## 5 Conclusions  
Three-dimension finite element models involving the interaction of soil-structure-equipment are established, and dynamic numerical simulation analysis of such models is performed. By changing some parameters such as ground motion input motions, properties of surrounding soils, burial depth, etc., the seismic performance of underground substations is proposed, and simulated results, such as acceleration amplification coefficient, displacement, stress and internal forces, are obtained from research. The findings are summarized as follows:  
1. The coupling boundary is selected as the boundary condition of soil-structure, which is feasible in a seismic response of an underground substation. \begin{table}
\begin{tabular}{c l c c c c c c} \hline  & & \begin{tabular}{c} **Top of** \\ **Floor 3** \\ \end{tabular} & \begin{tabular}{c} **Bottom of** \\ **Floor 3** \\ \end{tabular} & \begin{tabular}{c} **Top of** \\ **Floor 2** \\ \end{tabular} & \begin{tabular}{c} **Bottom of** \\ **Floor 2** \\ \end{tabular} & \
\begin{tabular}{c} **Top of** \\ **Floor 1** \\ \end{tabular} &
\begin{tabular}{c} **Bottom of** \\ **Floor 1** \\ \end{tabular} \\ \hline \multirow{3}{*}{M1} & Static ((\(10^{5}\) N/m) & 3.53 & 4.20 & 5.59 & 8.20 & 11.18 & 16.88 \\  & Dynamic (\(10^{5}\) N/m) & 4.57 & 5.46 & 7.29 & 10.58 & 14.41 & 21.56 \\  & Amplification (\%) & 29.51 & 30.02 & 30.43 & 28.94 & 28.85 & 27.71 \\ \hline \multirow{3}{*}{M2} & Static (\(10^{5}\) N/m) & 6.19 & 6.42 & 7.72 & 10.42 & 13.61 & 20.15 \\  & Dynamic (\(10^{5}\) N/m) & 7.74 & 8.12 & 9.77 & 13.18 & 17.16 & 25.41 \\  & Amplification (\%) & 24.88 & 26.46 & 26.56 & 26.48 & 26.11 & 26.10 \\ \hline \multirow{3}{*}{M3} & Static (\(10^{5}\) N/m) & 8.74 & 9.03 & 9.94 & 12.70 & 16.06 & 23.43 \\  & Dynamic (\(10^{5}\) N/m) & 11.06 & 11.26 & 12.61 & 16.17 & 20.40 & 29.78 \\  & Amplification (\%) & 26.62 & 24.68 & 26.86 & 27.36 & 26.98 & 27.09 \\ \hline \multirow{3}{*}{M4} & Static (\(10^{5}\) N/m) & 11.39 & 12.46 & 12.49 & 15.40 & 18.98 & 28.43 \\  & Dynamic (\(10^{5}\) N/m) & 14.24 & 15.00 & 15.74 & 19.68 & 24.29 & 35.85 \\  & Amplification (\%) & 25.06 & 20.35 & 25.96 & 27.77 & 27.99 & 26.11 \\ \hline \multirow{3}{*}{M5} & Static (\(10^{5}\) N/m) & 13.54 & 14.96 & 14.53 & 17.41 & 21.09 & 30.13 \\  & Dynamic (\(10^{5}\) N/m) & 17.22 & 18.50 & 18.56 & 22.37 & 26.91 & 38.52 \\ \cline{1-1}  & Amplification (\%) & 27.18 & 23.69 & 27.70 & 28.46 & 27.61 & 27.85 \\ \hline \end{tabular}
\end{table} 
Table 14: Shear forces of internal side wall 3 in different buried depths. 2. The seismic response of an underground substation is more sensitive to vertical earthquake motion than the seismic response of normal structures. Therefore, vertical earthquake motions should be taken into account in the seismic design of such underground structures. 3. Burial depth and elastic modulus are the main factors for the seismic performance of the underground electrical substation. With the increase in burial depth, layer drifts of an underground substation first increase and then decrease. With the increase in the elastic modulus of soil, the constraint of soil on structure increases and the deformation of structure decreases. 4. The acceleration amplification factors of electrical equipment are obviously greater than those of the same layer of the structure, and the peak drift of equipment is not synchronous with structure. Therefore, the interaction between structure and equipment should be taken into account in the seismic design of underground substations and some seismic measures should be proposed to control the dynamic response of electrical equipment. 5. The bottom of the side walls is the weak part of an underground substation. Therefore, it is necessary to increase the stiffness of side walls and strengthen the connection between the bottom floor and side walls. The authors wish to express their gratitude towards the Major Program of the National Natural Science Foundation (No. 51590914) (in China), the National Natural Science Foundation Project (No. 51578450, No. 51378225) (in China).