Applicability of the enlargement law and the Weibull distribution to the breakdown in SF6

For insulation co-ordination of gas-insulated transmission lines (GIL), but also for large gas-insulated switchgear (GIS), the question arises, how the reduction of the electrical strength, due to the area effect, has to be taken in to account. From the statistical point of view the enlargement law has to be considered. However, it is very important to apply an adequate basic distribution function. Therefore investigations were undertaken with negative lightning impulse voltage in pure SF₆ with one and four cylinder/sphere gaps at gas pressures of 0,3 MPa and 0,5 MPa. The validity of the enlargement law was confirmed. Furthermore the breakdown probability distribution function was calculated with the volume-time-law and it is shown that the measured distribution functions can be matched by the Weibull distribution