# Preliminary Design of the Electrical Power Systems for DTT Nuclear Fusion Plant
## 3 Distribution System of DTT Facility  
### Electrical Distribution Network  
Because of the different nature of the steady-state and pulsed-power loads, the design choice was to split the DTT distribution network in two sections:  
* Steady-State Electrical Distribution (SSED) designated to supply only the steady-state loads.
* Pulsed-Power Electrical Distribution (PPED) designated to supply only the pulsed loads.  
This means that the SSED and the PPED sections are electrically coupled only at the HV level, while their downstream distributions are kept separated.  
As a general design choice, the MV level for the internal electrical distribution is fixed at 20 kV. To be precise, the power flow analyses identified 21 kV as the optimal value for the no-load secondary voltage at the central tap of the HV/MV transformers. However, such voltage will be simply denoted as 20 kV in the following.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**PBS** & **Name** & **Description** \\ \hline
1 & Physics & This PBS concerns the study of plasma physics, so no loads are foreseen \\
2 & Magnets & No (or negligible) loads are expected in this PBS \\
3 & Mechanical Structures & No (or negligible) loads are expected in this PBS \\ \hline
4 & Heating and Current Drive (H\&CD) & This PBS concerns the additional heating system and consists of three subsections, corresponding to the three heating systems (ECRH, ICRH, NBI) [10; 17; 18; 19] \\ \hline
5 & Auxiliary Plant Systems & This PBS includes the main components of the auxiliary systems of DTT \\  & Auxiliary Plant Systems & (Cryogenic System, Water Cooling System, Vacuum Pumping System, Fueling \\  & Control Systems, Baking and Cleaning Systems) \\ \hline
6 & Coil Power Supply Systems & This PBS collects all the components belonging to the subsystems (power \\  & & converters) to feeding the coils [10; 14; 21; 22] \\ \hline
7 & Building Layout and Services & This PBS includes the service loads such as light panel, FM panel and Heating, \\  & & Ventilation and Air Conditioning (HVAC) system \\ \hline
8 & Diagnostics and Control System & This PBS contains information about diagnostics, instrumentation and control \\  & & system, telecommunication and security system \\ \hline
9 & System Level Engineering & No (or negligible) loads are expected in this PBS \\ \hline
10 & Remote Handling & This PBS is about remote handling systems (to handle components inside the \\  & & vessel or in harsh environments) \\ \hline \hline \end{tabular}
\end{table}
Table 1: PBS organization and description.  
Based on this design approach, as shown in Figure 2, the HV/MV substation SS0 includes six 150/20 kV transformers in order to balance the load requirements and to guarantee an adequate level of reliability and redundancy in the event of a failure of one of the transformers.  
In particular, there will be:  
* Four three-phase two-winding power transformers with a rated power of 63 MVA (and an overload capacity up to 100 MVA for 100 s) to supply PPED loads.
* Two three-phase two-winding power transformers with a rated power of 63 MVA to supply SSED loads, in a double redundant configuration.  
As shown in Figure 2, the two SSED transformers are equipped with on-load tap changers (OLTCs). Such a solution was not adopted for the PPED section because, since the OLTCs cannot follow the fast load variations of the PPED loads, the limited advantage does not compensate the risk of instabilities and the possible reduction in the transformers' reliability.  
The distribution system, downstream from the SS0, is arranged in different voltage levels that are: 20 kV, 6 kV and 400 V (230 V single phase).