## 1 Introduction
As an example, in [21] a family of converters based on an inductive link is discussed, which provided significant advantages in efficiency and in avoiding bulky capacitors; however, it was pointed out that it was not easy to extend the number of ports. In order to overcome such a limitation, a first study was presented in [10; 11], where different topologies were compared and evaluated considering components, control algorithms, and fault behaviors for a possible application to LVDC systems. The management of the state of active parts towards earth can be a critical issue for LVDC systems. In fact, when MCs and electronic power converters in general are used to interface two systems, whether they are both AC, both DC, or one AC and the other DC, a discontinuity in the state of active parts toward earth is introduced (e.g., the AC neutral wire and/or the DC middle point are interrupted). This issue, along with its effects on the management of the state of active parts towards earth, requires further discussion. Additionally, high-frequency common-mode disturbances and earth currents, both during standard operation and under fault conditions, cannot be addressed considering the AC and DC parts separately, as was highlighted in [37; 38; 39; 40]. Some papers propose to introduce metallic isolation by mean of specific converters [41; 42], which usually helps to avoid unnecessary disturbances while allowing for correct earthing management. However, this approach exhibits two major drawbacks: on the one hand, it requires many earthing policies, in that each section of the distribution system is isolated from the others. On the other hand, it requires unnecessary cascaded conversion stages, which increases power losses and system complexity, with these issues also driving interest towards MCs. In addition, the controllability of the converter currents under external fault condition is of great importance for protection design and for their integration with the converter control algorithm. While a certain number of papers consider the eventuality of a fault on the DC side of VSCs, both in high- [43; 44; 45] and low-voltage [46; 47; 48] applications, and provide extensive analysis of fault currents and converter behavior in fault conditions, the concept of fault current control is still not completely developed. Some papers introduce converter topologies and controls designed to be tolerant to DC-side faults on VSCs, both for high- [49; 50; 51; 52; 53; 54] or low-voltage [7; 23] applications, or devices capable of limiting short-circuit currents [55; 56; 57; 58]. However, these approaches, even if effective, do not directly include fault management in converter control algorithms, limiting converter effectiveness in these regards. In order to overcome all of the aforementioned issues, a new MC topology for hybrid distribution systems is introduced in this paper. In particular, two main contributions for MC applications are introduced in this paper. First, a new modular MC topology is presented in detail; its control principles, based on a direct current-tracking approach (for higher dynamic performances) are discussed; and an innovative control structure based on two control layers is proposed. Second, to overcome the lack of effective DC circuit breakers, which represents a traditional issue for DC systems, the proposed control approach was developed with particular attention to current controllability during faults, resulting in converters tolerant with respect to external faults, which can be integrated into the system's protection [25]. A set of numerical simulations is presented to support the presented discussion, including MC operation in steady-state conditions, under abrupt load/generation variations, and in the case of pole-to-pole faults on a port in order to highlight the strengths of the proposed MC topology and control.