Effects of Temperature Gradients on Breakdown Characteristics of Tri-Post Insulator in HVAC GIL

Tri-post insulators are the core component in gas-insulated transmission lines (GILs). When subjected to the combined effect of electrical, mechanical, and thermal stresses, occurrences of bursting breakdown of the tri-post insulators are frequent, posing a threat to the safe operation of the electric power system. Based on a downsized GIL model, the impact of temperature gradient on the multifield distributions and the breakdown characteristics of tri-post insulators were investigated numerically and experimentally. Results show that upon cooling to room temperature, the shrinkage ratio of epoxy resin surpasses that of aluminum, resulting in residual thermal stress around the conductor and inserts of the insulator. With an increase in conductor temperature, the relative permittivity increases but the elastic modulus of epoxy resin decreases at the high voltage (HV) side of the insulator. Consequently, the local electric field and tensile stress get relaxed, leading to an extended time to breakdown and a prolonged surface crack extending into the belly region of the insulator. Furthermore, the presence of defects amplifies the electric field and mechanical stress around them, thereby increasing the risk coefficient of insulators, with the risk coefficients of metal tips, cracks, and bubbles decreasing in that order. The outcomes of this study can provide references for the reasonable design and safe operation of GIL.