Abstract
Triple junctions, where gas, metal, and solid dielectric meet, exhibit strong electric field enhancement and are critical for discharge inception. This study investigates a typical triple junction found in medium-voltage GIS in open air under lightning impulse stress of both polarities. Experimental results are compared with simulations based on a model incorporating the statistical likelihood of a starting electron within the critical volume. When the electrode at the critical triple junction acts as the anode, breakdown is governed by electron detachment from negative ions. For a cathodic electrode, breakdown is dominated by the quasi-homogeneous field between the electrodes rather than the triple junction, due to limited start electrons availability, despite the critical volume being of significant size. The results highlight the importance of electron availability and field distribution in predicting polarity-dependent breakdown.