# Optimization of DC Energy Storage in Tokamak Poloidal Coils
## 8 Simulation Campaign  
Even though some preliminary indications on the best compromises were obtained by Formulas (16) and (17), the useful optimizations must be verified by simulations.  
For the simulations, \(V_{\rm max}=1.8\) kV or \(3.5\) kV was selected, to allow the use of commercial IGBTs with performances at the switching frequency suitable for the application. For instance, 3.5 kV can be sustained by a single 6-kV IGBT for railway applications, by a series of three IGBTs having the very common rating of 1.7 kV, and so on.  
The PF PSs, each with an installed power up to 90 MW, can operate with an input power lower than 3 MW. In particular, the optimizations were analyzed under three different limits of \(P_{\rm gridmax}\) (the input power factors are negligible in this analysis):  
1. Maximum 1 MW.
2. Maximum 2 MW.
3. Maximum 3 MW.  
Under each of these three different power limits, the study compared two opposite conceptual topologies:  
1. Independent DC links.
2. Shared DC links.  
In the first case, each PS operated with its own dedicated DC link (DC storage bank). This configuration served as a baseline for comparison. The second case, schematized in Figure 13, exploits the benefits of a DC link shared among PSs feeding different PF coils.  
Figure 11: Logic of the controlled current source implementing the charger in circuital simulations.  
Figure 12: Block scheme of the H-bridge control.  
benefits in terms of energy and dimensions, so it is better to exploit the economies of scale of designing them like the others; this is also a safety margin for operations.  
Table 3 provides significant insights into the advantages and limitations of sharing DC links. One of the most notable findings was the substantial benefits observed for the coil pairs PF1-PF2 and PF5-PF6:  
* For PF1-PF2, the shared storage configuration could potentially halve the size required for the storage bank.
* For PF5-PF6, the reduction in storage requirements was even more pronounced, with a potential decrease by about two-thirds.
* On the other hand, the pair PF3-PF4 showed no significant advantage from transitioning to a shared DC storage configuration. In this case, there is no reason to move from the simpler layout.  
Table 4 expresses these results in terms of efficiencies, using the definitions (18) and (19). As anticipated, in some cases the efficiencies can exceed 100%.  
Figure 14: Comparison between the case in which the PF1 and PF2 PSs share the DC storage and the case in which the same DC storage only feeds the PF2 PS.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{\(P_{\text{gridmax}}\) = 1 MW for Each Coil} & \multicolumn{2}{c}{\(P_{\text{gridmax}}\) = 2 MW for Each Coil} & \multicolumn{2}{c}{\(P_{\text{gridmax}}\) = 3 MW for Each Coil} \\ \cline{2-7}  & **Independent** & **Shared** & **Independent** & **Shared** & **Independent** & **Shared** \\  & **DC Links** & **DC Links** & **DC Links** & **DC Links** & **DC Links** & **DC Links** & **DC Links** \\ \hline  & 1.8 kV & & 1.8 kV & & 1.8 kV & \\
**PF1** & 68 F & 3.5 kV & & 58 F & 44 F & \\  & 110 MJ & 30 F & 92 MJ & 17 F & 70 MJ & 15 F \\ \hline  & 3.5 kV & 184 MJ & 3.5 kV & 104 MJ & 3.5 kV & 92 MJ \\
**PF2** & 35 F & & 34 F & & 30 F & \\  & 215 MJ & & 209 MJ & & 184 MJ & \\ \hline  & 3.5 kV & & 3.5 kV & & 3.5 kV & \\
**PF3** & 28 F & 3.5 kV & & 25 F & 18 F & \\  & 172 MJ & 3.5 kV & 154 MJ & 3.5 kV & 110 MJ & 3.5 kV \\ \hline  & 3.5 kV & & 368 MJ & 3.5 kV & 337 MJ & 3.5 kV & 245 MJ \\
**PF4** & 32 F & & 28 F & & 22 F & \\  & 196 MJ & & 172 MJ & & 135 MJ & \\ \hline  & 3.5 kV & & 3.5 kV & & 3.5 kV & \\
**PF5** & 55 F & 337 MJ & 3.5 kV & 337 MJ & 3.5 kV & \\  & 337 MJ & 38 F & 38 F & 30 F & 154 MJ & 3.5 kV \\ \hline  & 1.8 kV & 233 MJ & 1.8 kV & 184 MJ & 1.8 kV & 154 MJ \\
**PF6** & 155 F & & 120 F & & 105 F & \\  & 233 MJ & & 260 MJ & & 176 MJ & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of the differences between the voltage, capacitance, and energy of SC banks in the case of independent and shared DC links.