Determination of waiting times between successive breakdown experiments

A common duty in high voltage engineering is the experimental determination of the breakdown voltage probability distribution of gaseous insulated electrode configurations. Due to electron detachment from negative ions in the case of electron attaching gases, the breakdown probability does not only depend on the gas and its density, but also on the ion density in the gas. If a disruptive discharge occurs in a gas gap, the number of ions in the gap is significantly increased compared to the equilibrium state at zero voltage. If the time until the next voltage application is too short, the statistical time lag will be decreased and, as a consequence, the probability of breakdown will be increased. To perform statistical independent breakdown experiments it is necessary to ensure identical ion densities in the gas gap before every new voltage application. In the literature waiting times mostly around six minutes are proposed as sufficient for ion density equalization in sulfur hexafluoride (SF₆). However, simulations of the behavior of SF₆ performed for the present contribution reveal that six minutes are not always sufficient to achieve statistical independent results. Especially at higher pressures longer waiting times are necessary. To investigate this problem, measurements of the natural ion pair generation in SF₆ were performed at different laboratories. Based on this data the temporal development of the ion distribution in the gas is calculated. Two cases are of interest: the changes in ion density after a disruptive discharge and in case the gap withstood the applied voltage and therefore all ions are swept out. After transformation of the temporal ion density into breakdown probability curves it is possible to estimate the necessary waiting time.