The support capacitor stack is used widely in AC and DC filters of UHV substation. The seismic responses of the support capacitor stack is remarkable due to its mass of more than dozens of tons. For studying its properties of seismic responses

the support capacitor stack at one UHV substation is taken as the study subject in this paper. It is found fisrtly that the torsion effect of the equipment under seismic action is negligible on the basis of finite element model in combination of modal analysis and dynamic time history analysis

and its first vibration mode response is predominant. On this basis

7 ground motions with different field characteristics are selected for horizontal bidirectional input.The bidirectional sesmic coupling effect of the equipment is calculated by the simplified response spectrum method based on the first vibration mode and the the time-history method respectively. It is found through comparing the result of the two methods that the bidirectional coupling seismic effect can be fully considered on the base of the unidirectional first vibration response

which is calculated by the response spectrum method

times an amplification factor. The amplification factors for the displacement responses and the stress responses of the insulators are recommended as 1.15 and 1.20

respectively.