## 5 Future Trends and Opportunities--Modular Multicell Rectifier  
Among the aforementioned P2H converter topologies, the three-winding LFT is mandatory in the system. It is usually the bulkiest component in such a P2H converter \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Topologies**} & **Power** & \multirow{2}{*}{**Efficiency**} & \multirow{2}{*}{**Cost**} & \multirow{2}{*}{**Reliability**} & **Control** \\  & **Quality** & & & & **Complexity** \\ \hline
12-TR & \(-\) & \(+\) & \(+\) & \(++\) & \(+\) \\ \hline
12-DRMC & \(\Delta\) & \(\Delta\) & \(+\) & \(+\) & \(\Delta\) \\ \hline
12-TRASPF & \(+\) & \(\Delta\) & \(\Delta\) & \(\Delta\) & \(-\) \\ \hline AFE & \(++\) & \(\Delta\) & \(-\) & \(\Delta\) & \(-\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison among topology candidates. \(++\)—superior; \(+\)—satisfactory; \(\Delta\)—neutral; —unsatisfactory. Figure 9: Performance comparison of THDi over the possible operating conditions: (**a**) at the beginning of the electrolyzer’s lifetime; (**b**) at the end of the electrolyzer’s lifetime. system and brings challenges in transportation, installation, and footprint occupation. Therefore, eliminating the bulky LFT from such a P2H converter system is one of the primary emerging research topics. One promising topology solution is the modular multicell rectifier system, as shown in Figure 10. Modular configurations are employed, and each converter cell is implemented by the front-end ac/dc stage followed by a dc/dc stage with galvanic isolation. The highly modularized design enables high scalability and flexibility for the rectifier system. Figure 10 demonstrates a star-connected, input-series-output-parallel (ISOP) configuration to interface with the MV grid [41; 42]. Due to the cascaded configuration of converter cells per phase, 1200/1700 V Si IGBT and SiC MOSFET can be used ahead of the dc/dc galvanic isolation in each converter cell [43]. Moreover, a star-connected, input-parallel-output-parallel (IPOP) configuration is depicted in Figure 10. It is noted that the ac side can be configured as either a star or delta connection, and the star type features lower voltage stress for each converter cell [44; 45]. When connecting the IPOP type to the MV grid, a single conver  
Figure 10: Circuit diagram of modular multicell rectifier. (**a**) The circuit architecture in a star-connected ISOP configuration. (**b**) The circuit architecture in a star-connected IPOP configuration. (**c**) The ac/dc stage is realized by a single-phase, full-bridge circuit. (**d**) The dc/dc stage is realized by a dual active bridge circuit. with the MV grid is mandatory, and the 10-kV SiC MOSFET is found to be the significant enabling device [46]. A 25 kW 7 kV/400 V ac/dc converter using 10 kV SiC MOSFETs is successfully demonstrated in [47], and challenges in terms of flashover fault, high switching losses, MV cable oscillation, and dielectric dissipation are addressed. In the near future, the magnetic components and dielectric materials should be promoted to fully release the potential of this MV SiC-based, IOP-type modular multicell converter. Furthermore, the IOP type makes it compatible when connecting the modular multicell rectifier to a low-voltage power grid, e.g., 400 V, using 1200 V voltage class devices. The realizations of each ac/dc and dc/dc stage are well established. The single-phase ac/dc stage can be implemented by a full-bridge converter with four active switches, as shown in Figure 10c. A soft-switching variant with the zero-voltage switching capability is proposed in [47]. Moreover, it is noted that the three-phase ac/dc stage using a B6 topology is also promising for connecting to LV grids [48]. Under the case where each converter cell is designed to bear high power rating, the three-phase ac/dc gains advantage in terms of a higher efficiency. The dc/dc stage can be realized by multiple converter topologies, such as the dual active bridge [49] (as shown in Figure 10d) and LLC converters [50].