## 5 The Impact of Various Factors on the Lightning Performance of High-Voltage Substations  
In this section, the impact of various factors on the expected failure rate of the substation depicted in Figure 1 is examined. A lightning flash strikes either a phase (shielding failure) or an overhead ground wire (backflashover) of the incoming transmission line, taking into consideration that the substation is adequately protected against direct lightning strokes. Note, that lightning strikes the second-to-last metallic tower near the substation. The performed sensitivity analysis considers the installation position of the arresters, the length of the underground cable and the TFR. Table 2 depicts the substation outage rate when a lightning hits a phase conductor of the connected overhead transmission line. It must be noted that the terminal tower is bonded to the grounding system of the substation and that the TFR does not influence the outage rate for a lightning stroke to a phase conductor. The results reveal the upgrade of the lightning performance of the examined substation that metal oxide surge arresters provide and emphasizes the impact of the installation position of the arresters and the length of the cable. Table 3 presents the results obtained considering a lightning stroke to the ground wire of the connected overhead transmission line. The computed outage rate indicate that low TFR values reduce the probability of a backflashover. Moreover, long cables provide high protection against the incoming surges and contribute to the decrease of the expected failure rate. In addition, the installation of surge arresters near the equipment is required for the upgrade of the lightning performance of the substation. Note that for long cables and arresters installed at the high voltage wirings of the power transformer, \(R_{ph}\) is almost eliminated. Table 4 presents the total substation outage rate. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Cable Length (ET)**} & \multirow{2}{*}{**No Arresters**} & \multicolumn{2}{c}{**Distance between Position I and T**} \\ \cline{3-4}  & & **0 m** & **20 m** \\ \hline
0.2 km & 0.04718 & 0.00042 & 0.00154 \\
1 km & 0.03504 & 0.00035 & 0.00107 \\
2 km & 0.03126 & 0.00028 & 0.00102 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: R\({}_{\text{ph}}\) in function with the cable length and the installation position of the arresters. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Cable Length (ET)**} & \multirow{2}{*}{**No Arresters**} & \multicolumn{4}{c}{**Distance between Position I and T**} \\ \cline{3-10}  & & & **0 m** & **20 m** & **No Arresters** & **0 m** & **20 m** & **No Arresters** & **0 m** & **20 m** \\ \hline
1 \(\Omega\) & 0.0054 & 0.00061 & 0.00214 & 0.00411 & 0.00041 & 0.00154 & 0.00915 & 0.00044 & 0.00161 \\
5 \(\Omega\) & 0.0165 & 0.00188 & 0.00589 & 0.01653 & 0.00124 & 0.00892 & 0.01647 & 0.00131 & 0 The extracted outcomes prove the upgrade of the lightning performance of the system, after the installation of surge arresters, revealing, simultaneously, the effect of the TFR (that determines the occurrence or not of backflashover phenomena), the installation position of the arresters and the length of the cable. The installation of the arresters close to the power transformer and the use of long HV XLPE cables reduce the failure rate, even for high values of the TFR values. Table 5 depicts the substation outage rate (\(R\)) in the case that the distance between the positions \(I\) and \(T\) is zero (\(IT\) = 0 m), assuming that in the majority of the substations (especially for new ones) the arresters are directly connected with the power transformer. Nevertheless, in older substations, arresters implemented away from the equipment to be protected can be met.