High voltage alternative current (HVAC) gas insulated transmission line (GIL) bears a strong electric field and complex mechanical stresses during operation. As one of the main components of the GIL, tri-post insulators are often subjected to bursting breakdown accidents, which threatens the reliability and safety of HVAC GIL. In this study, based on a scaled-down HVAC GIL model, the mechanism of breakdown of tri-post insulators under the action of electromechanical coupling was experimentally studied. Also, the influences of external force, fixing mode, and defect location on the breakdown morphologies were investigated. The results show that partial discharges (PDs) caused by the initial defects of the tri-post insulator can develop into bursting breakdown, producing internal cracks inside the post and open cracks on its surface. Tensile stress promotes the defect evolution, while compressive stress does the opposite. The mechanical stress distribution inside the tri-post insulator is influenced by the external force and the fixing mode, resulting in distinct breakdown morphologies. Due to the stronger electric field, defects on the ground (GND) electrode are more prone to develop into breakdown than that on the HV electrode. Results of this study can provide a reference for tracing the origin of bursting breakdown faults as well as optimizing the parameters for designing tri-post insulators.
