A degraded cantilever model for post insulators under cyclic loading

Support-type tubular busbar structures widely used in substations are inherently vulnerable to earthquake-induced damage. This study experimentally characterizes the load-resisting behavior and deformation mechanisms of post insulators under quasi-static cyclic loading representative of 220 kV substation conditions. Quasi-static test results revealed that the porcelain body responded almost elastically, whereas localized plastic deformation developed at the porcelain-to-cast-iron-sleeve interface. The effective viscous damping ratio of the post insulator remained below 1.5%, indicating limited energy dissipation capacity within each loading-unloading cycle. A progressive increase in the top horizontal displacement induced pronounced stiffness degradation in the post insulator. Relative to the initial elastic value, the secant lateral stiffness decreased by 48% at a top displacement of 50 mm and by 67% at 100 mm. Building upon these observations, a composite cantilever-beam model incorporating stiffness degradation was proposed to represent the dynamic response of post insulators, and the associated dynamic equilibrium equation was formulated. Incorporating plastic deformation at the porcelain-sleeve interface into seismic design calculations increases the required flexural strength of electrical equipment by approximately 10-12%, thereby markedly improving its seismic resilience.