Partial Discharges and Breakdown Voltage in Designs with Triple Junctions under AC Stress.

Today, manufacturers of medium voltage equipment are searching for alternatives to sulfur hexafluoride (SF$_6$) as the insulating gas. The advantageous thermal and molecular properties make it suitable as an insulating gas, however, it is a strong green house gas. With the present climate challenges, one of the measures of reducing emission of climate gases is to phase out the use of SF$_6$-insulated medium voltage switchgears. An environmentally friendly alternative is air, which has a lower withstand voltage than SF$_6$. For backwards compatibility, the primary manufacturers in this development is optimization (to compensate for lower withstand) of current designs without increasing the dimensions. This work have investigated the breakdown voltage and optical detected partial discharges activity during AC stress in ambient air. The measurements have been carried out for two cylindrically symmetric test objects, containing triple junctions. These measurements have been supplemented by electric field simulations carried out in COMSOL Multiphysics. Various gap distances between the electrodes have been tested. Correlation between gap distance, breakdown voltage, partial discharge activity, and inception voltage have been determined. The withstand voltage of the investigated test objects proved to have a higher value than the dimensioning withstand voltage for streamer propagation in a sphere-plane arrangement. Characteristic pattern of the partial discharge activity, optically detected with a photomultiplier tube, align with the commonly observed phase-resolved pattern for gliding discharges along a surface of a solid dielectric in air. Streamer inception criterion based on applying the background field for calculating resulted in a dimensioning criteria where the withstand voltage of the investigated test objects is lower than the dimensioning inception voltage from calculations of the streamer inception criteria