## 1 Introduction  
In order to meet the requirements of the rapid growth of urban power load under the constraint of densely populated urban land, the number of underground electrical substations has been increasing in recent years. The disaster prevention and mitigation issue of underground structures, including with regard to underground electrical substations, is receiving increasing attention. Therefore, many scholars have focused on the interaction of structure-equipment and the seismic performance of underground structures. [PERSON] and [PERSON] (1989) [1] first developed the interaction principle of primary-secondary structure and obtained response spectrum curves based on the different floors by a simple model involving equipment-structure interaction. [PERSON] (1996) [2] established a simulation model of a San Francisco substation for seismic reliability analysis and compared the numerical substation damages with the real damages caused by Loma Prieta earthquake in 1989. [PERSON] (2009) [3] illustrated concepts interpreting and supplementing the seismic provisions of Manual 113 ([PERSON], 2008 [4]) which defined seismic design spectra as simultaneous occurrence of ground motion in three mutually perpendicular directions and estimation of deflections under seismic loading. Poor seismic performance of these system components could result in extended power outages that might propagate far beyond the local epicentral region. [PERSON] and [PERSON]. (2009) [5]proposed a Seismic Vulnerability Reduction Program (SVRP) design method to improve the seismic weak parts of substation and present several suggestions on the seismic design of a transformer substation. The international atomic energy agency (IAEA, [6]) provided seismic evaluation criteria on the interaction of structure-equipment in a nuclear power plant. Furthermore, from the study of the interaction between the main plant and electrical equipment (EE) in transformer substations, it was found that the seismic response of the main structure and equipment was not usually synchronized and equipment had a negative effect on the seismic ability of the substation ([PERSON] and [PERSON], 2013 [7]; [PERSON] et al., 2015 [8]). To date, many researchers have focused on the interaction of soil-structure. Assuming that interaction force is transformed by a spring and damper, [PERSON] (1970, [9]) introduced the soil-structure interaction principle. [PERSON] (1985, [10]) analyzed the static interaction between soil and the tunnel lining and found that the force between soil and structure had a significant relationship with contact surface roughness. [PERSON] (2000, [11]) developed an analytic method involving shear interaction between soil and structure. Based on [PERSON]'s studies, [PERSON] (2000, [12]) established a hysteretic non-linear soil model to investigate the load transfer mechanisms between underground structure and surrounding soil. Therefore, dynamic finite element analyses were conducted to investigate the load transfer mechanisms between underground structure and surrounding soil. [PERSON] (2002, [13]) studied the responses of an underground structure under vertical seismic excitation through the shaking table test, analyzing the dynamic interaction of soil and underground structure. Based on the background of metro construction in Nanjing city, China, a shaking table test was conducted to model the interaction of soil-metro structure with a liquefiable soil effect. [PERSON] and [PERSON] (2006, 2007, [14; 15]) studied the dynamic damage characteristics of underground structures in far-field earthquake motions and near-field earthquake motions. According to the static and dynamic coupling effects on the underground structure and surrounding rock system, [PERSON] (2009, [16]) proposed stress viscoelastic artificial boundary conditions to establish an underground structure-soil computational model.