## 4 Results of the Analysis
### The Results of the Critical Equipment Analysis  
By using sensitivity analysis, critical equipment can be identified. By changing the fragility parameters (median capacity and increased by 20%) of related equipment one by one, a set of system fragility curves is obtained. The changes in the system's median capacity and log standard deviation are shown in Figures 19-22. The impact of the 110 kV equipment and IS on the parameters related to the substation fragility curves is insignificant (less than 1.5% overall) because these pieces of equipment have a relatively high median capacity.  
In addition to the 110 kV LA and IS, the 110 kV CB, DS, CT, and PT show an overall increase in the reliability of substations when \(F_{\rm S}=9/10\). This is because the transformer and line-in unit are redundant with little \(PGA\), and the 110 kV connection unit and the line-out unit include a CB, a PT, a CT, and three DSs, so the variation in the 110 kV DS is the largest. Improving the seismic performance of the 110 kV DS can significantly improve the system reliability for \(F_{\rm S}=9/10\). At this time, the capacity of the substation is determined by the 110 kV connection unit and the line-out unit, which is consistent with the results in the previous section.  
Figure 19: Changes in the median capacity of the 110 kV equipment and IS.  
Figure 20: Changes in the logarithmic standard deviation of the 110 kV equipment and IS.  
Among the 220 kV equipment, the CT had the greatest impact on the median capacity of the substation system's fragility curve, which varied between 4% and 7% with a decrease in \(F_{\rm s}\). The influence of the CB is slightly lower, varying between 3% and 6% with the decrease in \(F_{\rm s}\). The impacts of the remaining equipment in descending order of size are the impacts of the PT, DS, TR, and LA. When \(F_{\rm s}\) = 9, the variation in the 220 kV equipment is significantly reduced, which verifies the analysis in the above figure. The CB, CT, DS, and PT also greatly influence the log standard deviation of the system in different ways; that is, the CB and CT reduce the log standard deviation and are positively correlated with \(F_{\rm s}\), and the DS and PT increase the log standard deviation and have a negative correlation with \(F_{\rm s}\). The LA, PT, and TR have a very limited impact on the median capacity (<1.5%) of the substation system due to their higher median capacity and lower participation. This also indicates that the 220 kV LA and PT and the individual TR equipment have high seismic redundancy. The analysis results in Section 4.1 show that the transformer unit has a higher fragility; however, the transformer is not the most important piece of equipment.  
This is because there are only three transformers; the number of other pieces of equipment far exceeds this number, and the transformers are power-redundant as two transformers are sufficient to ensure normal operation (in an earthquake). Therefore, the 220 kV CT, CB, and DS are the most critical facilities in the line-in unit, 220 kV connection unit, and transformer unit. Improving their seismic performance will significantly improve the reliability of substations.  
Figure 21: Changes in the median capacity of the 220 kV equipment.  
Figure 22: Changes in the logarithmic standard deviation of the 220 kV equipment.