Towards 500 kV power transformers damaged in the 2022 Luding earthquake: field investigation, failure analysis and seismic retrofitting

The Ms 6.8 Luding earthquake struck Sichuan province in China on September 5, 2022, triggering many failures of electrical installations in power grids. All high-voltage bushings of transformers in a 500 kV substation were damaged during the earthquake. Subsequent field investigation of the transformers was carried out, and a simulation model was developed to study the failure mechanism using the acceleration time histories recorded by a seismograph station near the substation. An equivalent modeling technique in the simulation model for the rotational stiffness of the flange joint per relevant code was proposed. Both the modal analysis and dynamic amplification analysis were then performed. After that, two types of improved flange configurations were suggested for newly designed and existing transformer bushings, respectively. Their effectiveness on stress optimization was validated. The results reveal that the first eigenfrequency of the transformer is close to the highest spectral peak of the recorded acceleration time histories, indicating obvious amplification effects. The stress distributions of the flange of the simulation model well match with the engineering failure, and the hitch lugs of the connection flanges are the leading cause that results in the cracks. Adopting the flange with stiffeners and adding axial curved shells and new hitch lugs between the flange plates can enhance the bending stiffness of the HV bushing and dramatically reduce the stresses of the flange at the positions where the engineering cracks propagated. Moreover, some instructive suggestions were proposed for assisting the seismic design and retrofitting of transformers.