## 4 Model Description
### Investment Strategies and Associated Costs  
To improve the reliability of the analyzed semi-urban and rural distribution networks, specific investment strategies were implemented to reduce _SAIDI_, _ENS_, and _COST_ indicators. These strategies include the construction of additional power lines, the replacement of old components, and network upgrades through the installation of automated switching devices. The investment options and their estimated costs, based on average market prices in Serbia, are provided in Table 7.  
Investment strategies for the semi-urban network:  
1. Construction of new power lines to form semi-rings at critical points in the network, specifically between nodes 12-32 (L1), 8-36 (L2), and 4-40 (L3), ensuring alternative supply paths. It was assumed that new lines are ideally reliable after installation.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{3}{c}{**Semi-Urban Network**} \\ \hline Feeder 2 & CB2, TS5, TS6, TS7, TS8 \\ \hline Feeder 5 & CB5, TS17, TS18, TS19, TS20 \\ \hline Feeder 7 & CB7, TS27 \\ \hline Feeder 9 & CB9, TS33, TS35 \\ \hline \multicolumn{3}{c}{**Rural network**} \\ \hline Feeder 1 & CB1, CB6, D4, F9, F10, F11 \\ \hline Feeder 2 & CB25, CB45, F33 \\ \hline \hline \end{tabular} Legend: CB—circuit breaker, D—disconnector, F—fuse, TS—equipment in 10 kV/0.4 kV TS (disconnectors in line bays, and disconnector and fuse in the transformer bay).  
\end{table}
Table 6: Components that exhibit higher failure rates in semi-urban and rural networks.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Component**} & \multicolumn{1}{c}{**Failure Rate**} & \multicolumn{1}{c}{**Failure Rate of Old**} & \multicolumn{1}{c}{**Repair Time**} \\  & **[faults/yr]** & **Component [faults/yr]** & **[h]** \\ \cline{2-4}  & **[Min, Mean, Max]** & **[Min, Max]** & **[Min, Mean, Max]** \\ \hline Circuit breaker & [0.0272, 0.034, 0.0408] & [0.068, 0.17] & [9.6, 12, 14.4] \\ \hline Recloser & [0.0272, 0.034, 0.0408] & [0.068, 0.17] & [9.6, 12, 14.4] \\ \hline Disconnector & [0.00224, 0.0028, 0.00336] & [0.0056, 0.014] & [8.4, 10.5, 12.6] \\ \hline Fuse & [0.0032, 0.004, 0.0048] & [0.008, 0.02] & [1.6, 2, 2.4] \\ \hline Cable line (per km length) * & [0.032, 0.04, 0.048] & - & [16; 20; 24] \\ \hline Overhead line (per km length) * & [0.0728, 0.091, 0.1092] & - & [9.6, 12, 14.4] \\ \hline \end{tabular} Legend: * Components for which aging was not assessed (Table 6).  
\end{table}
Table 5: Values of reliability indicators of components.  
2. Replacement of old components listed in Table 6.
3. Installation of remotely operated disconnectors (RODs) with fault indicators in all TSs on Feeder 3 and Feeder 7 to enable faster fault isolation. Investment strategies for the rural network:
1. Formation of a semi-ring by constructing a new power line between the endpoints of lines 24 and 32 (L50) and installing a recloser at the beginning of line L29.
2. Replacement of old components listed in Table 6.
3. Replacement of overhead lines with underground cable lines for L1, L2, L3, L25, L26, and L27 to improve reliability.
4. Installation of a recloser at the beginning of line L12 to enhance switching operations.  
Based on the assumption that in many energy markets, particularly in developing countries, there is no explicit regulation defining the cost of unsupplied electrical energy, leaving it to be determined through agreements between customers and suppliers, fixed unit monetary values of unserved energy are adopted: 30 EUR/kWh for industrial consumers, 15 EUR/kWh for commercial consumers, and 2 EUR/kWh for residential consumers. For practical implementation, the determination of fixed values is left to the respective DSO, which can define them based on the specific network segment being assessed for reliability and the values agreed upon with consumers in that area.