## 6 Heuristic Method for the Upgrade of the Lightning Performance of High-Voltage Substation  
The analysis presented in the previous sections reveals that the adjustment of the parameters that influence the expected failure rate of the substation, in a technically and financially affordable way, is vital for power utilities, in order to enhance the lightning performance of the substations and increase the reliability of the electrical network. To this direction, an appropriate method is developed to select the arresters' installation position, the cable length and the TFR that ensure that the substation outage rate will not exceed the defined acceptable restriction. Figure 4 illustrates the flow-chart diagram of the proposed methodology. The developed algorithm includes the following steps:  
1. data input (number of towers, span length, type of tower, geometrical characteristics of towers, _BIL_, conductors' cross section of the transmission line, TFR of each tower, cable length, conductors' cross section of the cable, electrical characteristics of power transformer, electrical characteristics of surge arresters, installation position of surge arresters, characteristics of lightning current). 2. calculation of \(R\) according to the procedures presented in Section 3. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & \multicolumn{8}{c}{**Cable Length (ET)**} \\ \hline  & \multicolumn{2}{c}{**0.2 km**} & \multicolumn{2}{c}{**1 km**} & \multicolumn{2}{c}{**2 km**} \\ \hline  & \multicolumn{8}{c}{**Distance between Position I and T**} \\ \hline
**TFR** & **No Arresters** & **0 m** & **20 m** & **No Arresters** & **0 m** & **20 m** & **No Arresters** & **0 m** & **20 m** \\ \hline
1 \(\Omega\) & 0.0524 & 0.00103 & 0.00368 & 0.03911 & 0.00076 & 0.00308 & 0.04014 & 0.00071 & 0.00264 \\
5 \(\Omega\) & 0.0635 & 0.00230 & 0.00743 & 0.05153 & 0  
* [Step 3]: if \(R<0.005\) then end, else go to the Step 4. * [Step 4]: set \(IT\) equal to zero and recalculate \(R\). * [Step 5]: if \(R<0.005\) then end, else go to the Step 6. * [Step 6]: Set the grounding resistance of the towers (except from the first tower near the substation) equal to 80% of its initial value and recalculate \(R\). * [Step 7]: if \(R<0.005\) then end, else go to the Step 8. * [Step 8]: Set the cable length equal to 200% of its initial value in the case that 300 m \(<ET\leq 400\) m or equal to 165% of its initial value in the case that 400 \(<ET\leq 600\) or equal to 135% of its initial value in the case that 600 m \(<ET\leq 1000\) m and recalculate \(R\) or equal to 117% of its initial value in the case that 1000 m \(<ET\leq 1500\) m and recalculate \(R\) or equal to 108% of its initial value in the case that 1500 m \(<ET\leq 2000\) m and recalculate \(R\). * [Step 9]: Calculation of \(R\), then end. Figure 4: Flow chart diagram of the developed heuristic method. The proposed first reformative adjustment is the installation of the arresters near the equipment to be protected (_IT_ = 0 m), considering that this modification is effective, technically easy and low-cost. If the compliance of the substation outage rate (_R_) with the defined limit is not achieved, a reduction of 20% of the TFR is imposed, considering technoeconomical criteria, since further improvement of the grounding resistance can be technically difficult and uneconomical. The increase of the length of the underground cable is the last corrective action to improve the lightning performance of the substation, since the extension of the installed underground cable and the abolishment of tower is a challenging work. The percentage increase for each cable length area is determined considering the topology of common substations, the cost of the cable installation (including the cost of the cable per km, the joints, the labor cost), and the need for reconfiguration of the system. Even if \(R\) is greater than 0.005 in Step 9, the procedure is completed, since further reduction of the grounding resistance or increase of the cable length is disadvantageous (in cost-benefit terms).