# Preliminary Design of the Electrical Power Systems for DTT Nuclear Fusion Plant
## 4 Operating Scenarios: Grid Calculations and Studies
### Fault Analysis for DTT Power System Design  
The short-circuit calculations were performed, in accordance with the standard IEC 60909-0, for all types of faults (three-phase, line-to-line, line-to-line with earth connection and line-to-earth) and for each node of the grid. The analysis was carried out for the most severe scenario: Distribution network operating in single radial configuration without power factor correction. For each simulation, the goal is to identify/verify the rated short-circuit withstand values and the breaking capacity of switchboards, transformers, cables and circuit breakers. The most important outcomes of the analysis are:  
* \(I_{\text{kss}}\) initial symmetrical short-circuit current. * \(i_{\text{p}}\) peak short-circuit current. As a further design criterion related to the short-circuit withstand, the equipment is selected assuming a 15% margin for both the peak and the symmetrical current and for all the types of fault. At this stage, because of the level of the data reliability and the design choices, which are not yet final, it was not possible to proceed with the preliminary sizing and choice of the protection system. However, the short-circuit currents were evaluated to verify the correct design of the switchboards. Tables 13-16 report the withstand values, grouped by categories, that shall be compared with the outcomes of the simulations to verify the correctness of the selected fault withstand ratings: rms value \(I_{\text{k}}\) of the permanent short-circuit current for the duration \(t_{\text{k}}\) of the event, its maximum allowable value \(I_{\text{kmax}}\) and the peak short-circuit current. In \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Section [mm\({}^{2}\)]** & **Cores and Laying Method** & **Rated Current [A]** & \begin{tabular}{c} \(R^{\prime}\) (20 \({}^{\circ}\)C) \\ [\(\Omega\)/km] \\ \end{tabular} & \
\begin{tabular}{c} **Max \(R^{\prime}\) (90 \({}^{\circ}\)C)** \\ [\(\Omega\)/km] \\ \end{tabular} &
\begin{tabular}{c} \(X^{\prime}\) [\(\Omega\)/km] \\ \end{tabular} \\ \hline
120 & Single core—direct in the ground & 156 & 0.155 & 0.198 & 0.12 \\
150 & Single core—direct in the ground & 175 & 0.126 & 0.161 & 0.11 \\
185 & Single core—direct in the ground & 197 & 0.102 & 0.13 & 0.11 \\
300 & Single core—in air & 602 & 0.0633 & 0.081 & 0.11 \\ \hline
400 & Single core—direct in the ground & 313 & 0.0508 & 0.065 & 0.099 \\ Single core—in air & 692 & 0.0508 & 0.065 & 0.099 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Electrical data and technical characteristics assumed for 20-kV cables. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Transformation Ratio 0.4 \(\pm\) 5\%/0.42–0.24 kV** \\ \hline
**\(S_{\text{n}}\) [kVA]** & \(P_{\text{0}}\) [kW] & \(P_{\text{k}}\) [kW] & \(V_{\text{k}}\) [\%] \\ \hline
500 & 0.9 & 3.9 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Electrical data of the LV/LV transformers. particular, a maximum temperature of 250 \({}^{\circ}\)C was considered for the calculation of \(I_{\text{kmax}}\) for the cables (insulated in EPR). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Busbar/Swichboard** & **Rated Voltage [kV]** & \(I_{\text{kmax}}\) [kA rms] & \(i_{\text{p}}\) [kA] & \(t_{\text{k}}\) [s] \\ \hline HV Busbars—SS0 & 150 & 31.5 & 80 & 1 \\ MV Busbars—S0 & 20 & 31.5 & 80 & 1 \\ MV Busbars—LCs & 20 & 31.5 & 78.75 & 3 \\ LV Switchboards & 0.4 & 70 & 164.5 & 1 \\ LV Auxiliary Switchboards & 0.4–0.23 & 20 & 41 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Sizing values of busbars and switchboards. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Section [mm2]** & **Rated Current [A]** & \(I_{\text{k}}\) [kA rms] & \(t_{\text{k}}\) [s] & \(I_{\text{kmax}}\) [kA rms] \\ \hline
150 & 325 & 19.39 & 1 & 21.45 \\
300 & 602 & 19.39 & 1 & 21.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Electrical data assumed for 6 kV cables.