (PD) inception and breakdown voltage characteristics of CO₂/N₂/SF₆ gas mixture under nonuniform field. These voltage characteristics were investigated with AC high voltage by changing the mixture rate of each gas of CO₂, N₂ and SF₆ gas and the gas pressure from 0.1 MPa to 0.6 MPa. It was found that adding a small amount of CO₂ gas into N₂/ SF6 gas mixture causes the drastic increase in the breakdown voltage: for instance, when the mixture rate of SF6 in N₂/SF₆ gas mixture is 50%, with 1% CO₂ addition the maximum breakdown voltage becomes 1.31 and 1.15 times higher than that of 50%N₂/50%SF₆ gas mixture and pure SF₆ gas, respectively. Moreover, those voltage characteristics of CO₂/N₂/SF₆ gas mixture were also investigated by changing the electric field utilization factor as well as by applying positive and negative standard lightning impulse voltages in order to enable to discuss the corona stabilization effect which seems to be one reason for drastically increasing the breakdown voltage. These results and breakdown mechanism of CO₂/N₂/SF₆ gas mixture were discussed on the basis of the corona stabilization effect and the dissociation energies of the component gases by observing PD light images, PD light intensities through a blue and red filter and PD current waveforms.