## 2 Materials and Methods
### Simulated Conditions  
The technical specifications of the electrical transformer enclosure are outlined in Table 6. The proposed ventilation system has a total electrical power of 3250 W and can achieve flow rates of up to 2200 m\({}^{3}\)/h. For the simulations, two flow rates were selected: a minimum of 1100 m\({}^{3}\)/h and a maximum of 2200 m\({}^{3}\)/h. Additionally, the ventilation system must ensure that the indoor temperature remains below 40 \({}^{\circ}\)C, and that the air velocity does not exceed 6 m/s in the main ducts and 4.5 m/s in the secondary ducts. Two possible scenarios can be considered for the transformer enclosure installation: a confined layout, with the transformer enclosure insulated from the indoor environment using panes, and an open layout, with the transformer enclosure simply placed at the center of the building. The layouts will be described in detail in Paragraph 2.6.1. A total of 6 simulations were performed, as can be seen in Table 7.  
Simulations C-2 and C-3 will not be discussed in this paper but are reported in the ESM (Figures S20-S27). The decision to include these simulations in the ESM is due to their intermediate behavior compared to C-1 and C-4, which is not worth a dedicated discussion considering the inadequate results obtained at the maximum flow rate. The other imposed boundary conditions are reported in Table 8 and are the same for each simulation (open and close layout and mass flow rate). The conditions were chosen based on the worst-case scenario measured during the experimental campaign. In the numerical model, the cabin was modelled by using the heat source that emits the real load in the cabin (3250 W). \begin{table}
\begin{tabular}{c c c c c} \hline
**Type of Power Supply** & **Power** & **Frequency** & **Three Phase Voltage** & **Distribution System** & **Neutral State** \\ \hline MT & 500 kW & 50 Hz & 20 kV/400 V \(\pm\) 5\% & TN-S & Distributed \\ \hline \end{tabular}
\end{table} 
Table 6: Electrical transformer enclosure characteristics. \begin{table}
\begin{tabular}{c c c} \hline
**Simulation** & **Electronic Supplementary Material (ESM)** & **Layout** & **Mass Flow Rate** \\ \hline C-1 & Only velocity results & 1100 m\({}^{3}\)/h \\ C-2 & All data & 1650 m\({}^{3}\)/h \\ C-3 & All data & Confined transformer enclosure & 1925 m\({}^{3}\)/h \\ C-4 & Only velocity results & 2200 m\({}^{3}\)/h \\ O-1 & Only velocity results & 1100 m\({}^{3}\)/h \\ O-2 & Only velocity results & 2200 m\({}^{3}\)/h \\ \hline \end{tabular}
\end{table} 
Table 7: Simulation layouts and mass flow rate. Figure 9: Temperature (**left**) and relative humidity (**right**) results on the longitudinal section. Simulated Conditions  
Figure 10a illustrates the layout of the open transformer enclosure without the air extraction system. Figure 10b shows the air extraction system applied to this layout. The system includes a duct network with an air extraction fan located on the ceiling. Fresh air is drawn in from outside through a grated opening in the entrance door. Air extraction occurs by pushing the air through an opening in the grate positioned above the entrance door (see Figure 10b). The confined layout utilizes a specialized enclosure (Figure 11a) constructed from rigid foam insulation panels that are 8.0 cm thick, with a thermal conductivity of 0.026 W/m\(\cdot\)K [33; 34; 35]. This envelope is designed to shield the building from thermal stress resulting from the energy dissipated by the electrical equipment in the transformer enclosure. Additionally, the design of the panels facilitates the direct inspection of the transformer enclosure, as they can be moved along rails for easy access to the interior. This feature is crucial for ensuring maintenance access while keeping the two environments as separate as possible. In this layout, the air ducts connect to the internal environment of the transformer enclosure through a dedicated pipeline (Figure 11b).