## 1 Introduction  
The ongoing changes in distribution systems are creating new interest in innovative solutions to enhance system performance. In particular, hybrid AC/DC distribution systems are a promising solution for obtaining high performance in terms of dynamic stability, voltage quality and reliability; in this context, multiport converters (MCs) are considered with particular attention, in particular when an integrated electronic transformer can be adopted [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. Indeed, these converters are particularly useful when it is necessary to interface different energy sources, storage devices, and loads with one integrated topology [10, 13, 14, 15, 5, 7, 18], while the traditional approach would require many two-port converters. For this task, MCs allow for a reduction in the number of components and cascaded conversion stages with respect to an equivalent system of two-port converters, resulting in reduced complexity, dimensions, and costs as well as improved reliability and enhanced efficiency [12, 13, 14, 15, 16, 17, 18, 19, 20, 24]. MC centralized control allows for better system dynamics and higher power quality and continuity [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25]. As all necessary communication channels can be embedded in the converter itself, the control effectiveness does not depend on communication infrastructures to achieve efficient coordination among the different ports. Furthermore, it can be easily integrated in advanced grid management, including hierarchical control structures [26; 27; 28]. The wide interest in MCs is proven by a significant amount of research work available in the literature covering a wide range of applications, from renewable energy sources, distributed generation, storage device integration, and distribution systems [5; 6; 12; 14; 18; 19; 20; 21; 24; 29; 30; 31] as well as automotive [13; 15; 32; 33; 34] and even high-voltage power systems [35; 36; 37]. As a consequence, MCs are identified as the ideal building block for high-performance distribution systems due to their properties in terms of controllability both under normal and fault conditions. Two main categories of MCs can be recognized based on their circuit topologies: MCs based on a capacitive internal bus [5; 6; 7; 8; 9; 10; 11] and MCs based on an inductive internal bus [11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. Considering that the DC coupling can be realized by means of a high-frequency transformer, the second category is then split into three sub-categories: isolated MCs [12; 13; 18; 19], non-isolated MCs [21; 22; 23], and MCs with a mix of isolated and non-isolated ports [14; 15; 16; 17; 18; 19; 20]. Furthermore, considering the wide range of possible applications, some MCs exhibit modular topologies [5; 6; 7; 8; 9; 10; 11; 22; 23], while others are designed with a specific number of ports and are not easily expandable [12; 13; 14; 15; 16; 17; 18; 19; 20]. From the control point of view, most MC controls are based on voltage-modulation techniques, often based on commutation phase-shift control [12; 13; 14; 15; 16; 17; 18; 19; 20], which is the standard for isolated converters comprising a high-frequency transformer. Some other studies, on the contrary, base their control algorithms on mean value quantities [5; 6; 7; 8; 9; 21; 22; 23], manipulating the duty cycle through suitable current and voltage loops. However, few papers consider converter behavior and control under external or internal fault conditions [7; 10; 11; 23; 25]. When considering the application of MCs to low-voltage DC (LVDC) distribution systems, the following three issues should be examined and discussed in detail: modularity; the state of active and exposed conductive parts with respect to earth; and controllability under fault conditions. A modular architecture is of high importance in distribution system applications; however, this issue is often overlooked in general designs for MCs.