SF6 dielectric behaviour in a high voltage circuit breaker at low temperature under lightning impulses

This work concerns the study of the dielectric and thermodynamic behaviour of SF₆ in a 145 kV circuit breaker subjected to lightning impulse voltages in both polarities at temperatures varying from ambient (20°C) to -45°C. The filling of the circuit breaker with SF₆ is carried out at the reference point: T=15°C, P=6 bars abs. and d=39.6 kg/m³. Devices allowing detection of the appearance of local condensation of SF₆ and to identify the breakdown between the circuit-breaker contacts and the flashover outside the circuit-breaker, are presented. The experimental results show a general tendency to a decrease in the U_50% voltage when the temperature is decreased at constant density (a decrease by about 5% when the temperature was decreased from +20°C to -25°C). Such a variation would not have a significant influence in industrial practice. Even if this variation is very weak, it also appears from the obtained results that the gas density is not the only parameter which has an influence on the electric strength of the gas. However, to compare results at different temperatures, it would be better to make it at constant density and not at constant pressure, to avoid the dependency of the ratio (E/N) on N (where E is the electric strength and N the density). On the other hand, it seems that the presence of the liquid in the circuit breaker is not a penalizing parameter. The distribution of the electric field corresponding to the geometrical configuration of the studied circuit-breaker is also presented