## 3 The Seismic Performance of an \(\pm\)800 kV Suspended Filter Capacitor
### The Influence of Pre-Tension Force on Seismic Response  
To avoid the negative axial force of suspended insulators, the influence of pre-tension force on the seismic response of the filter capacitor was investigated in this section. Because the artificial wave was generated according to the site condition that can better reflect the real situation and induce the real responses, this analysis only focuses on the structural responses under the artificial wave. The studied suspended filter capacitor in this paper adopts the pre-tension force of 20 kN. Thus, a tension amplification coefficient (TAC) is defined based on the maximum axial force of the suspended insulators under the pre-tension force of 20 kN, as shown in Equation (1):  
\[\text{TAC}(F)=\frac{\text{max}N(F)-\text{max}N(20)}{\text{max}N(20)} \tag{1}\]  
where max\(N(F)\) represents the maximum axial force of the suspended insulators under an earthquake when the pre-tension force is \(F\). TAC(\(F\)) illustrates the increase percent of the maximum axial force of the suspended insulators with the increase of the pre-tension force. When TAC(\(F\)) is less than 0, it indicates that the maximum axial force of the suspended insulators is less than that under the pre-tension force of 20 kN. Then, this study investigates the influence of PGA on the axial force and TAC. Under PGAs of 0.2 g, 0.3 g, and 0.4 g, the maximum axial force of the suspended insulators and the TAC are shown in Figure 8.  
It can be found from Figure 8a that the maximum axial force of the suspended insulators increases dramatically as the PGA increases. Furthermore, when the pre-tension force increases, the maximum axial force shows a growth trend overall. The filter capacitor is a suspension structure that exhibits an obvious geometry nonlinearity. One of the main features is the structural stiffness that will change with the deformation of the filter capacitor. Therefore, there are two main factors that influence the horizontal stiffness of the suspended filter capacitor. The first one is that the pre-tension force gives an initial structural stiffness, and the second one is that the axial force in the suspended insulators caused by earthquakes would bring in some stiffness contributions. When the PGA is relatively small, such as 0.2 g, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ground Motion** & **The Bottom Insulators (kN)** & **The First-Layer Insulators (kN)** \\ \hline El Centro & 6.51 & 10.43 \\ Taft & \(-\)10.95 & \(-\)3.65 \\ AW & \(-\)5.07 & 0.76 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristics of the selected earthquake. the influence of earthquakes on the structural stiffness is relatively small so the influence of the pre-tension force dominates. That is why the axial force increases continually as the pre-tension increases, and the TAC also significantly increases in Figure 8b. However, when the PGA is relatively large, such as 0.4 g, the impact of the pre-tension force on the stiffness increase is less than the strong seismic action. Therefore, Figure 8b shows a slight change of TAC with the increase in the pre-tension force under the 0.4 g artificial wave, and the TAC is in the range of 0-0.1 when the pre-tension force is enhanced. The influence of the pre-tension force on the minimum axial force and the maximum displacement is shown in Figure 9, in which the negative axial force represents the pressure. From Figure 9a, we can understand that the minimum axial force gradually reduces as the PGA increases, and the minimum axial force increases with the increase in the pre-tension force. When the pre-tension force is larger than 40 kN, the insulators will not produce pressure, which meets the requirements of IEEE 693 std [50]. Therefore, the pre-tension force is recommended to be adjusted so that the pressure generating can be avoided in the suspended insulators. Figure 9b has plotted curves of the maximum displacement of the filter capacitor versus the pre-tension force under various PGAs.