Seismic response of electrical equipment subjected to high–frequency ground motions

Abstract

Recent ground motion studies conducted at nuclear facilities indicate that in Central and Eastern United States, the ground motion response spectra exceed the safe–shutdown earthquake spectra at high frequencies. Even though high–frequency ground motions do not cause structural damage, high–frequency accelerations may propagate through the structure and interfere with the output of the safety–related equipment such as relays required to ensure safe shutdown of the plant during a seismic event. Hence, it is essential to seismically qualify electrical equipment subjected to high–frequency accelerations. The seismic qualification is conducted by comparing in–cabinet response spectra evaluated at the equipment locations with the capacities of equipment obtained from shake table tests. The in–cabinet response spectrum is obtained from analysis of nuclear power plant building and electrical cabinet in which the equipment are mounted. This study is based on the hypothesis that the high–frequency motions do not reach the equipment as the small displacements induced by such motions are filtered out by the geometric nonlinearities. The effect of two different types of nonlinearities are studied: (1) a gap in the connection between electrical cabinet and floor; (2) sliding friction between electrical cabinet’s base and the floor. The results from an analyses of various different cases show that the high–frequency motions do not reach the relays if the maximum displacement of building floor is less than the gap. Even if the displacement is larger than the gap, the in–cabinet spectral accelerations are not excessively high. On the contrary, results from a conventional linear analysis give excessively high unrealistic spectral accelerations.