## 2 Innovative MC Configuration  
The proposed MC topology is based on an extension of the converter based on a capacitive internal DC link presented in [5] and further discussed in [11], taking advantage of the enhanced controllability and fault-insensitivity properties of this family of converters. The development of the innovative MC configuration presented here is driven by the rationale of providing a symmetrical structure with a common reference for AC and DC line voltages, thus unifying the AC neutral and DC middle point wires. This is a very significant feature for fault analysis and protection design, as the presence of a common neutral/midpoint wire keeps the fault response consistent among AC and DC ports, which is not the case with standard power converters, nor is it the case with the MC discussed in [5; 11]. Thank to this solution, it is possible to have both AC and DC earthing with no functional problems; in addition, for each port, a low-impedance (virtually zero)closing path for common-mode currents is present, such that the issues related to common-mode (CM) disturbances discussed in [37; 38; 39; 40] are strongly mitigated. The proposed MC structure is shown in Figure 1, in general form, including M four-wire AC ports, an internal DC link with an arbitrary number of equilibrators, and N three-wire DC ports. The M four-wire AC ports can be used, indifferently, to connect the MC to an AC source, such as the public AC distribution network, or to fed AC loads. In the latter case, improved reliability can be obtained by integrating storage devices in the MC, realizing UPS functionality. The equilibrators maintain voltage balance between the positive and negative internal DC busbars. Their function is not related to any particular port, hence their locations are purely arbitrary, and their number is defined on the basis of the expected level of unbalance in DC loads. Lastly, the three-wire DC ports have a two-fold scope: first, they allow for the distribution of two voltage levels, which provides greater flexibility for DC loads; second, the distributed neutral wire allows for unified earthing management among AC and DC ports. Moreover, analogous to its two-wire counterpart discussed in [11], the diodes are connected in parallel to the DC port capacitor to prevent the transitory negative voltages that can lead to capacitor damage and a loss of controllability [25]. Thanks to the proposed MC topology here proposed, only three module types are needed, as follows:  
1. four-wire AC port module, represented in Figure 2a, including two semiconductors, each required to withstand the full DC link voltage;
2. equilibrator module, represented in Figure 2b, including two semiconductors, each required to withstand the full DC link voltage;
3. three-wire DC port module, represented in Figure 2c, including four semiconductors, each required to withstand one half of the DC link voltage. Figure 1: Three\(-\)wire, capacitive\(-\)coupled MC topology. As a result, the circuit topology presented in Figure 1 is completely modular, allowing for flexible operation in the ever-changing modern distribution systems. From a design perspective, since the maximum voltage that DC ports need to withstand is equal to one half of the maximum voltage that AC ports and equilibrators need to withstand, it seems unwise to use equal power electronic building blocks (PEBBs) for all ports. Indeed, due to circuit similarities between the proposed DC port structure and three-level, neutral-point-clamped (NPC) inverter legs, introducing three-level NPC AC ports and equilibrators may be a promising solution to further improve performance and modularity. Nevertheless, the advantages possibly granted by this solution rely on sizing and economic issues, which fall outside the scope of this paper.