# Optimization of DC Energy Storage in Tokamak Poloidal Coils  
[PERSON]  
1 Energy and Sustainable Economic Development (ENEA), Italian National Agency for New Technologies, 00044 Frascati, Italy; [EMAIL_ADDRESS]
2 DIT S. c. a. r. l, 00044 Frascati, Italy 2  
[PERSON]  
3,4 Ocean Power Electronics, 40056 Valsamogia, Italy; [EMAIL_ADDRESS] or [EMAIL_ADDRESS] (R.T.); [EMAIL_ADDRESS] (B.G. ); [EMAIL_ADDRESS] (S.I.) [PERSON]  
3,4 Ocean Perimeter Corporation  
PF coils can demand powers on the order of 1 GVA from the grid [9]. Even though the total energy produced at the tokamak output could compensate for these demands [10; 11], the technical and economic effectiveness of the process is difficult to sustain. Moreover, the huge and variable power flow at the interconnection nodes may be a problem rather than a resource for the external electrical grid [7; 12]. The limited variations and voltages required in the TF coils allow some options for PS optimizations. For example, traditional PSs could be substituted by contactless systems, known as flux pumps, able to minimize the losses in superconducting circuits in stationary conditions [13]. For the PF PSs, an identified improvement consisted of introducing a buffer of energy to limit the power exchange between the coils and the external grid. In principle, different topologies and technologies are possible. An energy storage was also proposed for the TF circuit at the AC side [14]. However, the most promising solution appears to introduce the energy storage in the DC link of the PF PSs [15] and sometimes of other coils. The technical reasons for that will be clear throughout the paper. It is evident that these kinds of problems are generally encountered in other energy sources and power conversion applications, where energy storage plays a pivotal role in ensuring stability and performance, becoming more and more essential. Nevertheless, the PF PSs are characterized by peculiar features: while the average energy and the duty cycles can be relatively small (especially in the present experimental plants), the power peaks are extremely high; many fast variations and charge/discharge operations are requested every day. These features suggest that supercapacitors (SCs), with their high power density and rapid charge-discharge capabilities, could be the suitable energy storage device [16; 17; 18; 19; 20; 21; 22]. Therefore, SC banks were inserted in support of the tokamak PSs, in particular in DC links between AC/DC and DC/DC converter stages, as is explained in the paper. After identifying this reference solution, the design challenge consists of minimizing the physical footprint of these DC storage systems while maintaining their functional integrity and reliability. In fact, as the SC banks may be the most expensive and largest part of a new PS system, an excessive size would cancel its potential advantages. It is important to stress that a good optimization requires using the DC link differently from the standard mode in electronics, where the DC link is designed to operate at a rather constant voltage. In practice, a standard DC link is not a storage device because only a fraction of its energy, which is already small for a traditional capacitor, can be used. On the contrary, the voltage across the SC banks must change between the minimal values to implement the desired scenarios and the safe values for the hardware. Since the interactions among circuit parameters and the coupling with the other tokamak elements are complex and time-variable, it is rather impossible, or at least impractical, to approach the design and optimization from an analytical point of view. Nevertheless, some models and formulas were developed to provide a starting range for the procedure and to include all the tokamak elements in a circuital model. This model can be used to validate the design. As the primary focus of the study was the DC link optimization, the PS circuits are based on a single H-bridge converter [15].