Failures of the metal flange were observed in transformer bushings in previous earthquakes. In order to evaluate the influence of the flange on the seismic responses of 1,100‐kV ultra‐high voltage (UHV) bushing, two types of flanges were selected, which used different materials and layouts. The finite element (FE) and theoretical analyses were first carried out. The results indicate that there is an out‐of‐plane rocking effect at the flange bottom plate, and the flange will be damaged if the stiffeners are improperly designed. Then two identical bushings using different flanges were tested on the shaking table. The bottom plate of the cast aluminum flange cracked along the stiffeners accompanied by oil leakage. However, the UHV bushing with stainless steel flange survived in the test. It is concluded that the properly designed flange could improve seismic performance of UHV transformer bushings.

To assess the seismic failure risk of post electrical equipment, an assessment method considering different sorts of uncertainty of the post electrical equipment was proposed. The aleatory and epistemic uncertainties were considered in the assessment, and the parameters in seismic hazard function of different sites were fitted. To investigate the effects of the supporting structure on the seismic fragility and risk of post electrical equipment, case studies on ultra-high-voltage gas insulated switchgear bushings mounted on two different supporting frames were conducted. The results indicated that both the maximum tensile stress of the bushing and the maximum displacement of the supporting frames should be considered as the limit statuses of the post electrical equipment. The different sorts of uncertainty increase the seismic failure risk of the post electrical equipment, and should be considered in the assessment. Besides, for the pillar-type porcelain element equipment, the supporting frames with lower lateral stiffness generated higher seismic failure risk. In constrast to the case of retrofitting the supporting frame to increase the lateral stiffness, the annual seismic failure risk of the GIS bushing mounted on the retrofitted supporting frame decreased 101%. Besides, the seismic failure risk of the post electrical equipment is not positive correlated with the peak ground acceleration. In the performance-based seismic design of the electrical substation, the seismic failure risk of the electrical equipment should be considered.