Limiting Values of Critical Electric Stress, and the Design Prediction of EHV/UHV SF6 Insulation Breakdown

An analysis of SF6 Paschen curves in the EHV/UHV range has been made. It is observed that the limiting value of critical electric stress is not the stress at which the apparent ionization and attachment coefficients are equal, that is, it is not the limiting value found for lower voltage treatments. Paschen departures, concluded to be a genuine phenomenon, are found to occur as an hyperbolic function of gap spacing. The Paschen curve, including departures, yields two new values of limiting critical stress. From those values, an equation is developed which may be used to predict insulation breakdown in uniform field configurations. Using a field intensification factor, the equation is adapted to the important engineering configuration of coaxial cylinders. The resultant equation gives good correlation with the breakdown behavior found by several groups using differing electrode geometries. Prediction of breakdown voltage is more accurate than that obtained by the widely used law of similarity. The breakdown equation is in a form suitable for use in prediction of EHV/UHV SF6 breakdown in engineering insulation design situations.